Behaviour and design resistance of long bolts in tension

Behaviour and design resistance of long bolts in tension Neda Jankovi´ c a,* , Filip Ljubinkovi´ c a , Jorge Conde b , Jordi Costa c , Luís Sim˜ oes da Silva a,1 a University of Coimbra, ISISE, ARISE, Department of Civil Engineering, Portugal b Universidad Polit´ ecnica de Madrid, Departamento de Física y Estructuras de Edificaci´ on, Av. Juan de Herrera, 4, 28040 Madrid, Spain c Tornillería Industrial, S.A. c/ Catalunya 11, P.I. Can Oller, 08130 Santa Perpetua de Mogoda, Barcelona, Spain ARTICLE INFO Keywords: Long bolts Bi-directional bolts Design tensile resistance Stiffness Ductility Eurocode ABSTRACT Long bolts, also known as bi-directional bolts, have multiple applications in structural engineering, particularly as an instrumental part of the adaptability and reusability of steel members. Despite that, the long bolt design is not properly covered by current standards. This paper describes an experimental and analytical study to characterize the long bolts in tension for their inclusion in the component method precluded by Eurocode 3. The study is part of a wider campaign (including the behaviour in shear, preloading, and relaxation of the bolts) developed in the framework of the European Research Project CONNECT4C. The study shows that the current design expressions for bolt resistance and stiffness can be used for long bolts in tension, regardless of bolt material or finishing surface. A bolt material model is proposed for possible inclusion in steel design standards. Finally, it is also recommended to adopt double nuts in long bolts to avoid premature thread stripping. 1. Introduction In many structural systems, long steel bolts may be advantageous. They allow the connection of parts that may not be in contact and are increasingly used in demountable solutions, potentially leading to improved adaptability and reusability in construction. Several innovative applications have profited from the deformability of long bolts, such as self-centring structural solutions [1,2]. They provide an excellent fastening solution whenever the grip length exceeds the available lengths for standard structural bolts, for instance, the connection across concrete-filled tubes (Fig. 1(a)), joints connecting very thick plates (Fig. 1(b)), foundation anchors (Fig. 1(c)). In the context of this paper, “long bolts”are defined as structural bolts that are manufactured from circular steel rods instead of a steel coil and are not subjected to heat treatment, in contrast to standard structural bolts. This type of threaded rods with metric threads is covered by EN 15048 [4], where they are referred to as “stud bolts”. They are needed whenever a bolt head is undesirable or longer lengths are required because standard structural bolts are usually available with maximum lengths (e.g. standard structural bolts according to EN 14399–4 [5] are specified in this standard with maximum lengths that may vary from 95 mm for an M12 diameter bolt up to 200 mm for an M36 diameter bolt, although it may be possible to order them with longer lengths). Hence, the behaviour of long bolts may differ from standard structural bolts although their design properties must comply with EN 1993–1-8 [6]. The ongoing RFCS CONNECT4C European Project [7] is focused on developing demountable and adaptable structural solutions that promote the reuse of reclaimed steel members. A key aspect of the project relates to the development of innovative joints that provide very large tolerances to accommodate reclaimed steel members without the need to cut or extend them [8]. These joints comprise the extensive use of long bolts that are dominantly subject to pure tension with or without shear. However, long bolts are not well covered in the literature and are not clearly supported by specific product standards. To fill this knowledge gap, this paper aims to verify and characterize their behaviour in tension and assess whether the design expressions for tension resistance and stiffness in EN 1993–1-8 [6], calibrated for standard structural bolt assemblies, can be applied to long bolts. To this end, an experimental campaign of long bolts in tension is reported. Secondly, a material law is proposed for the steel bolt material. Finally, the applicability of the design expressions in EN 1993–1-8 [6] is performed and a reliability assessment is carried out to validate the choice of the partial factor γ M2 * , in line with the target reliability index of EN 1990, Annex D [9]. * Corresponding author. E-mail address: [email protected] (N. Jankovi´ c). 