Analysis of Selected Life Factors of Wooden Bridges

This book was supported by project LTI20004 "Environmental Research and Development Information Centre" funded by Ministry of Education, Youth and Sports of the Czech Republic, program INTER-EXCELLENCE, subprogram INTER-INFORM. III Contents Abstract .................................................................................................................... VII 1 Introduction to the Problem ............................................................................... 1 1.1 History ......................................................................................................... 3 1.1.1 Historical Development of Wooden Bridges ....................................... 3 1.1.2 Bridges in Antiquity ............................................................................. 3 1.1.3 Bridges in the Middle Ages .................................................................. 5 1.1.4 Bridges in Renaissance ........................................................................ 5 1.1.5 Wooden Bridges in Medieval China .................................................... 7 1.1.6 Swiss Bridge School ............................................................................. 8 1.1.7 Wooden Bridges in America ................................................................ 9 1.1.8 Wooden Bridges at the Beginning of the Railway ............................... 9 1.2 Material .....................................................................................................12 1.2.1 Wood Construction ...........................................................................12 1.2.2 Submicroscopic Wood Construction .................................................12 1.2.3 Microscopic Wood Construction .......................................................13 1.2.4 Macroscopic Wood Construction ......................................................16 1.2.5 Wood Properties ...............................................................................19 1.3 Wood in Numbers .....................................................................................26 2 Research Question and Aims of the Work ........................................................29 3 Hypothesis .........................................................................................................31 4 Methodology and Method of Organizing the Collection and Acquisition of Experimental Data .....................................................................................................33 4.1 Empirical Methods ....................................................................................34 4.1.1 Observation Methods ........................................................................34 4.1.2 Measurement Methods.....................................................................35 4.1.3 Experimental Methods ......................................................................36 4.2 Theoretical Methods .................................................................................37 4.2.1 Theoretical Methods Based on the Logic of Thinking .......................37 4.2.2 Theoretical Methods Based on Analogy and Modeling ....................37 IV Contens 5 Analysis of Selected Bridge Structures in the Czech Republic...........................39 5.1 Detailed Analysis .......................................................................................40 5.1.1 ...................................................................................40 5.1.2 .................................................................................................47 5.1.3 ................................................................................................54 5.2 Analysis of Humidity Conditions ................................................................61 5.2.1 Lenora ................................................................................................63 5.2.2 ...........................................................................................64 5.2.3 Modrava ............................................................................................65 5.2.4 Polka ..................................................................................................66 5.2.5 .......................................................................................67 5.2.6 novice .......................................................................................69 5.2.7 ...............................................................................70 5.2.8 ................................................................................................71 5.2.9 bor .................................................................................................72 5.2.10 .......................................................................................73 5.2.11 ................................................................................74 5.2.12 ..........................................................................................75 5.2.13 ..........................................................................................76 5.2.14 -Brno ....................................................................................76 5.2.15 ...............................................................................................78 5.2.16 ..........................................................................................78 5.2.17 ................................................................................................80 6 Possible Solutions of Short Life .........................................................................81 6.1 Timber-concrete Bridge Constructions .....................................................83 6.1.1 Laboratory Measurements ................................................................85 6.2 Composite Materials .................................................................................94 6.2.1 Laboratory Measurements ................................................................95 6.2.2 IN-SITU .............................................................................................100 7 Discussion, conclusion .....................................................................................105 Contens V 7.1 The Quality of the Wood Used ................................................................107 7.2 Environmental Impact .............................................................................108 7.3 Design Solution ........................................................................................113 7.4 Closing Word ...........................................................................................114 8 Literature .........................................................................................................115 VII Abstract The first part of the monograph contains basic information about the historical development of bridge construction not only from the environment of Central Europe. In the first block, the basic facts about the material, its properties and structure are also given. The main part of the monograph includes the original results of research on real structures and laboratory samples in order to confirm or refute the hypothesis of this work. The research results contain sets of evaluated data mainly from bridge constructions from Central Europe. Regional targeting is deliberate with regard to climate and wood-destroying factors. The monograph brings new approaches to the solution of modern wood-based bridge structures. It is mainly the design, development and testing of wood-concrete load-bearing system for bridges and insulation system for bridge construction. This work is based on historical structural and material facts about wooden bridges and footbridges and on the failures of existing structures, in order to bring a new modern solution for the development of the field of wood-based bridge construction not only for Central Europe. 6 Chapter 1 2022 a) b) Fig. 1.6 bridge designs a) A. Palladio, b) Fausto Verantio. [11] It is clear from fig. 1.6 that the Renaissance builders had already thought through the bridge construction systems that are still in use today, but they did not yet have our current materials and technologies available. The Palladian wooden covered bridge from 1567 Ponte deli Alpini near Bassano has been preserved to this day, fig. 1.7. According to Leonardo da Vinci's drawings, models of wooden bridges were made, fig. 1.8." [11] Fig. 1.7 Ponte deli Alpini in Bassano (A. Palladio - 1567). [11] Introduction to the Problem 7 Analysis of Selected Life Factors of Wooden Bridges Fig. 1.8 Leonardo da Vinci bridge design models (around 1500). [11] 1.1.5 Wooden Bridges in Medieval China "At the end of the last century, written records and the remains of unique wooden bridges were discovered in China. These so-called "Rainbow Bridges" only existed in China 900 years ago [3]. These were arch bridges, in which the arch was created in a special way of "tangled" straight logs, fig. 3.9. The bridges were uncovered, and about a hundred of them were built. In 1999, American and Chinese experts, using medieval techniques, managed to restore the "Rainbow Bridge" in Jinzh, fig. 1.9. There was also a covered variant of these bridges (Lounge Bridge), of which approximately 400 were built. These bridges were built up to a span of 30 m. Figure 1.10 shows an example of such a bridge. The Xidong Bridge was built in 1746, with a length of 41.7 m, a width of 4.9 m, and a span of 25.7 m." [11] a) b) Fig. 1.9 a) diagram of the wooden supporting structure, b) the reconstructed "Rainbow Bridge" in Jinzh. [11] 8 Chapter 1 2022 Fig. 1.10 Xidong Bridge in China from 1746. [11] 1.1.6 Swiss Bridge School "The construction of wooden bridges has a long and rich tradition in Switzerland. The most famous is probably the covered wooden bridge over the Rhine near Schaffhausen (fig. 1.11) built by the carpenter Hans Ulrich Grubenmann (1709-1783). With a span of 119 m with internal support, the bridge was built in 1758 and was destroyed by Napoleon's troops on April 13, 1799. The builder wanted to build the bridge without central support, but the city councilors were forced to insert the pillar. After completing the bridge, he removed the middle support and thus proved his mastery. Brothers Hans and Jean Grubenmann built other covered bridges, such as the bridge at Wettingen (1778) with a span of 110 m (this bridge was also destroyed in 1799) and the covered bridge at Reichenau (fig. 1.12)." [11] Fig. 1.11 Bridge over the Rhine at Schaffhausen (1758-1799). [11] Fig. 1.12 Bridge over the Rhine near Reichenau. [11] Introduction to the Problem 9 Analysis of Selected Life Factors of Wooden Bridges 1.1.7 Wooden Bridges in America "Hundreds of wooden road bridges, but especially railroad bridges, were built in America during the 19th century. Very well known is the so-called "Colossus" (fig. 1.13) - a wooden bridge over the river Schuykill with a flat lattice arch with a span of 104 m, with a cross-section consisting of six beams with a cross-section of 150x300 mm. The bridge was built in 1812 by Lewis Wernwag, who emigrated to the USA from Reutlingen, Germany. To date, there are nearly 500,000 bridges in the United States, mostly of smaller spans, of which 7% are wooden and another more than 7% have at least a wooden bridge deck." [11] Fig. 1.13 Bridge in the USA from 1812. [11] 