Post-fire Mechanical Properties Research Articles

Research on post-fire mechanical properties of thin-walled cold-formed steel and its influence on the Σ-shaped columns after fire exposure

This paper conducts a comprehensive investigation into the post-fire mechanical properties of thin-walled cold-formed steel (CFS) and evaluates the design method for the post-fire load-bearing capacity of CFS lipped channel section with web stiffener (Σ-shaped) columns. A total of 144 tensile tests were conducted to determine the mechanical properties of CFS after exposure to heating treatment. The study considered the effects of varied material strengths (Q235 and Q420), plate thicknesses (1.5 mm and 2.5 mm), extraction positions (flat plate, 90° corner, and 45° corner), as well as heating temperatures ranging from 20 °C to 800 °C. The research indicates that the heating temperature does not have a prominent influence on the elastic modulus of CFS, whereas its impact on yield strength is pronounced. After exposure to 800 °C, the yield strength of CFS can be reduced by 40 %, but the strain-hardening effect by cold-working still existed, which enhanced the yield strength by up to 20 %. This paper presents a predictive formula for the reduction factor of the mechanical properties. To assess the influence of post-fire mechanical properties on the load-bearing capacity of CFS columns, a finite element model of Σ-shaped columns was developed and calibrated. The simulation results indicate that the post-fire strain-hardening effect at corners would enhance the load-bearing capacity by less than 6 %. Finally, based on a parametric analysis, it was demonstrated that the Direct Strength Method (DSM) applied for distortional-local interaction buckling columns was still effective once the post-fire mechanical properties were incorporated when slenderness factor λdl > 1.5.

Study on fire resistance of high performance concrete nonplanar beam-wall joints

The utilization of high performance concrete (HPC) in the core region of beam-wall joints can enhance the structural integrity and load-bearing capacity of prefabricated buildings. However, due to the high-temperature bursting characteristics of HPC, the mechanical properties of the joints degrade prematurely under fire. In this paper, the fire resistance of HPC nonplanar beam-wall joints was investigated using high-temperature experiments and numerical simulations. First, the prefabricated beam-wall joint and the cast-in-place beam-wall joint proposed in this study were subjected to high-temperature and post-fire static loading tests, enabling a comparative analysis of their fire resistance and load-bearing capacity. The results demonstrated that the utilization of HPC in the core region of the prefabricated beam-wall joints can enhance its load-bearing capacity and fulfill the fire resistance requirements. Subsequently, a finite element model of the nonplanar beam-wall joint was established based on test results to investigate the influence of different structural parameters on its post-fire mechanical properties. The results demonstrate that the diameter of vertical reinforcement in the wall can significantly influence the post-fire load-bearing capacity of the joints, while the concrete strength in the core area of the joints has little impact on the post-fire bearing performance of the joints. Compared to the conventional cast-in-place beam-wall joints with normal strength concrete, the prefabricated beam-wall joints have superior fire resistance and post-fire load-bearing performance.

Post-fire mechanical properties of titanium-clad bimetallic steel after cold-forming process: Experiment and constitutive model

Titanium-clad bimetallic steel (TCBS), composed of a titanium alloy cladding layer and carbon steel substrate layer, has been proposed to mitigate the effects of corrosion and enhance the durability of steel structures. This study conducted tensile coupon test to explore the coupling effect of the cold-forming process and fire exposure on the monotonic mechanical properties of TCBS. Following the cold-forming process and fire exposure, there was no visible cracking observed in either the cladding or the metallurgical bonding layer. Test results revealed a notable enhancement in the strength properties and a decrease in the ductility of the TCBS specimens after cold forming. Moreover, the study elucidated the effects of the exposure temperature and cooling method on the stress–strain curves and fractures of TCBS corner specimens. Three different fracture sequences of the TCBS corner specimens were observed during monotonic tensile test. The key mechanical parameters were concerned in this study, which included the yield and ultimate strengths, yield and ultimate strains, elastic modulus, elongation after fracture, and section shrinkage. Then, the variation trends of key mechanical parameters were clarified. Predictive equations were developed for these parameters of the TCBS corner specimens after fire exposure. Additionally, two constitutive models were developed for the corner TCBS stress–strain curves with and without the yield plateau, enabling their use in numerical and theoretical investigations of the post-fire performance of cold-formed TCBS components.

Experimental investigation on the post-fire mechanical properties and charring rate of inorganic-bonded bamboo composite exposed to fire

The fire resistance remains a challenging topic for engineered wood and bamboo structures due to the combustibility of wood and bamboo and the usage of organic adhesives, which are temperature-sensitive substance in fire. As an alternative solution, this paper investigated a novel bamboo-based composite prepared with magnesium oxysulfide inorganic adhesive (inorganic-bonded bamboo composite, InorgBam), and the fire tests were carried out to investigate the post-fire mechanical properties and charring behavior of InorgBam. The reduction effect of fire exposure on compressive or tensile strength parallel-to-grain was experimentally studied. The effects of fire exposure surface numbers, grain direction, geometric dimension and exposure time on charring rate were discussed. Results showed that the InorgBam can be maintained the post-fire strength under compressive or tensile loads parallel to grain without significant degradation when the exposure temperature below 250 °C. The temperature of the char front was determined to be approximate 350°C by using an interpolation method. The charring rate was mainly dominated by the exposure time and decreases nonlinearly with the increase of exposure time, and the charring rate with multi-sided fire exposure was approximately 1.1 times of that exposed to one-sided fire. Finally, a nonlinear model was proposed to predict the charring rate of InorgBam material, which provides a basic reference for understanding the fire resistance of InorgBam material exposed to fire.