1 The author Luís Sim˜ oes da Silva is an editor of this journal. In accordance with policy, Luís Sim˜ oes da Silva was blinded to the entire peer review process. Contents lists available at ScienceDirect Journal of Constructional Steel Research journal homepage: www.elsevier.com/locate/jcsr https://doi.org/10.1016/j.jcsr.2025.109492 Received 29 December 2024; Received in revised form 20 February 2025; Accepted 3 March 2025 Journal of Constructional Steel Research 229 (2025) 109492 Available online 10 March 2025 0143-974X/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). 2. Background 2.1. Geometry and normative framework In Europe, standard structural bolts must comply with product standards (EN 15048 [4] for non-preloaded structural bolting assemblies and EN 14399 [10] for high-strength structural bolting assemblies for preloading) and are supplied as an assembly that comprises the bolt, the nuts and the washers (the latter part of the assembly in case of EN 15048). These product standards refer to additional ISO standards that specify mechanical properties [11], dimensional properties [12–18], dimensional tolerances [19,20], coating systems and surface properties [21–23] and inspection procedures [24–26], as summarized in Table 1 [27]. Concerning high-strength structural bolting assemblies for preloading, two different systems are available, HR (EN 14399–3 [28]) and HV bolts (and EN 14399–4 [5]), which mainly differ in their failure mode under pure tensile force, particularly in terms of both residual strength Symbols and acronyms Acronyms B Black surface of the long bolt BF Bolt fracture failure mode G Galvanized surface of the long bolt FT Fully threaded bolts LC Load cell LVDT Linear velocity displacement transducers N1 Single nut configuration for long bolt N2 Double nut configuration for long bolt PT Partially threaded bolts TS Thread stripping failure mode Latin letters AGross cross-section area, Percentage of elongation after fracture A gt Elongation percentage corresponding to the tensile strength A f Elongation after fracture for full-size fasteners A s Tensile area of bolt dExternal diameter of the fully threaded bolt d s Diameter of the unthreaded shank d 1 Basic minor diameter of bolt threads d 2 Basic pitch diameter of the bolt thread d 3 Minor diameter of the bolt thread EYoung’s modulus of steel FForce, load F t,Rd Design tension resistance of bolt f yb Bolt yield strength f ub Bolt tensile strength HHeight of the fundamental triangle of the thread k 2 Adjustment factor for tension resistance of bolt l 15 Bolt length corresponding to 15 threads LTotal bolt length L b Bolt elongation length L g Grip length L g,N1 Operational grip length for tests with single nuts L g,N2 Operational grip length for tests with double nuts L t Distance between the nut face and the end of the threaded part (runout) L tl Thread engagement length PThread pitch R p Stress at 0.2 % non-proportional elongation R m Tensile strength R f Stress at fracture point r t,nom,i Design resistance function (nominal properties) t n Thickness of nut t w Thickness of washer uElongation of the bolt Greek letters ε sh Strain hardening strain ε u Ultimate strain γ M2 Partial factor Fig. 1. Applications of long bolts: (a) Concrete-filled tubes [3]; (b) Joint with large grip length (Centro de Arte y Tecnología, Segovia, Spain. Picture by the authors); (c) Foundation anchor (Bodega BRVS, La Rioja, Spain. Picture by the authors). N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 2 and deformation capacity at the point of collapse. The HR type assemblies achieve ductility primarily through the plastic elongation of the bolt, while the HV type assemblies attain ductility through the plastic deformation of the engaged threads [10]. Hence, the first type is characterized by a necking fracture in the threaded part of the bolt, while the second usually fails by nut stripping, without shank necking [29]. Fig. 2 illustrates the geometry of a typical fully threaded (FT) and partially threaded (PT) structural bolt assembly. It is composed of a bolt and a nut, with or without washers depending on the type of bolt (nonpreloaded or preloaded) and the joint material. In Fig. 2(a), the distance