1.1.8 Wooden Bridges at the Beginning of the Railway "In the 19th century, there was a rapid development of railways in Europe and on the American continent. This required many bridges, which were built of stone, brick, but in the beginning also of wood. Steel began to extrude other materials at railway bridges at the end of the 19th century in connection with the development of its industrial production when it was significantly improved and reduced in price. An example of a successful wooden structure of a railway viaduct can be the Portage Bridge (fig. 1.14), which served rail transport in the USA from 1852 to 1875. A lattice girder bridge 170 m long and 71 m high was supported by a system of five-storey wooden supports based on stone pillars." [11] 10 Chapter 1 2022 Fig. 1.14 Portage Bridge railroad viaduct in the USA. [11] "The information given in the previous chapters is just a very brief and incomplete list of the rich history and traditions of wooden bridges, which dates back to the beginnings of human history. After the decline in the construction of wooden bridges in the first half of the 20th century, caused by the advent of modern steel and concrete structures, a renaissance of wooden bridge structures has been taking place since the 1960s, especially in the area of road bridges and footbridges. This development was made possible by developing new wood-based construction materials, new types of joints, and design procedures, including modern computer technology and production technologies. Contemporary wooden bridges and footbridges are discussed in the following chapter." [11] There are about 20 historic covered bridge structures in the Czech Republic. Many of these buildings have not been preserved, as they were replaced by more modern structures or destroyed by bombing during World War II and floods, such as two river. Probably the oldest preserved bridge structure was built in 1719. The supporting construction of the bridge made of fir wood is still original. It is a two-pole beam bridge, the supporting elements of which are large-diameter rough-hewn beams. For this bridge in the 3rd century, it was necessary to repair mainly the cladding to protect the supporting structure from the weather. Introduction to the Problem 11 Analysis of Selected Life Factors of Wooden Bridges Fig. 1.15 H built-in 1721, the Rechle in Lenora from 1870, or the timbered covered bridge in region. All of these bridges have survived for centuries, and their longevity can inspire modern wooden bridge constructions. [11], [3] 12 Chapter 1 2022 1.2 Material 1.2.1 Wood Construction "The anatomical structure of wood largely determines its technical properties, and its knowledge is therefore essential in monitoring the behaviour of wooden elements. This issue can be found in more detail, for example, in the publications: [1] to [8]. The wood structure can be observed and described from the perspective of macroscopic, microscopic and submicroscopic point of view." [11] 1.2.2 Submicroscopic Wood Construction "Submicroscopic structure deals with the chemical composition of wood and molecular structure. The chemical composition of wood is practically the same for all trees species: 49.5% carbon, 44.2% oxygen, 6.1% hydrogen, and 0.2% nitrogen. Chemical elements then form several complex organic compounds: cellulose (40-50% of weight); hemicellulose (20-30% of weight); lignin (25-30% of weight); extracts (0-10% of weight). Cellulose (C6H10O5) n is a polysaccharide whose chain macromolecules consist of glucose units that are linked by 1-4 -glycosidic bonds. The cellulose macromolecule is linear, homogeneous (elementary fibril, microfibril). Free spaces between microfibrils (so-called intermicellar spaces) can hold small molecules (e.g., water), which causes the wood to swell. The average degree of polymerization of cellulose for average wood is given to 1000 or more. Cellulose is a relatively less reactive component of wood. Cellulose has high tensile strength, comparable to ordinary steel. The modulus of elasticity along the cellulose chains reaches up to 130 GPa; the modulus of elasticity perpendicular to the fiber axis is approximately 30 GPa. Hemicellulose is also a polysaccharide with a shorter chain of heterogeneous linear macromolecules. The average degree of polymerization for hemicellulose is 150-250. They are more often represented in deciduous wood than conifers. They are divided into hexosans (C6H10O5) n, which occur mainly in conifers, and pentosans (C5H8O5) n, which predominate in deciduous trees. Hemicellulose forms the link between cellulose and lignin. Hemicellulose and cellulose form a polysaccharide component of the wood mass, which is collectively called holocellulose. Lignin is a macromolecular substance of aromatic isotropic nature with a modulus of elasticity of only around 2000 MPa. Its macromolecules are large, three-dimensional, and highly reactive. Lignin is the cause of the elastic-plastic properties of wood. Introduction to the Problem 13 Analysis of Selected Life Factors of Wooden Bridges Hemicellulose and lignin significantly affect the compressive strength of wood fibers. Reducing the content of hemicellulose and lignin in the wood leads to a reduction in the strength of the wood. Extract substances, which are accompanying substances that can be separated by extraction, are not directly parts of the wood structure. Common extract substances are volatile acids, essential oils, alcohols, dyes, mineral compounds, etc. However, they significantly affect the physical properties of the wood. They increase the density of the wood and reduce the level of equilibrium moisture of the wood. The content of volatile extracts increases the durability of wood. The most crucial extractant is resin." [11] 1.2.3 Microscopic Wood Construction s are common to many different tree species. The cell wall skeleton is formed by cellulose, which is joined into larger structural units, so-called elementary fibrils. These are grouped into so-called fibrous microfibrils. Each microfibril contains approximately 100 to 2000 cellulose chains, as well as hemicellulose and lignin. Figure. 1.16 shows the structure of the cell wall of the wood fiber schematically. There is a layer of the so-called middle lamella (ML) between the individual cells, which connects the cells to the tissue. It consists predominantly of lignin and pectin and contains almost no cellulose. The cellulose microfibrils form an irregular lattice in the primary wall (P). The thickness of this primary wall is approximately 5% of the total cell wall thickness. The secondary wall in normal wood It consists of lamellae of microfibrils formed by cellulose chains. The lamellae are helically deflected to the left and clockwise from the longitudinal axis of the cell by about 50 to 700. The thickness of layer S1 is approximate twice the thickness of the primary wall. A substantial part of the secondary wall (85% of the thickness) consists of a layer S2 wit clockwise lamellae with minimal deviation from the longitudinal axis of the fiber (10300). The inner layer S3 is similar to layer S1. The microfibrils of this layer are also helically deflected in the opposite direction by 60 to 900. 14 Chapter 1 2022 Fig. 1.16 Schematic structure of the cell wall. [11] When observing wood under a microscope (at a magnification of at least 20 times), it can be verified that the wood consists of cells of various shapes and sizes, with other specific features invisible to the naked eye. The basic cell types are vascular cells, tracheids, libriform fibers, and parenchymal cells. Depending on their size and storage method, their function is different in living wood and still forming and in inanimate wood, which is already dead. However, it should be borne in mind that the individual cell types do not occur in isolation. They form more or less homogeneous groups tissues characterized by the same orientation and specific arrangement of cells. Figure 1.17 shows the basic cell types. The vascular cells are relatively broad, cylindrical cells. They are perforated at the ends to allow contact with adjacent vascular cells. Contact with the cells surrounding the vascular cells on their longitudinal sides is provided by thinning of the cell walls - dots and colons. In some types of wood, there is a helical reinforcement on the inner surface; in other species, there are membranes in the vascular cells (tulles) closing the cell cavity of the vascular cell. Vascular cells occur only in deciduous trees. The basic building block of conifers is 95% of the stem volume of long homogeneous cells - tracheids. However, they also occur in deciduous trees. These are elongated cells - tubes of square cross-section with rounded corners 2 to 5 mm long and 10 to are oriented parallel to the longitudinal axis of the trunk. During the transition from spring wood to the summer wall, the cells thicken; while the diameter of the cells decreases, the length of the cells increases. This creates a different density of spring Introduction to the Problem 15 Analysis of Selected Life Factors of Wooden Bridges and summerwood in a ratio of approximately 1: 3. However, radially oriented tracheids (so-called tracheid rays) also occur in the wood structure, interrupting longitudinal tracheids at certain intervals and ensuring radial distribution of nutrients. Tracheid beams are shorter than longitudinal tracheids, measuring only about 0.1 to 0.2 mm in length. Tracheids are adapted to conduct water and nutrients. The exchange of substances (thus also moisture) between the cells occurs by thinning in the cell walls - dots. In coniferous wood, the predominant type is the colon. Tulles are not formed in tracheids; helical reinforcements on the inner surface of the walls have only those types of wood that contain them in the vascular cells. Fig. 1.17 basic types of wood cells: a) wide vascular link with perforations, b) narrow vascular link with perforations and helical reinforcement, c) narrow vascular link with ladder perforations, d) tracheid, e) tracheid with helical reinforcement, f) wood parenchymal cells, g), h) ray parenchymal cells - horizontal and upright, i) libriform fiber, j) fibrous tracheid, k) lattice perforation of the vascular cell. [11] Libriform fibers are long and thin, mostly relatively thick-walled, spindle-shaped cells. Simple fine thinning - slit colons - are visible on their walls. The length of these cells is variable within the annual ring; the shortest are in springwood, the longest are in summerwood. They occur only in deciduous trees. Parenchymal cells have a different character from all previously described cell types. They are cubic, even elongated or even irregular, prismatic in shape, relatively thinwalled with a wide cell cavity. The walls contain a large number of simple thinning - dots. They are adapted to conduct and store nutrients. Their length is about 0.1-0.2 mm, and their thickness is about 0.01-0.05 mm. In addition to basic cell types, transient cell types (e.g., fibrous tracheids, fibrous parenchymal cells, etc.) also occur in some deciduous woods. 