ANALYSIS OF THE STRUCTURAL PERFORMANCE OF REINFORCED CONCRETE UNDER FIRE LOADING

SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 68 ANALYSIS OF THE STRUCTURAL PERFORMANCE OF REINFORCED CONCRETE UNDER FIRE LOADING D. Imamaliev1, A. Sazairov2, R. Akbarli3 Department of Construction and maintenance of automobile roads, Tashkent State Transport University, Tashkent, Uzbekistan1 Department of Mechanics, Azerbaijan University of Architecture and Construction, Baku, Azerbaijan2,3 https://doi.org/10.5281/zenodo.17702731 Abstract. This study examined the behavior of reinforced concrete structures when exposed to high temperatures resulting from fire. Deterioration in material strength due to fire exposure alters a reinforced concrete structure’s load-bearing capacity and overall behavior. Elevated temperatures negatively affect key material properties of reinforced concrete, including density, coefficient of thermal expansion, thermal conductivity, and elastic modulus. As a result, if a structure experiences fire either concurrently with or prior to an earthquake, these changes in material properties will significantly influence its dynamic performance. For the numerical simulation, the selected structure was designed with a formwork plan and load-bearing system in accordance with earthquake-resistant design principles. Based on this design, fixed and variable loads acting on the beams were assigned. By promoting resilient infrastructure capable of withstanding severe environmental conditions such as earthquakes and fires, this study contributes to the achievement of sustainable development goals. It underscores the necessity of integrating fire resistance into earthquake-resistant design to foster disaster-resilient urban development. The findings may encourage more flexible and sustainable construction practices aligned with SDGs 9 (Industry, Innovation and Infrastructure), 11 (Sustainable Cities and Communities), and 13 (Climate Action). Keywords: high temperatures, concrete reinforcement, load-bearing capacity, coefficient of thermal expansion, temperature-dependent variations, resilient urban development, sustainable infrastructure, earthquake engineering, fire resistance, and the effects of climate change on structures. Introduction Many countries have reinforced their fire-safety regulations following major fires and conflicts. For instance, Italy has intensified its regulatory framework, and the Regulation on Fire Protection of Buildings now establishes key architectural requirements in addition to provisions for detection and suppression systems. Recent studies further reflect this shift: [13] investigates the impact of fire cut-offs on airflow within ventilated facade cavities, providing engineering assessments of solid and perforated cut-offs in Azerbaijan and underlining the need for updated design solutions. Other research [6,14] employs ANSYS finite element modelling to evaluate the flexural response and ductility of reinforced concrete beams with GFRP reinforcement, demonstrating that higher GFRP ratios improve load-carrying performance. Post-earthquake fire (PEF) effects on buildings are examined in several studies [1–4], which emphasize that fire loads applied after seismic and gravity actions significantly reduce the residual capacity of damaged structures, highlighting the need for further investigation in SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 69 seismically active regions. Additional findings in [7] confirm that sequential fire loading further weakens already compromised elements. Recent work also demonstrates the potential of datadriven approaches: for example, machine-learning models have been successfully used to predict the properties of aerated concrete incorporating ash–slag waste [15], offering new opportunities for performance assessment in environments prone to both seismic and fire hazards. Further studies [16–19,20] investigate advanced reinforcement strategies and the elastoplastic behavior of structural elements under complex loading, incorporating hardening, softening and damage evolution-factors essential for understanding the interaction between seismic and fire effects. Research on temperature-induced structural transitions [21] likewise clarifies how thermal exposure alters material properties, providing important insights for fireaffected infrastructure. Recent material innovations