between the inner faces of two washers, denoted as L g , represents the grip length, i.e. the thickness of the parts being fastened, whereas the distance between the nut face and the end of the threaded part (runout) is denoted by L t .L,L b and L tl represent the total bolt length, the bolt elongation length (defined as the grip length +washer thicknesses + half the combined height of the bolt head and the nut) and thread engagement length (defined as the axial distance over which the internal and external threads of a fastener overlap, ensuring the distribution of load), respectively. Ais the cross-section area of the shank while A s denotes the crosssection area of the threaded length, given by Eqs. (1a) and (1b) according to ISO 898-1 [11]. A= π 4d2 s(1a) As= π 4(d2+d3 2)2 (1b) where d s is the diameter of the shank, d 2 is the basic pitch diameter of the bolt thread, and d 3 is the minor diameter of the bolt thread. These and other relevant parameters (d 1 - the basic minor diameter of bolt threads, Pthread pitch and Hthe height of the fundamental triangle of the thread) are explained in standards ISO 724 [13], and ISO 68-1 [31]. Fig. 3 illustrates the geometry of a long bolt assembled with single nuts. Unlike standard hexagonal head structural bolts, it does not exhibit a bolt head because of the manufacturing process, so nuts are required at both ends. In this study, long bolts are fully threaded rods, although they can also be manufactured as partially threaded. The total length of the bolt, denoted as L, is only limited by the length of the rod. Additionally, L b refers to the bolt elongation length, L g is the grip length, L tl is the thread engagement length, and dis the external diameter of the bolt. 2.2. Behaviour of bolts in tension 2.2.1. Standard structural bolts Fig. 4 depicts an example of the force-elongation curve of a bolt tested in tension. In the initial part, the curve exhibits a linear elastic behaviour, followed by the development of plastic deformation with a progressive reduction of stiffness (k b ) until the failure of the bolt. It is noted that the resistance of carbon steel bolts loaded in tension is dictated by the threaded part since it is the weakest part, with the minimum cross-section area, approximately given by A s /A=0.78 [32]. The behaviour of standard structural bolts in tension has been studied by many authors for many years. Recently, Strangh¨ oner et al. [32] reassessed the resistance of carbon steel bolts subject to tension, shear, and combined forces. Besides carrying out a large test campaign, the authors collected the available experimental results in the literature and concluded that the design expressions in EN 1993–1-8 [6] could be improved while still satisfying the target probability of failure of the EN 1990, Annex D [9]. It is well known (Kulak et al. [33]) that the bolt tensile resistance depends on the way the tension is applied. Tightening the nut introduces torsional stresses and more complex combined tension-torsional stress condition in the bolt. Torquing a bolt until failure results in a reduction of both the ultimate load and the ultimate deformation as compared with the corresponding values determined from a direct tension test. Additionally, in the case of subsequently loading the bolt in direct tension after it has already been loaded by tightening the nut, there is not a significant drop in ultimate strength. This is relevant for the requirements for preloaded and non-preloaded bolts and justifies the need for two distinct product standards [4,10]. Two distinct failure modes are observed in bolts subjected to tension: (i) bolt fracture, and (ii) thread stripping (nut or bolt). Grimsmo et al. [30] studied the influence of the length L t (see Fig. 2), on the bolt failure mode. They concluded that the probability of the thread failure can be reduced by increasing this length. In addition, the authors simulated the influence of high nuts, designed according to ISO 4033: Hexagon high Table 1 Summary of standards for structural bolts. Product standard Symbology and general requirements Mechanical properties (bolts, nuts, washers) Dimensional properties Dimensional tolerances Coating systems Inspection Non-preloaded bolt assemblies in carbon steel and alloy steel EN 15048 ISO 225 ISO 898 ISO 261 ISO 965 ISO 4042 EN 15048–2 ISO 724 ISO 3269 ISO 888 High-strength bolting assemblies for