22 Chapter 1 2022 wood species moisture deformation coefficient perpendicular to the fibers parallel to the fibers tangentially radially conifers 0,32 0,16 0,01 deciduous trees 0,40 0,20 0,01 plywood 0,30 chipboard 0,45 - 0,70 Tab. 1.1 Average values of the moisture deformation coefficient when the moisture content of wood changes by 1%. [11] Fig. 1.22 Deformation of cross-sections cut from different log parts after drying. [11] The most important properties also include thermal conductivity and longitudinal thermal expansion. The wood itself has low thermal conductivity. The thermal conductivity of wood is low due to its low bulk density and porosity. However, it is different in the direction of the fibers and perpendicular to the fibers (it is approximately half) the thermal conductivity of wood increases with increasing bulk density and increasing humidity. Thermal elongation is lower than for metals and concrete. For spruce wood, it is 6x106 K-1 along the fibers and 34x10-6 K-1 perpendicular to the fibers; for oak wood, it is 4x10-6 K-1 along the fibers and 28x10-6 K-1 perpendicular to the fibers. Dry wood is a very good insulator. The specific electrical resistance of wood is smallest in the direction parallel to the fibers. It is approximately doubled perpendicular to the fibers. The electrical resistance of wood is inversely proportional to temperature and humidity. The electrical resistance of wood is the most sensitive to changes in humidity. This fact is used in electrical resistance hygrometers. Introduction to the Problem 23 Analysis of Selected Life Factors of Wooden Bridges The critical parameters for the design of structures are mainly the mechanical properties of wood (especially flexibility, strength) express its ability to withstand external loads. It is necessary to distinguish between the properties of perfect wood (i.e., basically wood mass) and structural wood. The properties of perfect wood show considerable dispersion, which is exacerbated by growth irregularities, dimensions, humidity, temperature, nature of the load (static, dynamic, impact), duration of load effects, negative environmental influences. The wood properties show a considerable variance between individual trees of the same species of wood but also within the cross-section of the trunk, both in cross-section and along the length of the trunk. The wood shows very different properties with a significant variance even within the annual ring the differences are between spring and summerwood. Due to its anisotropy, wood shows different mechanical properties in the direction of the tree main axis (longitudinal - parallel to the fibers, perpendicular to the fibers - in the radial and tangential direction to the annual rings - see fig. 1.23). Normal and tangential stresses in the directions of the main axes are shown in fig. 1.23. For wood, the strengths are determined in: bending (fm); tension parallel to the fibers (ft, 0) and perpendicular to the fibers (ft, 90); pressure parallel to the fibers (fc, 0) and perpendicular to the fibers (fc, 90); shear (fv) and torsion (ftor). For wood, three different moduli of elasticity E (EL, ER, ET), three moduli of elasticity in shear G (GLR, GLT, GRT), and six Poisson's factors of are introduced (LR, LT, RL, RT, TL, TR). Of these 12 elastic constants, 9 are independent of each other. Only constants for directions along and perpendicular to the fibers will suffice in the practical assessment of the elasticity and strength of wooden elements of structural dimensions. The differences in the radial and tangential direction to the annual rings are smaller orders of magnitude and usually neglected. When designing elements of wooden structures, the most important are the strength and deformation characteristics of materials. 24 Chapter 1 2022 Fig. 1.23 Orientation of stress in wooden cross-section. [11] The properties of perfect wood are essentially the properties of the wood mass because their strength and deformation characteristics are determined on small samples (e.g., for the flexural strength test, the sample dimensions are: 20/20/300 mm), which do not contain natural wood imperfections. In contrast, structural wood (dimensions of real structural elements) has these imperfections (knots, fiber deflection, cracks) arising during the wood's growth and subsequent processing to the extent that they occur in common materials produced by manufacturers of wood materials. Until relatively recently, the material characteristics of wood were determined on small, perfect samples, and the results were then converted to structural wood of real dimensions. Eurocodes (EC5) introduces methods for determining the material characteristics of structural elements. Fig. 1.24 schematically shows a working diagram of perfect wood stressed in parallel with the fibers - i.e., the dependence between tensile/compressive stress and relative deformation. It is clear from the diagram that the deformation line of the tension runs almost linearly until the brittle failure. Significant nonlinear behavior before failure occurs at pressure. A dashed line marks the actual course of the working diagram; a solid line marks the idealized course for practical calculations. Moisture significantly affects the mechanical properties of wood; as the moisture content of the wood increases, the strength and stiffness of the wood decrease. After reaching the fiber saturation point, there is practically no further decrease. Introduction to the Problem 25 Analysis of Selected Life Factors of Wooden Bridges Graph 1.2 Working diagram of wood for tensile stress (ft) and compressive stress (fc) parallel to the fibers. [11] The effect of moisture on the mechanical properties of perfect wood is given in tab. 1.2. Strength and deformation characteristics are determined for wood moisture of 12%. In practice, a linear relationship between mechanical properties and moisture content can be considered for wood moisture in the range of 8-20%. mechanical properties change of strength [%] pressure in the direction of the fibers 6 pressure perpendicular to the fibers 5 bend 4 tension in the direction of the fibers 2,5 pull perpendicular to the fibers 2 shear perpendicular to the fibers 2,5 impact toughness 0,5 modulus of elasticity in the direction of the fibers 1,5 Tab. 1.2 Change in the mechanical properties of perfect wood with a change in humidity of 1%. [11] Further information on the structure of wood and its properties can be found in the literary sources [3], [5], [8], [9], [11], [17], [20], [24], [26] [36], [38], [45]. 26 Chapter 1 2022 1.3 Wood in Numbers As a source of wood before agriculture (8,000 years ago), forests covered almost half of the land. According to the "Food and Agriculture Organization", the area of forests in 2011 was about 31% of the land, which is about 41 million km2. The largest percentage of the world's forests can be found in the Russian Federation, Brazil, Canada, the USA, and China. characteristics world EU woodiness [% ] 31 37,6 change in afforestation since 2000 - 1,2 2,8 Tab. 1.3 Forestry and afforestation in the world and the EU. [11] In the Czech Republic, the forest cover under Maria Theresa was about 14%, and in 2014 about 34% and is still growing. Finland leads in the EU forestry ranking with about 73%. The Czech Republic is in 11th place. The lowest afforestation in the Netherlands and Ireland is about 11%. The total supply of wood in the Czech Republic in 2016 was estimated at approximately 696 million m3, and the average supply per 1 ha of forest land is 265 m3. [44] Graph 1.3 Species diversity of forests in the Czech Republic in 2016. [44] The total species composition of forests in the Czech Republic in 2016 is shown in Graph No. 1, where the dominant species is Norway spruce, which also forms the dominant mined raw material. Among deciduous trees, beech and oak have the largest representation. Norway spruce pine other broadlea ves beech oak Larch birch other conifers %50,5 16,4 8,5 8,3 7,2 3,8 2,3 0,3 0 10 20 30 40 50 60 Introduction to the Problem 27 Analysis of Selected Life Factors of Wooden Bridges Graph 1.4 Species change of forest warehouses in the Czech Republic. [9] Spruce monocultures are gradually declining due to the long-term sustainable health of the forest. The increase in wood material in the Czech Republic in 2016 was approximately 17.9 million m3, and extraction this year was approximately 15 million m3. In the long run, logging does not reach annual increments. In 2018, mining was increased due to the bark beetle calamity. [44] 2010 2013 2014 2015 2016 coniferous 73,9 72,9 72,5 72,3 72,1 broadleaf 25,1 25,9 26,3 26,5 26,7 0 10 20 30 40 50 60 70 80 29 2 Research Question and Aims of the Work The main scientific question is to determine the reason why current wooden bridge structures, despite modern materials, technologies, and experience, in many cases do not reach even 10 years of life. quality of the tree species used; environmental impact; constructional solutions. 31 3 Hypothesis Based on historical experience, it can be said that wooden bridge structures are very durable, and their service life in the Czech Republic exceeds 300 years. There are still much older historic wooden bridge structures abroad, but wood is a local raw material with its specifics. Therefore it is necessary to focus on buildings made of local materials. In the EU, we can find modern wooden bridges older than 25 years, such European and Scandinavian countries (Tynset in Norway, the Reno Bridge in Norway, the Crest Bridge in France and others). Fig. en bridge. Therefore, it can be assumed that modern bridge structures, thanks to the experience and modern materials such as glued laminated timber, will have higher durability than historical bridges that have survived for centuries. 39 5 Analysis of Selected Bridge Structures in the Czech Republic To determine the main causes of failures and insufficient service life of some modern wooden bridge structures, a detailed analysis of existing bridge structures in the Czech Republic is crucial, as wood is a local raw material with specific properties for individual areas. [41] to [43] 40 Chapter 5 2022 5.1 Detailed Analysis Based on inspections of about 40 modern wooden structures in the Czech Republic, three were selected for detailed analysis in order to reveal the critical causes of failures of these structures. 5.1.1 With wooden bridges and footbridges, including historic buildings, we often come across one of the most dangerous wood-destroying fungi from the Gloephyllum family. This attacks coniferous and deciduous wood in Central Europe. Detecting an attack is difficult, especially in the first phase. A standard diagnostic method used in practice is the so-called acoustic percussion method, which has been used since ancient times. Because it is a very aggressive and rapidly spreading species of wooddestroying fungi, its early detection is crucial for the possibility of remediation and prevention of serious failures or accidents. To determine the occurrence of this dangerous species of fungus, it is currently possible to use many diagnostic methods through acoustic, culture, and DNA tests. crossing the Vltava River. Because it is a building in a protected area, the wood and design were chosen as a ith the historical center (see fig. 5.1). Fig Bridge. It is a two-span continuous bridge with an upper deck. The bearings are sliding steel tangential. There are no bridge closures on the bridge-building. The load-bearing steel structure is made of - Faltus" elements. The grating structure consists of six main I-shaped welded beams. Rolled L-shaped profiles ensure horizontal reinforcement. The bridge deck is made of spruce crossbeams (prisms) of thickness 160 mm and a height of 140 mm; the bridges are oak with a thickness of. 0.05 m. The crossbars are formed by three elements of Analysis of Selected Bridge Structures in the Czech Republic 41 Analysis of Selected Life Factors of Wooden Bridges different lengths, as can be seen in fig. 5.2. The longest elements, with their cantilever, form a support for the railing construction. Medium-length crossbeams also have a static diagram of a continuous beam with overhanging ends. The shortest crossbeams are stored as continuous beams. The bridges are placed obliquely over the crossbars, as can be seen in fig. 5.2. Fig. 5.2 Scheme of distribution of wooden elements of crossbeams and bridges. The wooden structure was protected with impregnating agent KORASIT CK. It is a fluid based on chromium and copper compounds. The compound is a water-soluble salt. The impregnating agent acts against fungi and wood-destroying insects. Impregnation was applied using vacuum-pressure technology. Four years after commissioning, fungi of the genus Gloeophyllum appeared on some crossbeams. These findings indicated an advanced stage in the attack on the bridge structure. Fig. 5.3 Infestation by wood-destroying fungus. 