also contribute to improved fire resistance and structural performance: 3D-printed LC3-based engineered cementitious composites [22,23] show enhanced beam behavior, mechanical properties and controlled anisotropy, demonstrating promising approaches for developing more resilient and energy-efficient structural systems. Fire Curves Fire temperatures over time are commonly estimated using zone models, room-fire models and standard fire curves. Among these, ASTM E119 and ISO 834 are the most widely applied, representing the standard forms of the natural and experimental curves described by Buchanan (2001). These models assume a uniform gas temperature within the compartment and do not account for flame spread or smoke movement, making them most suitable for post-flashover conditions. The ISO 834 temperature–time relationship used to define high-temperature degradation of concrete properties is expressed by Equation (1): 𝑇 = 𝑇0+345𝑙𝑜𝑔⁡ (8𝑡 + 1) (1) Fig. 1. Temperature Variation Over Time in Relation to the Duration of the Fire. Understanding fire-induced temperature development and the resulting degradation of material properties is essential for evaluating structural performance at high temperatures. Although concrete is non-combustible, its mechanical and physical characteristics deteriorate as temperature increases. This degradation becomes particularly pronounced above 600 °C, at which point concrete may lose nearly half of its strength—a level commonly referred to as the critical temperature [8–10]. This study advances the field in three key aspects. First, it introduces a temperaturedependent modelling framework for reinforced concrete frames subjected to ISO 834 fire, directly linking material degradation in SAP2000 to global seismic response indicators. Second, it quantifies the combined influence of elevated temperature and seismic loading on lateral displacements, base shear and internal forces, and evaluates the resulting drift demands against typical code-based limits for RC structures. Third, it provides practical guidance by identifying SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 70 temperature ranges in which seismic performance remains within acceptable drift limits, while safety margins notably diminish, thereby indicating when second-order effects and soft-storey behaviour may become critical in fire-affected RC frames. Material and Method We’ll look at a building meant for residential use. The structure has four stories, three xdirectional spans, two y-directional spans, and three spacing between them. The SAP2000 software will be used to model and analyze the structure. C30/37 is the concrete class selected for the design. Table 1. Gives general details about the planned structure. Property Value Building Use Purpose Residential Number of Floors Ground + 3 Floors Floor Height 3 m Flooring Ribbed Slab Foundation Continuous Foundation Concrete Class C30/C37 Steel Class B420C Beam Dimensions 𝑏𝑤=25 𝑐𝑚, ℎ=40 𝑐𝑚 Column Dimensions 𝑏=30 𝑐𝑚, ℎ=60 𝑐𝑚 𝑏=60 𝑐𝑚, ℎ=30 𝑐𝑚 Soil Class ZB (Slightly weathered, moderately strong rocks) Building Use Class 3 Building Importance Factor (I) 1 Concrete Elastic Modulus (E) 32000 MPa Live Loads (q) In Rooms: 2 kN/m2 On Stairs: 3.5 kN/m2 On Balconies: 5 kN/m2 A formwork plan was developed in accordance with earthquake-resistant design principles, and the architectural layout was prepared within the scope of the study. Preliminary sizing of structural elements followed the relevant standards and regulations. The structural system was then modelled in SAP2000 using the selected section dimensions, calculated beam loads and defined material properties. Seismic analysis was performed in SAP2000 using the Modal Combination Method [11,12]. Fig. 2. Model of the modeled structure created in Sap2000. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 71 For each floor level, the dynamic analysis uses three dynamic degrees of freedom, which represent rotational motion around the z-axis and translational motion in the x and y directions. Consequently, each floor level has a total of three degrees of freedom. A four-story building has twelve dynamic degrees of freedom in total. Therefore, there should be twelve mode shapes in the structure. The table contains the periods and additional details about the modes gleaned from the modal analysis. Table 2. The building’s natural vibration frequencies and periods. Modal Periods and Frequencies Eigenvalue (rad2/sn2) Period (s) Frequency (Hz) 1 0.932045 1.07291 0.932045 2 0.815862 1.22569 0.815862 3 0.752464 1.32897 0.752464 4 0.263753 3.79143 0.263753 5 0.232894 4.29379 0.232894 6 0.216319 4.62281 0.216319 7 0.127348 7.85249 0.127348 8 0.112993 8.85008 0.112993 9 0.105233 9.50271 0.105233 10 0.081042 12.33924 0.081042 11 0.07197 13.89459 0.07197 12 0.066917 14.94389 0.066917 Temperature Assignment to the Structure: When evaluating reinforced concrete structural elements exposed to high temperatures (fire), it is essential to account for the temperature-dependent degradation of material properties. In this study, a standard compartment fire was represented by the ISO 834 fire curve. The fire was assumed to affect only the structural elements at the ground floor, where the columns are critical for the global stability of the frame, while the upper-story elements remained at ambient temperature (20 °C). In SAP2000, the fire scenario was modeled by assigning temperature-dependent concrete properties to the ground-floor columns at discrete temperature levels (20–680 °C). For each level, the corresponding reductions in Young’s modulus and density from Table 3 were applied only to the heated columns, while all other elements retained their original properties. Each analysis case therefore represents the same gravity and seismic loading, but with progressively reduced stiffness and mass in the fire-exposed columns. This procedure provides a direct and consistent link between the temperature-dependent material data in Table 3 and the displacements and internal forces presented in the Results section. Table 3. Temperature-dependent material properties of concrete used in the analysis for the fire-exposed ground-floor columns. Time (min) Temperatu re (°C) Young's Modulus (E) (MPa) Density (ρ) (kg/m3) 0 20 32000 2500 1 349 22450 2394 2 445 16352 2365 3 502 12653 2353 SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 72 Time (min) Temperatu re (°C) Young's Modulus (E) (MPa) Density (ρ) (kg/m3) 4 544 9991 2344 5 576 7910 2336 6 603 6200 2331 7 626 4750 2326 8 645 3491 2321 12 705 0 2308 20 781 0 2292 30 842 0 2278 40 885 0 2269 50 918 0 2262 60 945 0 2256 The values of Young’s modulus and density at each temperature were used directly in the SAP2000 model for the fire-exposed ground-floor columns. Other thermal parameters (such as specific heat and thermal conductivity) were not required in the structural analysis and are therefore not listed here. All figures in this paper are original and were prepared by the authors based on their own numerical simulations in SAP2000. Results The mass and elastic modulus of the material decrease with increasing temperature, resulting in an increase in the structure's first periods in both the x and y directions. Up to 300°C, this increase is slight, but after that, it has greatly increased, especially at 600°C, when it is roughly twice as high as it was at the beginning. Table 4. Joint’s displacements based on temperature (mm). Node 20°C 100°C 200°C 300°C 400°C 500°C 600°C 680°C 1 0.56 0.56 0.55 0.61 0.70 0.86 1.21 2.70 2 1.48 1.49 1.47 1.62 1.86 2.26 3.19 7.08 3 2.31 2.31 2.29 2.52 2.89 3.52 4.95 11.00 4 2.92 2.92 2.89 3.18 3.65 4.44 6.25 13.89 Fig. 3. Joint displacements as a function of temperature (mm). Modal effective masses have decreased as a result of rising temperatures and longer structure periods, which has decreased the structure’s base shear forces. Although this decline is not very noticeable until 300°C, it has since accelerated, especially reaching about 46% at 600°C. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 73 Table 5. Base shear force. Temperature (°C) 𝑽𝒕x (kN) 𝑽𝒕𝒚 (kN) 20 58 66 100 58 66 200 57 65 300 51 59 400 45 52 500 36 43 600 27 31 Conclusion The reduction in lateral stiffness leads to increased nodal displacements, even though base shear decreases with temperature. This increase is modest up to 300 °C but becomes more pronounced at higher temperatures, reaching about 2.16 times the initial displacement at 600 °C (Table 4). At 680 °C, the maximum roof-node displacement is 13.89 mm compared to 2.92 mm at 20 °C. For the 12 m building height, this corresponds to a drift ratio of approximately 0.12 %, which remains well below typical code limits of 1–2 % for RC structures. 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