preloading EN 14399 ISO 8992 ISO 3508 ISO 4759-1 ISO 6157 ISO 14399-2 ISO 4014 ISO 4017 ISO 10683 ISO 3269 ISO 4753 Fig. 2. Geometry of a standard structural bolt: (a) Lengths definition; (b) Partially and fully threaded specimens [30]. N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 3 nut [34]. The nut height had a major impact on the results since the bolt showed a more ductile behaviour than with a regular nut, switching the failure mode from thread stripping to bolt fracture. Similarly, Alexander [35] claims that as the number of threads is reduced, the failure mode changes from bolt fracture to stripping. Consequently, by increasing the nut height, the probability of thread stripping decreases, and the probability of bolt fracture increases. Tartaglia et al. [36] confirmed that nut stripping failure is more characteristic of HV bolts, while HR bolts usually fail by bolt fracture, according to their design (EN 14399–1 [10]). Despite experiencing different failure modes, both bolt types showed similar evaluation of forces in the performed tests. The influence of the use of double nuts was experimentally studied by Grimsmo et al. [37], showing that it prevented thread stripping. Concerning the influence of fully threaded vs partially threaded bolts, Yang et al. [38] numerically studied the fracture behaviour of the PT and FT bolts under tensile loading. PT bolts show higher yield and ultimate strengths, but smaller deformation capacity compared to FT bolts, confirming earlier results by Grimsmo et al. [30]. Hu et al. [39] studied the influence of surface finishing on failure by thread stripping. They concluded that the nuts with zinc plating surface led to the thread stripping failure due to the threads of the nuts being over-tapped (by 0.4 mm) for the coating process. They propose the use of nuts one material property class higher than the bolts to decrease the probability of failure by nut stripping. 2.2.2. Long bolts Unlike conventional bolts, long bolts are not very well covered in the literature. Most references deal with long bolts as a part of a bolted endplate beam to concrete-filled hollow section column connections [3,40,41]. In this case, long bolts are subject mainly to shear and are not relevant for this paper. Fransplass et al. [42–44] studied long bolts manufactured from threaded rods subject to tension and combined tension and shear at low and elevated strain rates. The tests were carried out using a purposemade fixture and by fixing this element in different positions various grip lengths were studied, in the range of 0.41 mm to 9.8 mm. It was concluded that the number of threads in the grip length influenced the failure of the bolt. In most cases, the failure mode was by bolt fracture, but bolts with shorter grip lengths showed significantly lower ductility compared to the ones with longer grip lengths. Additionally, based on the analytical models developed by Alexander [35], they propose modified equations for the prediction of the thread stripping resistance [42]. However, since they tested small grip lengths and small diameter rods in mild steel, M5 class 4.6, the validity of their conclusions for the present study may be limited. Finally, Loureiro et al. [45] carried out an initial study aiming at assessing the influence of using long threaded structural bolts in unequal-depth internal node steel joints instead of standard structural bolts. Threaded bars were used in the tensile and compression parts of the connection. They concluded that the threaded bolt joints led to an increase in resistance and rigidity of 59 % and 24.5 %, respectively. 2.3. Manufacturing of long bolts The production of standard structural bolts follows four steps: i) selection of the steel coil based on chemical composition, ii) stamping process (usually cold forging), to shape the head geometry and production of the thread by means of threading dies; iii) heat treatment (annealing +tempering), by furnace or induction, to obtain the desired mechanical properties; iv) durability coating. The production of long bolts presents some differences when compared to standard structural bolts, namely: i) they are not manufactured from steel coil but from a long calibrated rod (usually 6 m long); ii) they are not subjected to headshaping cold forging, only to threading; iii) they are not subjected to heat treatment process; iv) after production, the bar is cut