42 Chapter 5 2022 The occurrence of wood-destroying fungi requires, above all, suitable conditions for growth. These include the humidity, temperature, and pH of the wood. In particular, the impregnation prevents the wood from sticking to and being attacked by wooddestroying fungi and thus perfectly protects the surface until its compactness is compromised. Cracks created only after impregnation can be a way to be attacked by wood-destroying fungi, including fungi of the genus Gloeophyllum. On the grew only from cracks in the wood mass. This attack was caused by the impregnation of wet wood, thanks to which the surface of the elements was perfectly protected at the time, but after the non-dried wood was incorporated into the structure, it dried quickly in the summer, which caused considerable drying cracks. Cracks occurred mainly on the sunlit upper crossbeams side. Thanks to which water and dirt got into the core of the cross-section and created ideal conditions for attachment and growth of Gloeophyllum fungi. Fig. 5.4 Method of violation. In the case of an attack of the impregnated element by Gloeophyllum fungi, which after installation contains drying cracks, the unprotected core is attacked, and the internal mass of the cross-section is destroyed. Due to the impregnation of the perimeter of the elements, no indications of infestation by this fungus are visible until the fruiting body appears. In most cases, the internal structure of the wood is irreversibly damaged. Early identification of fungal infestations from the Gloeophyllum family is crucial for the structure's life. These wood-destroying fungi (Gloeophyllum spp.) Attack both coniferous and deciduous trees. They need a temperature range of 5 - 60% for growth. This is a very dangerous sponge, especially for building structures. The danger is mainly associated with the destruction of the wood mass, which usually progresses outwards from the core of the cross-section. Gloeophyllum sepiarium - creates in the semicircular to circular solid fruiting bodies, grown laterally or in the middle. The fruiting bodies are roughly bristly on the surface, ocher rusted to dark brown, and often form strips. The pulp of the fruiting Analysis of Selected Bridge Structures in the Czech Republic 43 Analysis of Selected Life Factors of Wooden Bridges body is also rusty brown. It grows on fences, railings, and bridge structures. In houses, it is most often in window sills. Infected wood appears to be healthy, but inside it is damaged. Damage to the wood can be identified by tapping. The fruiting bodies grow from cracks and fissures. During prolonged drought, the mycelium dries and dies. The spores are cylindrical, smooth, colorless, size 4 Gloeophyllum abietinum (Bull.: Fr.) P. Karst.) has similar properties. In contrast to the disc beam, which is coarsely hairy on the surface of the fruiting bodies, the smaller fruiting bodies of the Gloeophyllum abietinum have a darker color and a smooth surface. The pulp of the ovary is light. The spores are smooth, cylindrically ellipsoidal, size 10- - Gloeophyllum trabeum (Pers.: Fr.) Murril.) It differs from fungi of the genus Gloeophyllum because the drape does not form leaves but large pores. The fruiting bodies are dark brown but initially resemble small buns and spill over the substrate, most often along cracks. The spores are cylindrical in size 7- - Diagnostics Three diagnostic methods are most often used, which differ in efficiency, evidence, and efficiency. The methods are chosen mainly according to economic and time requirements. acoustic percussion This method is one of the oldest and most often used in construction practice. This is mainly due to the minimal demands on technology and speed of identification. The method consists of tapping the monitored element with a mallet and monitoring the hollowness of the element using human hearing. Several factors can affect this method, such as the boundary conditions of the element, its humidity, and degree of damage. For bridge construction, where there are a large number of the same elements in terms of both storage and moisture, this method can be effective. acoustic ultrasonic Acoustic methods can also include ultrasonic diagnostic methods that can detect cavities in wood. These methods cannot be used on some structures mainly due to access to the monitored elements. cultivation In microbiology, cultivation means the targeted maintenance or multiplication of microorganisms in vitro. Bacteria are most often cultured on growth media, while viruses are cultured in cell cultures or embryos. Certain specific factors necessary for the growth of microorganisms or cell culture maintenance are necessary for cultivation. The main factors are temperature and carbon dioxide tension. [10] 44 Chapter 5 2022 Fig. 5.5 Culture dish with AGAR. It is possible to use AGAR type culture medium for cultivation. Wood samples are taken from the structure, C). After about 5 days, it is already possible to determine whether the structure is infested with a wood-destroying fungus. This is a laboratory method. DNA Deoxyribonucleic acid, commonly referred to as DNA, is a nucleic acid that carries the genetic information of all organisms except some non-cellular ones. [4], [10] This method enables fast and accurate but costly determination of the species of wood-destroying fungus. For these purposes, it is necessary to take a suitable sample of the wood mass. This is a laboratory method. The diagnostics aimed to determine the extent of damage to the wooden crossbeams method was the acoustic percussion method, which quickly detected damaged elements. The second method was the cultivation method, which aimed to confirm the percussion method and determine the exact extent of the infestation. Both methods were applied to all overhanging crossbeams that were endangered by running water and dirt. On each two-beam console, 4 seats were monitored. The first field was marked as section ABCD and section KLIJ, according to fig. 5.2. The second field was designated as the EFGH section and the OPMN section. In total, it was a set of 212 elements. The acoustic percussion method was applied using a mallet, which created an impact on the monitored element. A professional technician evaluated the subsequent acoustic reaction. The acoustic sound was always compared with the response of the surrounding crossbars for more optimal determination of results. The result was the determination of damaged elements, which is indicated in fig. 5.5. Analysis of Selected Bridge Structures in the Czech Republic 45 Analysis of Selected Life Factors of Wooden Bridges a) acoustic method b) cultivation method damaged wood infested wood healthy wood Fig. 5.6 Graphical representation of the result of diagnostic methods a) acoustic method b) cultivation method [10] The above graphic representation fig. 6.6a) shows that about 46% of the monitored elements were declared damaged by the acoustic percussion method. The occurrence of the beam determined the probable cause of the damage. Cultivation was chosen as a control method. Samples were taken from all monitored elements with a core drill and separated for subsequent laboratory evaluation of the occurrence of the beam. From the above graphic representation, fig. 6.6b) it follows that the cultivation method revealed beam infestation in about 83% of the monitored elements. Both methods were applied to the same elements but with different results. Determination of damage to the elements by the acoustic hammer method showed that approximately 46% of the monitored wooden crossbeams are damaged. The remaining approximately 54% showed no signs of damage. The control culture method was then confirmed in about 41% of damaged elements identified by the first method of infestation with a wood-destroying fungus. In addition, it expanded the range of attacks on the monitored elements by about 37%. The remaining 17% was declared healthy wood. These connections are indicated in graph 5.1. A B C D E F G H I J K L M N O P 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 A B C D E F G H I J K L M N O P 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 46 Chapter 5 2022 Graph 5.1 Graphical representation of the extent of damage and infestation of wood. [15] Damaged elements diagnosed by the acoustic percussion method revealed approximately 6% of damaged structural elements that were not confirmed by the control culture method. This difference can be considered as an error, which could be caused by several factors in the diagnosis, including the extent and size of cracks, the structure of the wood, the influence of the environment, noise, and more. These elements were declared undamaged because they did not prove the presence of a wood-destroying fungus by the cultivation method. The above error could also be the error of the culture method, where the most likely cause appears to be an inappropriate sampling point. However, this factor is very unlikely, as sampling has always been carried out in such a way as to capture possible damage, but of course, it cannot be ruled out. From the point of view of economic comparison of both methods, the acoustic percussion method is more favorable, in a ratio of 1: 5. The cultivation method's economic complexity consists mainly of sampling, subsequent sorting, preparation for laboratory diagnostics, cultivation itself, and evaluation. The so-called DNA test is the most sophisticated and economically most demanding method of detecting a wood-destroying fungus. Wood-destroying fungi from the family of beams are one of the biggest risks, especially for wooden bridges and footbridges. Its danger is mainly in the difficulty of detection by ordinary visual inspection. This sponge mainly attacks the core of the cross-section, which is not as protected by impregnation as the envelope of a wooden structural element. After the attack, the internal structure of the wood mass of the cross-sectional core is very often destroyed, and only the impregnated Analysis of Selected Bridge Structures in the Czech Republic 47 Analysis of Selected Life Factors of Wooden Bridges envelope remains undamaged. Infestation with this wood-destroying fungus was monitored on about 10 structures of bridges and footbridges in the Czech Republic. On the monitored st the damage to wooden elements. For this purpose, the acoustic tapping method was used, which is a common diagnostic method for determining the damage to elements due to wood-destroying fungi from the beam family. Laboratory cultivation was chosen as the control method. The results show that the classical acoustic method by tapping is very accurate in terms of wood damage by wood-destroying fungi from the beam family. However, as demonstrated by the cultivation method, the extent of the infestation is much larger, mainly due to the different stages of beam growth in the individual elements. The research proved the effectiveness and efficiency of the acoustic percussion method. Nevertheless, it is appropriate to carry out a cultivation method to ensure the cause of the damage. [2], [4], [10], [14], [15], [16], [18], [26] to [33], [37] 5.1.2 V An essential part of any design of a wooden bridge structure is the choice of such an arrangement of elements that they do not cause the intake of rainwater into the load-bearing wooden or other components. Otherwise, there is an acute threat, especially from soft rot. Fig. supporting structure of the footbridge is a double wooden girder made of glued beams with an intermediate bridge deck. The footbridge is made of a system of simple beams mounted on pillars and supports on steel bearings with a Teflon surface. The footbridge consists of 5 fields, while the first and fifth fields are made of two uniform circular arches with a radius of 23 meters, fields 2, 3, and 4 are 54 Chapter 5 2022 Fig. 5.18 Leakage point due to slatted bevels. Fig. 5.19 Demonstration of a suitable and unsuitable solution. [11] 5.1.3 Wood can also be used to lighten the structure, making it possible to increase the resulting load-bearing capacity of the bridge structure. This solution mainly brings an increase in level, which affects subsequent communications. . [12], [40] Analysis of Selected Bridge Structures in the Czech Republic 55 Analysis of Selected Life Factors of Wooden Bridges The investigated bridge is designed as a vertical bridge with one span. The supporting structure consists of two main steel lattice girders. The bridge deck consists of a steel crossbeams system, LLD side members, and bridges. The cross members are made of two U280 profiles welded at the joints to the main beams at axial distances of 2720 mm. Longitudinals made of LLD GL24h with a rectangular cross-section with a width of 180 mm and a height of 360 mm are placed on crossbars at axial distances of 400 mm and 500 mm. The head of the longitudinal members is modified by a notch 180 mm high. Bridges made of grown oak dried wood D30 are placed on the longitudinal members. The connection to the side members is made using countersunk screws. At the same time, the bridges form a moving road. The substructure consists of two concrete abutments with sloping dilated wings. Bridge bearings are placed on stone storage blocks loosely mounted on storage sills. The substructure does not affect the load capacity of bridges. [12], [40] Fig. 5.21 Schematic section of the main bridge structure. [12], [40] During a visual inspection of the bridge, structural defects were found that could significantly affect its load-bearing capacity or further use. Longitudinal cracks were found on the glued laminated timber side members in the area of the notch in the place where the wooden side members were placed on the steel crossbeams. These cracks are a typical phenomenon when exceeding the shear capacity of wood. In this case, the phenomenon is exacerbated by notch, where shear strength decreases depending on the geometry of the notch. The formation of such cracks is typical for this treatment. [12], [40] Diagnostics The focus of the diagnostics was mainly on the primary failure, which was the failure of wooden elements by pulling perpendicular to the fibers due to shear, see fig. 5.22. 56 Chapter 5 2022 Fig. 5.22 Photo documentation of the crack in the longitudinal member. Based on the original static calculation, an assumption was discovered that could was an even distribution of the load from the road section to the side members. The road part was formed by oak transversely placed prisms D30, 120/160, which were screwed directly to the longitudinal members. Prior to the diagnostics itself, a calculation of the FEM aid was performed, the subject of which was to determine the load distribution from the vehicle through the bridge deck section to the side members. The FEM model is based on the real dimensions and materials of the damaged structure. [12], [40] 40] Analysis of Selected Bridge Structures in the Czech Republic 57 Analysis of Selected Life Factors of Wooden Bridges The solved longitudinal members were selected based on the performed load test indicated on the diagram (see fig. 5.24). Fig. 5.24 Scheme of distribution of sensors on monitored beams [12], [40] Fig. 5.25 Placement of string strain gauges on the longitudinal member. [12], [40] A total of 4 string strain gauges were used, with the following distribution: the first sensor recorded the stress directly at the point of presumed failure by pulling perpendicular to the fibers due to shear stress. The other 3 sensors were located on 58 Chapter 5 2022 the underside in the middle of the spans. The layout and designation of the sensors are shown in fig. 5.25. A loading vehicle according to the design of the FEM model was used for the load. The load distribution during the test was according to the diagram in fig. 5.26. Fig. 5.26 Schematic load distribution during load test. [12], [40] After evaluating the load test, see. [12], [40] the results were compared with the FEM model to determine the load distribution to individual longitudinal members. Analysis of Selected Bridge Structures in the Czech Republic 59 Analysis of Selected Life Factors of Wooden Bridges Chart. 5.2 Percentage distribution of the load on the monitored elements. The differences, which were determined based on the FEM model and the IN-SITU load test, are in the order of tens of percent. The cause of these significant differences may be the rigidity of the connection between the longitudinal members and the bridge deck part and the wood mass's inhomogeneity. However, the damage by soft rot was especially revealed during the subsequent reconstruction (see fig. 5.27). Fig. 5.27 Damage to monitored beams. [12], [40] The cause of the failure was then evaluated for soft rot, which formed under the insulating foil. These facts were not known during the load test or during the inspection of the structure. There was no indication of such a failure. A B C MKP 5,88 29,4 64,7 IN-SITU 3,4 17,9 78,7 0 10 20 30 40 50 60 70 80 90 60 Chapter 5 2022 Despite the significant influence of the failure of the beams by soft rot, it was proved that the load distribution on the longitudinal members from the bridge deck part is not uniform. This proved the crack, where the wood mass was broken by pulling perpendicular to the fibers due to shear forces. Analysis of Selected Bridge Structures in the Czech Republic 61 Analysis of Selected Life Factors of Wooden Bridges 5.2 Analysis of Humidity Conditions Wood moisture is one of the decisive factors in wooden elements' life and mechanical properties. Therefore, 17 wooden bridge structures, both modern and historical, were monitored, for which surface and internal humidity depending on temperature were recorded. The different humidity ratios of the massive loadbearing elements were mainly monitored. Internal humidity sensors, which are protected as intellectual property under number 32424, were used for monitoring. Fig. 5.28 shows a diagram of this sensor, which uses the non-conductivity of the Teflon layer to detect moisture at the tip end at a specific location. Fig. 5.28 Diagram of internal humidity sensor. [13] According to the diagram, these sensors were placed in the supporting longitudinal elements of the monitored structures (see fig. 5.29). Fig. 5.29 Scheme of measured points on monitored structures. In addition to the internal humidity, surface humidity was also monitored in the same place to determine the humidity gradient. The following equipment was used for this purpose: "Greisinger GMH3810 wood moisture meter". [6], [13] 62 Chapter 5 2022 a) b) Fig. 5.30a) BOSCH PTD1 b) Greisinger GMH3810. Another monitored variable was the relative humidity of the outside environment and temperature measured using: "BOSCH PTD 1 thermo-detector". An additional measurement was ultrasonic monitoring of the propagation speed of acoustic waves in the material. With a purpose to determine the estimate of possible internal damage and density of the material using: "Pulse ultrasound TICO" using 54 kHz probes. Fig. 5.31 Pulse ultrasonic apparatus TICO. [12] It is clear from the above diagram of fig. 299 location of the probes that direct pulsed ultrasonic measurement was used. Relationships define the propagation velocities of the longitudinal and transverse waves. (1) (2) Analysis of Selected Bridge Structures in the Czech Republic 63 Analysis of Selected Life Factors of Wooden Bridges In which: vp longitudinal wave propagation velocity (m.s-1), vs transverse wave propagation speed (m.s-1), 0 bulk density of the material (kg.m-3), Poisson's number (-), Young's modulus of elasticity (Pa). It follows from these relations that wave speed propagation does not depend on the frequency of the waves but only on the material parameters of the environment. Therefore, the choice of source frequency during measurements does not affect the absolute results but only the "quality" of the measurement. There is a loss of transmitted energy (absorption) in a given environment, which significantly depends on the frequency of the waves and humidity, temperature, and other parameters [19]. 5.2.1 Lenora It is a historic bridge whose load-bearing structure consists of rough-hewn wooden beams placed on a stone substructure. The beam construction is two-pole, composed of simple beams. The upper building is roofed and clad along its entire length, see fig. 5.32. Fig. 5.32 Lenora bridge construction after reconstruction. This building is located on the outskirts of the village of Lenora at an altitude of about 755 m above sea level and bridges the Warm Vltava in the South Bohemian Region. The structure was built in 1870, of which the substructure and main supporting beams have been preserved. The cladding and roofing were changed during several reconstructions. 70 Chapter 5 2022 Fig. 5.42 - advanced delamination. 5.2.7 This suspension bridge bridges the E50 road in the middle of the Buchlov hills. Its load-bearing structure consists of two curved beams of glued laminated timber placed on the reinforced concrete substructure. Transverse reinforcement is realized by the bridge deck part (steel crossbeams and cross-winding by rods - intermediate bridge deck); see fig. 5.43. The building lies at an altitude of about 546 m above sea level and is located in the South Moravian Region. The structure was built in 2011 and is made of spruce wood. The left truss at a distance of approx. 1 m from the closing wall in the direction of the rest stop was chosen as the monitoring point. Analysis of Selected Bridge Structures in the Czech Republic 71 Analysis of Selected Life Factors of Wooden Bridges Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 23/7/ 2018 15,3 19,9 55 12,67 25 - 220 20/11/ 2018 16 15,7 44 1,807 - 2,1 1840 220 Tab. 5.8 Recorded values from diagnostics. - advanced delamination. Advanced delamination is evident in the construction of the main beams, see fig. 5.44, which corresponds to the age of the structure. 