to the desired measure (usually 1 m pieces) and the cut ends are chamfered. If subjected to heat treatment, the long bolt recovers the initial curvature of the rod, thus requiring a subsequent straightening process which is complex and uneconomical. Fig. 3. Geometry of a structural long bolt: (a) Geometrical definition; (b) Example of M20 1000 mm long bolt. Fig. 4. Points of interest on the force-elongation curve of a standard bolt subjected to tensile force. N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 4 2.4. Design expressions Strangh¨ oner et al. [32] provide a review of the design expressions for the resistance of carbon steel bolts in EN 1993–1-8, AISC 360–22 [46] and AS4100 [47] codes. Table 2 summarizes the design expressions for resistance and stiffness of bolts in tension in EN 1993–1-8 [6]. L b is the bolt elongation length, measured from nut centre to head centre or midpoints between two nuts (in case double nuts are used), see Fig. 2 and Fig. 3. Eq. (3) is applied for a single bolt and hence, considers half the stiffness value of the component “Bolt in Tension”given in EN 1993–1-8 [6] (which is defined for a single bolt-row comprising two bolts). 3. Experimental programme 3.1. Definition of the test program The main objective of the first experimental campaign within the CONNECT4C research project [7] is to characterize the behaviour of long bolts in tension, by extracting F-ucurves (where Fis the applied tensile force and uis the elongation of the bolt, see Fig. 5), and comparing the obtained results of tensile resistance and initial stiffness with Eqs. (2) and (3). A total of 66 tensile tests were performed, covering the following parameters: •Bolt diameter (M20, M24, M27 and M30), •Bolt class (8.8 and 10.9), •Surface finishing (B –black and G –galvanized), •Number of nuts provided on each bolt end (N1 –single and N2 – double nut). Each test was labelled concatenating the letter ‘T’(for tension) with the specifications for its defining parameters. As an example, ‘T25M24–10.9-B-N1’corresponds to a tension test (T), order of test in the series total (25th), long bolt diameter M24 class 10.9, with black (B) surface finishing, and a single nut (N1) on each bolt end. This labelling system is used hereinafter. A summary of all tests is given in Table 3. Three repetitions were performed for each set of parameters to capture variability. Double-nut tests (N2) were initially planned only for bolts M20 and M27. After initial testing, it was observed that most bolts with a single nut (N1) experienced nut stripping failure. Since this is an undesirable failure mode [30], additional double-nut tests were performed for M24 and M30 bolts, except for M24 8.8 bolts, where failure by bolt fracture was observed in all single-nut tests (N1). These additional tests are indicated with parenthesis in Table 3. 3.2. Experimental setup The experimental setup is depicted in Fig. 6, with the relevant parts numbered in Fig. 6(b) and some additional pictures shown in Fig. 7. The test setup comprises a reaction frame (6) formed by vertical columns and a horizontal strong beam supporting a 6 MN jack. The jack piston was screwed to a pinned part (10) transferring load and displacement to a plate (9) bolted to the top beam (2), which was supported by a guiding system (4), sliding on the reaction frame columns (6) with the interposition of Teflon pads to minimize friction. A bolted connection tool (3) joined the top beam (2) to the long bolt specimen. The tool had a 36 mm diameter hole, and therefore an adjustment washer (5) was needed according to the long bolt diameter to provide conventional hole tolerance. The same connecting tool (3, 5) placed upside-down was used to fix the specimen to the bottom beam (1), whose vertical motion was prevented by two brackets (7) attached to the reaction frame columns (6), with load cells (8) interposed to measure the vertical reactions. The bottom beam (1) was supported by the same system as the top one (4, 6). When the jack piston moved upward, the top beam (2) moved with it, stretching the bolt which was fixed at the bottom beam (1). The total bolt length L(see Fig. 3) was constant for all tests, 1000 mm. Table 4 lists the geometry and resistance of all bolts, as well as the operational grip lengths L g,N1 (single nut N1) or L g,N2 (double nut N2), calculated according to the details of the bolt assembly, including washers, nuts, and the protruding threaded portion of the bolt beyond the nuts (approximately 5 threads, as shown in Fig. 7(c)). 