5.2.8 This bridge structure is located near the village of Kunovice. Its supporting structure consists of two steel trusses and crossbeams. In order to increase the load-bearing capacity, a wooden bridge deck part was designed during the reconstruction, which was made of longitudinal members made of glued laminated wood and bridges made of oak wood (see fig. 5.45). Fig. 5.45 bridge construction. 72 Chapter 5 2022 The building lies at an altitude of about 176 m above sea level and bridges a tributary of the Moravia renovated in 2016. The extreme longitudinal section at a distance of approx. 1 m from the closing wall in the direction from the village of Kunovice was chosen as the place of observation. Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 23/7/ 2018 16,6 21,1 55 13,63 26,3 - 250 20/11/ 2018 16 19,9 54 2,496 - 0,6 1610 250 Tab. 5.9 Recorded values from diagnostics. The construction is in perfect condition. 5.2.9 -bearing structure consists of two curved beams of glued laminated timber placed on the reinforced concrete substructure. Transverse reinforcement is realized by the bridge deck part (steel crossbeams and cross-winding by rods intermediate bridge deck); see fig. 5.46. Fig. 5.46 ridge construction. The building lies at an altitude of about 299 m above sea level and is located in the Moravian-Silesian region. The structure was built in 2015 and is made of spruce wood. The left truss at a distance of approx. 1.5 m from the closing wall in the direction of Analysis of Selected Bridge Structures in the Czech Republic 73 Analysis of Selected Life Factors of Wooden Bridges Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 4/ 8 / 2018 13,6 20,1 58 13,69 27,5 1890 220 220 Tab. 5.10 Recorded values from diagnostics. The construction is in perfect condition. 5.2.10 Thi -bearing structure consists of two straight beams of glued laminated timber placed on the reinforced concrete substructure. Transverse reinforcement is realized by the bridge deck part (steel crossbeams and cross-winding by rods intermediate bridge deck); see fig. 5.47. Fig. 5.47 . The building lies at an altitude of approximately 495 m above sea level and is located The left truss at a distance of approx. 1 m from the closing wall in the direction of the E442 road was chosen as the monitoring point. Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 4/8/ 2018 15,6 42,5 61 15,54 26,8 - - - Tab. 5.11 Recorded values from diagnostics. 74 Chapter 5 2022 Fig. - biological damage. Each supporting beam is composed of a pair of glued lamellar trusses, between which it flows, and thanks to this, there was an attack by wood-destroying insects, see fig. 5.48. 5.2.11 -bearing structure consists of two straight beams of glued laminated timber placed on the reinforced concrete substructure. Transverse reinforcement is realized by the bridge deck part (steel crossbeams and cross-winding by rods - intermediate bridge deck) see fig. 5.49. Fig. 5.49 bridge construction. The building lies at an altitude of about 484 m above sea level and is located in the The left truss at a distance of approx. 1 m from the closing wall in the direction of The construction shows no signs of damage. Analysis of Selected Bridge Structures in the Czech Republic 75 Analysis of Selected Life Factors of Wooden Bridges Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 4/8/ 2018 16,4 23,8 61 14,45 25,5 - - - Tab. 5.12 Recorded values from diagnostics. 5.2.12 v 1 This wood-concrete beam footbridge is located on the outskirts of the village wooden beams made of laminated wood and a concrete slab that can be driven directly. The horizontal structure is placed on the reinforced concrete substructure. The bridge deck part realizes transverse reinforcement (reinforced concrete slab); see fig. 5.50. Fig. 5.50 bridge construction. The building lies at an altitude of about 338 m above sea level and is located in the Central Bohemian Region. The structure was built in 2017 and is made of spruce wood. The right truss at a distance of approx. 1.5 m from the closing wall in the direction of Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 6/8/ 2018 11,3 18,5 74 20,69 28,5 1980 200 20/11/ 2018 15,6 14,1 41 2,074 0,6 1870 200 Tab. 5.13 Recorded values from diagnostics. 76 Chapter 5 2022 The construction shows no signs of damage. 5.2.13 This wood-concrete beam footbridge is located on the outskirts of the village orting structure consists of wooden beams made of laminated wood and a concrete slab that can be driven directly. The horizontal structure is placed on the reinforced concrete substructure. The bridge deck part (reinforced concrete slab) realizes transverse reinforcement (see fig. 5.51). Fig. 5.51 bridge construction. The building lies at an altitude of about 338 m above sea level and is located in the Central Bohemian Region. The structure was built in 2017 and is made of spruce wood. The right truss at a distance of approx. 1.5 m from the closing wall in the direction of Ul. surface Ul. inner Ul . air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 06/08/ 2018 10,9 18,2 74 22,69 30,2 1920 200 20/11/ 2018 14,3 14,4 54 2,435 - 0,9 1950 200 Tab. 5.14 Recorded values from diagnostics. The construction shows no signs of damage. 5.2.14 -Brno This arched footbridge is bridging the river Svitava. Its supporting structure consists of two arched beams of glued laminated timber placed on the reinforced concrete Analysis of Selected Bridge Structures in the Czech Republic 77 Analysis of Selected Life Factors of Wooden Bridges substructure. Transverse reinforcement is realized by the bridge deck part (steel crossbeams and cross-winding by rods - intermediate bridge deck); see fig. 5.52. Fig. 5.52 Brno bridge construction. The building lies at an altitude of about 272 m above sea level and is located in the South Moravian Region. The structure was built in 2012 and is made of spruce wood. The left truss at a distance of approx. 1.3 m from the closing wall in the direction of the center of Brno was chosen as the monitoring point. Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 23/07/ 2018 11,8 24,4 46 9,036 22,2 1530 280 20/11/ 2018 15,5 21,5 48 2,377 0,3 1540 280 Tab. 5.15 Recorded values from diagnostics. -Brno bridge construction - advanced delamination. 78 Chapter 5 2022 Advanced delamination is evident in the construction of the main beams, see fig. 5.53, which corresponds to the age of the structure. 5.2.15 It is a historic bridge whose load-bearing structure consists of rough-hewn wooden beams placed on a stone substructure. The beam construction is two-pole composed of simple beams. The upper building is roofed and clad along its entire length, see fig. 5.54. Fig. 5.54 bridge construction. m above sea level and bridges the Svratka River in the South Moravian Region. The structure was built in 1718. The cladding and roofing were changed during several reconstructions. The original wooden beam, which bridges the first field towards the center of the village and is located on the middle beam of the transverse span, was chosen for observation. Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 20/08/ 2018 16 22,6 50 8,648 20 1830 320 18/11/ 2018 17,3 16,3 49 2,44 0,4 1770 320 Tab. 5.16 Recorded values from diagnostics. 5.2.16 This beam footb of glued laminated timber placed on the reinforced concrete substructure. Transverse Analysis of Selected Bridge Structures in the Czech Republic 79 Analysis of Selected Life Factors of Wooden Bridges reinforcement is realized by the bridge deck part (steel crossbeams and crosswinding by rods - intermediate bridge deck); see fig. 5.55. Fig. 5.55 bridge construction. The building lies at an altitude of about 325 m above sea level and is located in the South Moravian Region. The construction was made of spruce wood. The left truss at a distance of approx. 1 m from the closing wall in the direction of Ul. surface Ul. inner Ul. air Ul. air te mperature ultrasonic propagation speed width dat e % % % g.m - 3 C m.s - 1 mm 27 /08/ 2018 11,9 21,6 50 8,543 19,8 - 220 20/11/ 2018 14,5 22,3 46 2,537 1,9 - 220 Tab. 5.17 Recorded values from diagnostics. Fig. 5.56 The bridge construction in ice delamination. 86 Chapter 6 2022 The performed experiment is based on static friction, which ensures operational safety, significantly higher load capacity, and lower deformation than in the case of kinematic friction. a) b) Fig. 6.7 a) axonometry of the experimental sample, b) section of the experimental sample, (1 U-shaped wooden element, 2 T-shaped concrete element, 3 prestressing elements. For the proposed experimental element, see fig. 6.7 uses the connection of a Tshaped concrete profile and a U-shaped wooden element, which forms a woodconcrete beam during the connection, while the wooden elements also form a hidden formwork. This solution makes it possible to introduce transverse prestressing forces after the concrete has hardened, which ensures the connection between the elements by friction. The friction force can be varied depending on the applied preload. Potefloned fasteners were used to allow controllable prestressing. The advantage of this solution is the removal of the lubricant, which can bind impurities and thus significantly change the resulting values of the applied prestressing forces using a torque wrench. If a lubricant is used, it often floats out, and subsequent tightening no longer guarantees the required prestressing forces. Potefloned means guaranteeing constant conditions during prestressing and the possibility of repeated processes over time. Possible Solutions of Short Life 87 Analysis of Selected Life Factors of Wooden Bridges Fig. 6.8 Teflonated fasteners. Before prestressing, tests were performed to calibrate the fasteners covered with Teflon. The same assembly that was used in the wood-concrete beam experiment was calibrated. The assembly consisted of a bolt, two large-format washers, and nuts. Fig. 6.9 Measuring set. The measuring set included a Wera 7100 series torque wrench, an Ahlborn Almemo 2690 control panel, and an FKA613 force sensor. The aim was to perform several successive tightenings on a series of 3 assemblies so that it was possible to compile 88 Chapter 6 2022 a calibration graph of the dependence of the tightening torque on the prestressing force. Graph 6.1 Dependence of the tightening torque on the prestressing force. Graph 6.1 shows that the 1.0 and 2.0 series are almost identical. The only difference is the 3.0 series, which was assembled from elements produced in a different series. Graph 6.2 Dependence of the tightening torque on the prestressing force of states 1-4. Possible Solutions of Short Life 89 Analysis of Selected Life Factors of Wooden Bridges Graph 6.2 describes the dependence of the prestressing force on the tightening torque of the individual states. Condition 1 describes the average values corresponding to the first tightening. Each additional condition corresponds to a subsequent release and retightening. The decrease in prestressing force due to wear of the friction surfaces between the first and second tightening (state 1 and state 2) corresponds to almost 30% (28.72%). Subsequent steps differ minimally, always with a decrease in prestressing force of about 1%. Thanks to the performed calibration of the tightening torque and prestressing force for Teflon joints, it was subsequently possible to perform a correct prestressing of the experiment. The experiment was set up based on preliminary calculations and numerical models to be performed in a controlled laboratory environment. The monitored sample from the KVH profile, strength class C24, consisted of three wooden elements measuring 60x280 mm x 2 and one element 60x180 mm. After assembly, these elements form a U-shape and lost formwork at the same time (see fig. 6.8). C40 / 45 T-shaped concrete mixer has board dimensions of 180x100mm and webs of 60x100mm. The concrete reinforcement was provided by a welded wire mesh of 100x100 mm with a diameter of 6 mm to ensure the transmission of shear forces (see fig. 6.8). Fig. 6.8 Reinforcement of the experimental wood-concrete sample. To determine the resulting dimensions and mass distribution of wood and concrete, the force sum conditions of the balance of forces in the cross-section were used: , (4) where the value of Fc1 corresponds to the compressive force in the concrete T-section slab. Fc2 indicates the compressive force in the web section of the concrete T section. In the profile, can be a pressure effect on the U-profile wooden part, which is introduced in the formula by the value Fw1. On the right side, while neglecting the possibility of tensile action of the concrete T-profile, only the drawn part of the 90 Chapter 6 2022 wooden profile U remains, which is introduced by the value Fw2. When placed in the force condition of equilibrium, we get the formula: , (5) where fck is the characteristic compressive strength of concrete, beff the elective width, and hc the slab thickness. Furthermore, the formula contains b - web width T cross-section, fc0k - characteristic wood compressive strength, ft0k - characteristic wood compressive strength, h1 - the height of the middle wooden element, h - the height of extreme wooden profiles U. The only unknown is the value x, which indicates the position of the neutral axis of the idealized cross-section. (6) Optimal dimensions and materials were designed based on the idealized woodconcrete cross-section's determined neutral axis. A neutral axis is also an ideal place for mounting the transverse prestressing of the profile. Fig. 6.9 Fitting the experimental wood-concrete sample into a test bench. The element designed in this way was fitted with a transverse preload at the location of the assumed neutral axis. Based on the assumption of elasticity theory, the prestressing elements were installed according to the values of longitudinal shear: , (7) where VEd is the design value of the shear force, Sc is the static moment of the area of the pressed part of the concrete slab to the neutral axis of the idealized crosssection, and I1 is the moment of inertia of the idealized cross-section. Another criterion for the placement of prestressing elements was the size of the washers and the distribution of prestressing forces in the wood-concrete interface. The maximum prestressing forces are always limited by the size of the base so that the wood mass Possible Solutions of Short Life 91 Analysis of Selected Life Factors of Wooden Bridges is not damaged by pressure perpendicular to the fibers. When using large-format thick-walled 60 HRC hardened steel washers, the average preload corresponded to approx. 17.5 kN with an average tightening torque of approx. 65 Nm. According to the distribution of prestressing elements and the introduction of prestressing, the theoretical friction force on the beam was determined to be approximately 146 kN. To ensure the proper functionality of the structure, the condition must be met: . (8) The primary goal of the wood-concrete beam experiment using frictional forces was primarily to determine the EIef value, which depends on many factors, including the properties of concrete and wood. The unknown factor is mainly the rate of friction sledding. Four-point bending was used for static loading of the experimental sample. This type of loading was used mainly due to the theoretically constant shear stress in the first and last third of the span, which is suitable for testing the coupling for which shear is crucial. The loading was realized using a hydraulic device in step force increments without a relief cycle. The individual steps were chosen at 20 kN until failure. At each step, the monitored values were read, including deflection in the middle of the span, pressure in the supports, compressive force, and tension of the lower and upper fibers (see fig. 6.9). Graph 6.3 Resulting graph of deformation versus load. 0 20 40 60 80 100 120 0 2 4 6 8 10 12 14 16 92 Chapter 6 2022 Graph 6.3 describes the dependence of the deformation in the middle of the beam on the total force of the hydraulic compression pistons during four-point bending. This graph describes an almost ideal linear curve terminated by slippage in the transverse prestressing friction joints at a force of 103 kN and deformation of 15.01 mm. During the slip, the sample did not break only in the abrupt change from static to kinematic. Graph 6.4 Resulting graph of stiffness and force dependence. An essential parameter is the element's stiffness under load, described in figure 6.4. The stiffness during the whole load is very stable, with a maximum range of 8.88% and an average value of around 6900 kN / m. compressive force sag rigidity - concrete - wood [kN ] [mm ] [kN/m ] [MPa ] [MPa ] 0 0 0 0 0 23 3,51 6553 - 10,28 6,41 43 6,25 6880 - 18,33 12,10 63 8,76 7192 - 27,56 16,87 83 11,83 7016 - 36,94 21,87 103 15,01 6862 - 47,33 26,31 Tab. 6.1 Data obtained from the load test of an experimental wood-concrete sample. Table 6.1 lists all the values obtained and calculated in the static load test of the experimental wood-concrete sample. 0 1000 2000 3000 4000 5000 6000 7000 8000 0 20 40 60 80 100 120 F[kN] Possible Solutions of Short Life 93 Analysis of Selected Life Factors of Wooden Bridges Based on this pilot experimental test, basic information and parameters for starting a series of tests were obtained. These are mainly the values needed for a more accurate design of other experimental tests such as (EI) ef. Fig. 6.10 Idealized stress diagram of wood-concrete composite elements. Suppose we use an idealized assumption about the behavior of wood-concrete composite elements. In that case, it is possible to determine by back analysis, for example, the values of the modulus of slip Kser. (9) When changing the preload, it is also possible to assume a change in the Kser slip module. The degree of prestressing applied depends mainly on the prestressing force, which can be a decisive factor compared to other coupling means. [1], [7], [21] to [23] 94 Chapter 6 2022 6.2 Composite Materials The best-known composite in construction is reinforced concrete, which uses the tensile strength of steel in the drawn part of the element, and concrete is used to transfer compressive forces. Together, these materials work due to similar thermal longitudinal expansion. Despite steel's high compressive and tensile strengths, reinforced concrete elements are more economically advantageous than if each material were used separately due to the connection with concrete. The same principle can be applied to increase the service life of wooden elements on bridge structures. The basic rule is to prevent rainwater from flowing into the wooden profiles, which subsequently increases the humidity and the possibility of attack by biological pests. The service life can thus be reduced by decades, as shown by long-term monitoring of selected structures (see chap. 5.1). Among the traditional ways to insulate wooden beams against rainwater is coating insulation (cardboard, foil) or plating. Insulation is always applied only from the top so that there can be an exchange of moisture between the wood and the environment. These common insulation systems can cause local failures, such as insulation failure. In some cases, moisture can evaporate from the wood element directly under the insulation, causing condensation to occur, which can cause an increase in humidity and subsequent failure. a) b) Fig. 6.11a) variant with insulating foil b) variant with plating. Fault under insulation is, in most cases, very problematic to identify. Due to condensation on the underside of the insulation, it flows into the lowest place or area. Possible Solutions of Short Life 95 Analysis of Selected Life Factors of Wooden Bridges Fig. 6.12 Fault under the insulation on the side member. Fig. 6.12 shows a fault that was caused by condensation under the insulator and subsequent biological attack by soft rot. This damage occurred after only 3 years of operation. In addition to weakening the beam, the joints of the adjoining elements were also loosened. A possible alternative that could eliminate the existing condensation problem is a composite composed of a carrier (wood) and a contact insulator (polyurea). The main requirements for the insulator are cohesiveness, sufficient flexibility, and water tightness. Polyurea material was chosen as the insulating element. It is a highly flexible, twocomponent polymer system that can be used on materials such as steel, concrete, and wood. The application is carried out using special high-pressure hot pumps. The mixing components (amines and isocyanates) ratio is 1: 1. The system is characterized by its fast curing in the order of several tens of seconds. The insulation has very good mechanical parameters (adhesion to the substrate, tensile strength, elongation). Another advantage is the UV resistance of the material. A composite composed of wood and polyurea may be suitable for wooden bridges and footbridges. Thanks to this solution, condensation, and subsequent faults should be prevented. The main benefit would be to increase the service life by decades. The uncertainty of the optimal function lies primarily in the airtight sealing of the wood surface. The prevention of the possibility of evaporation, and the ingress of moisture, depending on the external environment. 6.2.1 Laboratory Measurements Three types of experimental measurements were performed within the laboratory verification of the set assumptions. 102 Chapter 6 2022 The selected beam was chosen in the first field at the edge of the transverse span. This place was chosen because of the expected most significant load of weather and humidity. The measurement was started in November 2016 after the beam was installed in the structure. Graph 6.8 Course of humidity conditions on the monitored structure. When installing the sensors, the humidity of the entire selected beam was measured. Because the beam was directly exposed to heavy precipitation during transport, installing the insulation system on the drier side was necessary, where the humidity was around 14% during installation. On the opposite side of the cross-section, a value of approx. 46% was measured, see fig. 6.21. Fig. 6.21 Recording of excessive humidity before installation of sensors (45.6%). 