3.3. Test procedure and instrumentation The test was performed by applying a displacement to the top retaining beam, by means of a 6 MN hydraulic jack (see Fig. 7(b)) attached to the beam with a pin connection. The test speed was 0.025 mm/s. During the tests, the following data was acquired (see Fig. 8): •Applied force and reactions, using load cells (LCs), •Displacements, using LVDTs (Linear velocity displacement transducers). Two load cells with capacities of 2 MN were used for load measurements. They were labelled as LC-1 and LC-2 and positioned between the lower retaining beam and the reaction brackets, as shown in Fig. 8(a) and (c). In total, 12 LVDTs were used for measuring vertical absolute (label Table 2 Design specifications for bolts in tension in EC3–1-8:2005 and EC3–1-8:2024. Tension resistance (2) Fully and partially threaded bolts Ft,Rd =k2fubAs γM2 Initial stiffness (3) Non-preloaded bolts kb=0.8EAs Lb Preloaded bolts ∞ k 2 : reduction factor with k 2 =0.9 (exception: for countersunk bolts k 2 =0.63); in EN 1993–1-8:2024, the value of 0.9 (or 0.63) are given directly | f ub : nominal tensile strength of the bolt | A s : tensile stress area of the bolt | γ M2 : partial factor (γ M2 =1.25) | E: Young’s Modulus of the bolt material | L b : the bolt elongation length. Fig. 5. Tensile force application. Table 3 Summary of the performed tests. Bolt diameter Class Surface No. of nuts on each side Repetitions No. of tests M20 8.8 and 10.9 B and G N1, N2 3 24 M24 8.8 B and G N1 3 6 M24 10.9 B and G N1 (+N2) 3 6 (+6) M27 10.9 B and G N1, N2 3 12 M30 10.9 B and G N1 (+N2) 3 6 (+6) Total 66 N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 5 A) and relative (label R) displacements of parts of the test. Four of them (A - 1, 2, 3, and 4) were positioned on the top and bottom bolt-to-beam connection tools. These were later used for the calculation of the long bolt elongation. LVDTs A - 5, 6, 7, and 8 were positioned at the edges of the upper retaining beam to assess potential beam rotation during test execution. LVDTs A - 11 and 12 were positioned on the bottom retaining beam, to monitor potential beam bending or rotation. LVDTs R - 9 and 10, measured the relative displacement between the upper and bottom retaining beams and the bolt-to-beam connection tools, thereby detecting any openings between these elements. 3.4. Test results 3.4.1. General The experimental results comprise the results from the tensile tests of the bolts, following ISO 898-1 [11], clustered according to bolt class and bolt diameter. They are complemented by tensile tests for machined cylindrical steel coupons extracted from the long bolts, according to clause 9.7 of ISO 898-1, and geometrical measurements of the external (d) and internal (d 1 ) diameters, and pitch length (P). 3.4.2. Geometrical measurements The real geometrical properties (external and internal diameters, and Fig. 6. Test layout: (a) 3D view; (b) Elevation with marked parts; (c) Photo from the test. Fig. 7. Test layout details: (a) Bolt-to-beam connection tool; (b) Hydraulic jack; (c) Protruding part of the bolt. N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 6 pitch length) were measured with a calliper on a sample of long bolt specimens, sorted according to bolt diameter, class and surface finishing. For each group, 3 bolts were measured in 3 different places across their length. Hence, 9 measurements were performed per group, and, in total, 108 measurements for each property. These measurements are summarized in Table 5. An accurate measure of the internal thread diameter was not feasible due to the small thread pitch. Measurements of the thread pitch are based on the length covered by 15 threads l 15 . The tensile area A s was calculated using Eq. (1b). The ratio between as-measured properties and nominal properties is listed in Table 6, showing that the mean value for A s /A s,nom for black bolts is 0.968 and 0.980 for galvanized bolts. As expected, the ratios are slightly higher for galvanized bolts, due to the coating thickness. 