0 5 10 15 20 25 30 XI.16 I.17 III.17 V.17 VII.17 IX.17 XI.17 I.18 III.18 V.18 VII.18 IX.18 XI.18 XI.16 I.17 VIII.17 I.18 VII.18 XI.18 external 15,5 16,8 15,95 16,55 16,1 15,95 internal 13,9 15,8 26,15 24,05 21 19,65 Possible Solutions of Short Life 103 Analysis of Selected Life Factors of Wooden Bridges After installing the beam on the structure in the winter, when the relative humidity and temperature are low, the internal humidity remained around 16.5%, and the surface humidity gradually decreased. At the onset of high summer temperatures, the internal humidity shifted outwards from the profile. At the same time, the moisture hit the insulation system, thanks to which the humidity under the insulation increased abruptly. This moisture does not affect the cohesion of the composite, but long-term exposure could lead to infestation by wood-destroying pests. In the following period, the cross-section dried naturally, which also positively affected the internal humidity under the insulation. Assuming that the current trend remains, there is a presumption to prove the wood-polyurea composite's optimal functioning. Suppose the long-term functionality of this system is proven in practice. In that case, it will be possible to contribute to better protection and a significant increase in the service life of wooden bridge structures. 105 7 Discussion, conclusion This work tries to answer the fundamental question of the low service life of some wooden bridge structures in the Czech Republic. Despite the existing theoretical orientation procedures, how to determine the service life of wooden bridge structures, these methods are only a qualified estimate. One of these methods is, for example, the Factor Method (ISO 15686-1), which is described in detail in the literature. Based on this method, the service life of an uncovered structure is around 40 years and of a roofed structure around 150 years [27]. The probable cause of the low service life of some current wooden bridge structures is non-compliance with one or more rules for the durability of these structures, which are known from the preserved historic structures. These are mainly: selection of a suitable material (larch for structures, oak for the road part) ; ; ensuring optimal humidity (internal wo If these conditions are not ensured, there may be a significant reduction in service life, as with the structures listed in chap. 5.1. If the basic rules of design, construction, and administration are not observed, the service life is reduced to about 10 years. These constructions also include the construction of a footbridge in the village of Strunkovice, which was made of glued laminated timber, which ensured the condition of dryness of the built-in material. Fig. 7.1 Condition of the wooden footbridge in the village of Strunkovice after 8 years. However, two other conditions were not ensured, and therefore, after ten years, this structure was replaced by a steel structure. During the construction of the steel structure, the supporting structure was no longer included in the soil, which was the main cause of the destruction of the original wooden footbridge, and its service life is estimated at decades. 106 Chapter 7 2022 The knowledge of the professional public in the field of design and management of steel or concrete structures is probably decisive over the last more than 100 years of experience. Wooden bridges and footbridges in the Czech Republic have a modern history of approximately 20 years. I believe that this difference in the knowledge and experience of the professional public is a decisive factor in the issue of the service life of wooden bridges and footbridges in the Czech Republic. The following answers to the fundamental questions aims to contribute to the design and implementation of timber structures that will last for tens to hundreds of years, as history shows that timber structures can compete successfully with concrete or steel structures in the long run. Discussion, conclusion 107 Analysis of Selected Life Factors of Wooden Bridges 7.1 The Quality of the Wood Used Wooden bridge constructions are being built all over the world. From the available sources, it is clear [3], [11], [27] that wooden bridge constructions can last for hundreds of years. Wood is a local raw material that varies more or less by region. This work was focused mainly on the locality of the Czech Republic, where it is possible to find hundreds of wooden bridge structures and dozens of historic buildings whose age is more than 100 years. The quality of wood can be monitored from many perspectives, including mechanical properties, durability, resistance to biological pests, and others, described in detail in the literature [11], [27]. All monitored modern bridge structures in the Czech Republic are made of spruce wood, see chap. 5.5. The probable argument of this monopoly is the price and availability of spruce lumber in a given locality. Assuming the similarity of the hardness properties of spruce wood in Central Europe, which can be verified in the field using the ultrasonic method, it is possible to compare with historic structures, whose durability is proven by their existence. Modern wooden bridge structures whose age does not exceed 10 years were selected for monitoring, see chap. 5.2. There were a total of 8 monitored constructions with a maximum of 1980 ms-1 at the wood-concrete footbridge in -1 (2012). The average value for all monitored structures is the speed of propagation of ultrasonic waves 1780 ms-1. It is necessary to add that these constructions have no major defects and are at the beginning of their life. When compared to historic buildings, where the average value reaches 1840 ms-1, the quality of the material when comparing the speed of propagation of ultrasonic waves is to the detriment of modern buildings. This comparison is incorrect due to the construction of historic buildings hundreds of years ago from larch wood, whose strength and durability are higher than that of spruce wood [1]. Based on the monitoring of three historic bridge structures, it is clear that over time there is no significant decrease in the density of the material, so that it has a significant effect on the mechanical properties. 108 Chapter 7 2022 7.2 Environmental Impact As an organic, inhomogeneous, anisotropic and hygroscopic material, wood is influenced by the external environment. The moisture content of wood, which is long-term depending on the external environment, is a significant influence. The moisture content of the wood affects the mechanical properties and the service life of the structure. The risk limit of wood moisture is associated primarily with the occurrence of biological pests. This is the primary risk regarding the longevity of wooden bridge structures, which can cause serious failures in a relatively short period. The installation of undried wood is hazardous, despite modern impregnations, see chap. 5.1.1. Thanks to the gluing technology, modern constructions made of glued laminated wood ensure that the wooden elements are pre-dried before being built into the structure. The development of the external environment for the humidity of glued laminated timber structures was monitored on 14 modern bridge structures, which are up to 10 years old, and three historical structures, see chap. 5.2. The following graphs always show the values of surface moisture measured on the surface of the structure and the internal humidity, which was monitored inside the profile see chap. 5.2. Graph 7.1 Dependence of temperature on material humidity in summer. Graph 7.1 shows the external and internal humidity of the monitored structures at different temperatures in the summer, which brings higher absolute humidity. If we interpolate the points with a linear function, we decrease humidity with increasing temperature in both cases. Based on this graph, it is possible to consider a slight drying of the surface and internal wood mass. In the case of moisture above 30% of the internal materials, incipient defects were found in the structure. Discussion, conclusion 109 Analysis of Selected Life Factors of Wooden Bridges Graph 7.2 Dependence of absolute ambient humidity on material humidity in summer. Graph 7.2 shows the moisture content of the wood (surface, indoor) at the absolute humidity of the environment. The graph shows that the wood dries out even with increasing absolute humidity. From the above facts, it can be said that temperature is a significant factor influencing the humidity of wood in the summer. Graph 7.3 Dependence of temperature on material humidity in winter. 110 Chapter 7 2022 The dependence of external temper ature and humidity of wood is shown in graph 7.3. Here it is clear that as the temperature drops, the humidity also drops. This is the opposite phenomenon as in the summer period, where the humidity of the wood decreased with increasing temperature. Graph 7.4 Dependence of absolute humidity on material humidity in winter. Graph 7.4 records the wood moisture values for the respective absolute ambient humidity. It is clear from the graph that with decreasing absolute humidity, there is also a decrease in wood moisture, where a sharp decrease occurs in the humidity of the inner material. It is, therefore, possible to claim that, in addition to the decreasing temperature, the absolute environment humidity also affects the drying of wood. Important graph 7.5 shows the dependence of the wood moisture gradient (difference between summer and winter humidity) and the absolute ambient humidity gradient (difference between summer and winter gradient). From the linear curve corresponding to the external (surface) humidity, the increase in the wood moisture gradient in absolute value with the increase in the absolute humidity gradient of the environment is evident. The greater the drop in absolute humidity of the external environment, the greater the increase in humidity. While for indoor humidity, it is the opposite. The greater the absolute humidity gradient, the greater the moisture loss inside the materials. Discussion, conclusion 111 Analysis of Selected Life Factors of Wooden Bridges Graph 7.5 Dependence of the absolute humidity gradient on the wood moisture gradient. The above data also makes it possible to determine the difference between outdoor and indoor humidity in different seasons. This is an average difference of 8.88% for the summer period, while this difference in the winter period is only 5%. This condition is caused by an increase in surface moisture and a decrease in internal humidity in winter. Graph 7.6 Comparison of values from graph 1.1 with really measured data in summer. 118 Literature 2022 [35] RITTER M. A. 2005: Timber bridges: Design, Construction, Inspection and Maintenance Part 2, Honolulu Hawaii: University Press of the Pacific, p 453, ISBN 1-4102-2191-X. [36] frame work joint, Procedia Engineering 114: 132-139. [37] Shirouzu, T., Uno, K., Hosaka, K., Hosoya, K. 2016: Early-diverging wooddecaying fungi detected using free complementary sampling methods, Molecular Phylogenesis and Evolution 98: 11-20. [38] [39] -Press, 121-126, ISBN 807099-276-X. [40] ZP_2015009. [41] 2016: Monitoring and failures of footbridges made from glued laminated wood, Procedia Engineering, Vol. 142, pp. 87-91, ISSN 18777058, DOI: 10.1016/j.proeng.2016.02.017. [42] footbridges made from glued laminated wood. Procedia Engineering 142: 87-91. [43] . 2017: Dynamic effects on wooden footbridge, MATEC Web of Conferences 107: 00012. 16. [44] glued joints of timber beams and the influence of quality manufacturing on to their carving capacity, Wood Research 61(4): 573-581. [45] Wood, L. W. 1962: Relation of stregth of wood to duration of load, U.S. Forest Products Laboratory, Report No. 1916, 12/1951, reprinted 1962, https://www.fpl.fs.fed.us/documnts/fplr/fplr1916.pdf. Literature 119 Analysis of Selected Life Factors of Wooden Bridges [46] -807434-389-6.