3.4.3. Bolt material tensile tests Long bolts used in the experimental campaign were made of highstrength steel, classes 8.8 and 10.9, supplied and tested by bolt manufacturer FATOR. Table 7 provides a summary of the coupon tests, totalling 43 coupons. Material characterization was performed by coupon testing, according to clause 9.7 of ISO 898-1 [11] and ISO 6892-1 [48], providing the full stress-strain curves depicted in Fig. 9. Table 8 summarizes the results showing mean values per bolt diameter and the mean and coefficient of variation (CoV) across all coupons of the same material. In Table 8,R p,0.2 ,R m , and R r are the stress at 0.2 % non-proportional elongation, the tensile strength and the stress at the rapture point, respectively, A gt is the total percentage elongation corresponding to the tensile strength, Ais the percentage elongation at fracture. All values satisfy the minimum requirements of ISO 898-1 of R p,0.2,min ≥660 MPa , R m ≥830 MPa, A≥12 %, for class 8.8 and R p,min ≥ 940 MPa , R m ≥1040 MPa, A≥9 %, for class 10.9. 3.4.4. Force-displacement curves The force-displacement curves are presented in Fig. 10.Fig. 10(a) shows the curves for the tests performed with single nuts (N1), whereas Fig. 10(b) shows the equivalent curves for the tests performed with double nuts (N2). In all cases, the maximum load is indicated with the symbol that also corresponds to the failure mode (circle for BF failure, star for TS failure). The different diameters (M20, M24, M27, M30) and the two bolt classes (8.8, 10.9) are labelled in the plots and can be easily distinguished. Bolt finishing surfaces are indicated with full (black) and dashed lines (galvanized). Adopting the table organization for the test results presented in [32], Table 9 summarizes the test results for the tension tests of the long bolts. Table 4 Nominal geometry and resistance of the bolts. Bolt notation M20 M24 M27 M30 Bolt diameter dmm 20 24 27 30 Thread pitch Pmm 2.5 3.0 3.0 3.5 Tensile area A s mm 2 245 353 459 561 Nut thickness (nominal) t n mm 16 20 22 24 Washer thickness (nominal) t w mm 4 4 5 5 Operational grip length for 1000 mm bolt, single nut N1 L g,N1 mm 935 922 916 907 Operational grip length for 1000 mm bolt, double nut N2 L g,N2 mm 903 882 872 859 Class 8.8: Design tensile resistance, Eq. (2) F t,Rd kN 141.1 203.3 264.4 323.1 Class 10.9: Design tensile resistance, Eq. (2) F t,Rd kN 176.4 254.2 330.5 403.9 Fig. 8. Instrumentation: (a) Position of the LVDTs and LCs; (b) LVDTs; (c) Load cell. N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 7 The following observations can be derived from Fig. 10 and Table 9: •M20 8.8 bolts present a marked yield plateau, that is not apparent for all other bolts. •The plots confirm that all tests with double nut failed by Bolt Fracture (BF), and a large proportion of tests with single nut (N1) failed by Thread Stripping TS. In addition, tests failing by BF achieved larger ductility than those with TS failure and featured more consistent values of maximum force and corresponding displacement. •Tests failing by TS present a large scatter in terms of maximum force and corresponding displacement; about 50 % of the failures by TS for 10.9 bolts occur prior to bolt yielding. Table 5 Summary of geometrical characterization. Black (B) Galvanized (G) d(mm) P(mm) A s (mm 2 )d(mm) P(mm) A s (mm 2 ) M20 / Nominal 20.00 2.50 244.79 20.00 2.50 244.79 8.8 Measured, Mean 19.47 2.53 229.52 19.62 2.53 233.82 Measured, CoV 0.34 % 0.23 % 0.77 % 0.42 % 0.16 % 0.97 % Mean/Nominal 97.34 % 101.17 % 93.76 % 98.12 % 101.06 % 95.52 % 10.9 Measured, Mean 19.87 2.53 240.47 19.98 2.53 243.54 Measured, CoV 0.12 % 0.33 % 0.28 % 0.07 % 0.14 % 0.17 % Mean/Nominal 99.34 % 101.04 % 98.23 % 99.90 % 101.08 % 99.49 % M24 / Nominal 24.00 3.00 352.50 24.00 3.00 352.50 8.8 Measured, Mean 23.79 3.02 344.84 23.87 3.01 347.75 Measured, CoV 0.38 % 0.21 % 0.91 % 0.16 % 0.20 % 0.39 % Mean/Nominal 99.12 % 100.74 % 97.82 % 99.45 % 100.43 % 98.65 % 10.9 Measured, Mean 23.72 3.03 342.34 23.84 3.03 346.48 Measured, CoV 0.15 % 0.18 % 0.34 % 0.15 % 0.10 % 0.33 % Mean/Nominal 98.84 % 101.07 % 97.12 % 99.35 % 100.93 % 98.29 % M27 / Nominal 27.00 3.00 459.41 27.00 3.00 459.41 10.9 Measured, Mean 26.54 3.02 441.27 26.69 3.02 446.93 Measured, CoV 0.27 % 0.24 % 0.60 % 0.34 % 0.16 % 0.76 % Mean/Nom 98.30 % 100.83 % 96.05 % 98.84 % 100.62 % 97.28 % M30 / Mean/Nominal 30.00 3.50 560.59 30.00 3.50 560.59 10.9 Measured, Mean 29.75 3.51 549.82 29.86 3.51 554.44 Measured, CoV 0.11 % 0.15 % 0.26 % 0.24 % 0.16 % 0.55 % Mean/Nom 99.18 % 100.38 % 98.08 % 99.54 % 100.24 % 98.90 % All / Max CoV 0.38 % 0.33 % 0.91 % 0.42 % 0.20 % 0.97 % Table 6 Comparison of as-measured and nominal geometrical properties. Black (B) Galvanized (G) Total d/d nom (−)P/P nom (−)A s /A s,nom (−)d/d nom (−)P/P nom (−)A s /A s,nom (−)d/d nom (−)P/P nom (−)A s /A s,nom (−) Mean 0.987 1.009 0.968 0.992 1.007 0.980 0.989 1.008 0.974 CoV 0.74 % 0.35 % 1.72 % 0.63 % 0.35 % 1.45 % 0.74 % 0.35 % 1.69 % Max 0.995 1.014 0.988 1.000 1.014 0.997 1.000 1.014 0.997 Min 0.969 1.001 0.927 0.975 1.000 0.941 0.969 1.000 0.927 Table 7 Summary of coupon tests. Class Bolt No. of batches No. of coupons 8.8 M20 2 6 M24 2 6 M27 1 4 M30 1 4 10.9 M20 2 6 M24 2 6 M27 2 6 M30 2 5 Total: 43 Fig. 9. Stress-strain curves for long bolts: (a) Class 8.8; (b) Class 10.9. N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 8 •For tests failing by BF, 8.8 bolts present higher ductility than 10.9 bolts. For bolts failing by BF, it is observed that the descending part of the curve after the maximum load is significantly shorter compared to the standard structural bolts tested in the literature. This was due to the test layout, because the bottom beam (see (1) in Fig. 6), and its fittings were hanging from the long bolt. Although this effect was not relevant until the maximum load was reached (its self-weight of about 500 kg was deducted from the load cell measurements), it affected the behaviour after reaching this point, transforming the test from pure displacement control into a hybrid displacement/force control test, thereby preventing the capture of the descending equilibrium path. To demonstrate this, four additional tests were conducted using a standard universal tensile testing machine with a 600 kN capacity (see Fig. 11), following strictly the prescribed test specifications of ISO 898-1 [11]. It is noted that the total bolt length had to be reduced to 400 mm because of equipment limitations. To ensure BF failure mode, double nuts were used at each end. The test conditions remained consistent with the prior experiments, utilizing bolts from the same batches as in the previous tests and a loading speed of 0.025 mm/s. Fig. 12 compares the load-displacement behaviour of the additional tests with the corresponding tests described in Table 9 (T4, T16, T25 and T37) and two reference tests from the literature (Strangh¨ oner et al. [32]) on standard structural bolts. Normalized displacement and force values are used on the xand y-axis, given that 3 different bolt diameters are compared (Strangh¨ oner et al.’s tests were conducted on M16 bolts, whereas the tests carried out in the present study correspond to M20 and M24 long bolts). Comparing tests T4, T16, T25, and T37 with additional tests AT1 to AT4, much larger ductility ratios and ratios of elongation at fracture versus elongation at maximum load are observed for the latter tests. For bolt class 8.8, similar ductility is observed between tests AT1 and AT2 and Strangh¨ oner et al.’s test, while for bolt class 10.9, tests AT3 and AT4 exhibit about 50 % less ductility when compared to Strangh¨ oner et al.’s test. Table 10 compares the experimentally measured initial stiffness with the results from Eq. (3). A good match is obtained but a trend of decreasing measured stiffness with increasing bolt diameter is observed (0.915 <0.997 <1.015 <1.065). Additionally, comparing the use of one nut against two nuts, smaller initial stiffness is observed for double nuts, probably reflecting the assumption of measuring the bolt elongation length from the midpoint of the two nuts. 3.4.5. Failure modes The same failure modes for this type of load, as previously Table 8 Bolt coupon test results from machined test pieces. Class Bolt R p,0.2 =f y (MPa) R m =f ub (MPa) R r (MPa) A gt (%) A (%) 8.8 M20 Mean 833.6 950.6 630.4 9.13 17.15 M24 Mean 883.2 995.9 683.9 8.53 16.88 M27 Mean 930.3 1013.7 705.6 8.29 16.87 M30 Mean 895.9 984.7 672.1 8.13 17.28 Total Mean 880.3 983.6 669.8 8.58 17.04 Mean/Nominal 1.38 1.23 – – 1.42 CoV 4.32 % 3.45 % 5.27 % 5.63 % 5.00 % 10.9 M20 Mean 1109.4 1176.1 754.8 6.86 14.93 M24 Mean 1083.8 1166.6 797.9 6.47 13.73 M27 Mean 1107.0 1183.3 800.6 6.42 14.05 M30 Mean 1104.3 1187.0 859.6 6.08 12.77 Total Mean 1101.0 1177.9 800.8 6.48 13.92 Mean/Nominal 1.22 1.18 – – 1.55 CoV 2.29 % 1.83 % 5.12 % 10.00 % 7.09 % Fig. 10. Force-displacement curves: (a) Tests with single nut (N1); (b) Tests with double nut (N2). N. Jankovi´ c et al. Journal of Constructional Steel Research 229 (2025) 109492 9