Effects of cooling type on mechanical properties of concrete produced with slag-modified cement exposed to high temperatures

Effects of different cooling methods on residual mechanical properties of normal-strength concretes produced with slag-modified cements were not reported in previous literature. Therefore, a (2 x 2 x 4) factorial experiment was carried out in the present study to investigate the compressive strength and elastic modulus of slag-modified cement concretes with different strength levels (characteristic compressive strength of 20 and 30 MPa) subjected to different maximum temperatures (200, 400, 600, or 800ºC) and cooling procedures (slow or fast cooling). According to analyses of variance (ANOVA), air-cooled specimens showed higher residual mechanical properties. Higher residual elastic modulus was observed in concretes with higher initial strength, whereas residual strength was only affected by initial strength for higher temperatures. Effects of different cooling methods were more pronounced in slag-modified concretes than in concretes produced with ordinary Portland cement, especially for temperature up to 400ºC. Since slag-modified concretes have lower calcium hydroxide content, volume expansion and cracking propagation due to lime rehydration during slow cooling were mitigated, leading to higher post-fire mechanical properties. In contrast, these types of concrete exhibited significant temperature difference along their cross-section when fast cooling was used, so that a substantial thermal shock caused limited post-fire mechanical properties.

Post-fire mechanical properties and constitutive model of Q690 high-strength structural steel

Fire hazard is one of the main threats to the integrity of steel structural buildings. Developing an accurate post-fire constitutive model for structural steel is crucial for enhancing the failure assessment of steel structures exposed to fire. In this study, the post-fire mechanical behavior of Q690 high-strength structural steel is investigated. Eighteen round bar specimens made of Q690 steel were heated with temperatures ranging from 20 °C to 900 °C. Tensile tests were conducted on these specimens to obtain the post-fire stress-strain curves and associated mechanical parameters of the material. To accurately characterize the post-fire behavior of Q690 steel, a temperature-dependent elastoplastic constitutive model combined with its numerical implementation algorithm is proposed. This model consists of four components: a temperature-dependent elasticity criterion, the von Mises yield criterion, an associated flow rule, and a temperature-dependent isotropic hardening law. The proposed constitutive model has a notable feature, i.e. the material parameters in the model can be defined as a function of temperature according to the test results. As a result, when utilizing this model for post-fire assessments of steel members, only three routine parameters, i.e. elastic modulus at room temperature, Poisson's ratio, and the temperature experienced by the material, are required in the numerical simulation to generate the material’s stress-strain relationships. To verify the proposed constitutive model, the new model was introduced into the numerical simulations of the tested round bar specimens after exposure to various temperature conditions. Both the load-displacement curves and local deformation of each round bar specimen obtained from the numerical simulations are compared with the test results to validate the accuracy of the new model. The constitutive model presented in this study provides a useful tool for assessing the post-fire behaviors of steel components.

Post-fire properties of lightweight 3D printed concrete containing expanded perlite

There has been an increase in construction with three-dimensional (3D) printing technology but 3D printed (3DP) structures also require weight reduction similar to conventional reinforced concrete structures. In addition, the behaviour of this type of structure against fire needs to be investigated. The number of printed layers and the time gap between layers for the 3DP specimens were among the variables examined. The test results demonstrate that as the replacement percentage of natural sand (NS) with expanded perlite (EP) increased, at 25% volume of replacement, the interlayer bond strength increased on average by 18.6%, while at the highest replacement level, of 75%, the strength decreased on average by 5.8%. Additionally, by incorporating EP, the compressive and flexural strengths of 3DP specimens declined on average from 9% to 29.7%, and from 39.3% to 49.3%, respectively. As the replacement level of NS increased, residual compressive and flexural strengths increased after exposure to 800°C. Furthermore, it was demonstrated that exposure to high temperature had the least effect on interlayer bond strength, whereas it significantly reduced the compressive and flexural strength. The results show that increasing the time gap between layers reduced interlayer bond strength and flexural strength while negligibly affecting compressive strength.

Bilayer coating strategy for glass fiber reinforced polymer composites toward superior fire safety and post-fire mechanical properties

The inherent heat and fire vulnerability from the polymer matrix of fiber-reinforced polymer composites (FRPs) contributes to fire propagation, matrix decomposition and especially loss of structural integrity during fire hazards. Applying surface fire protective coating is a popular approach without sacrificing mechanical properties. In this work, the influence of fire protective coating on the fire safety and post-fire mechanical properties of FRPs with an ingeniously designed coating bilayer composition. The combustion behavior study showed that the bilayer substantially postpones the decomposition of the polymer matrix through the thickness, showing a reduction of 50 % and 65 % for peak heat release rate and total smoke production respectively. Fire insulation tests showed good thermal insulation capabilities with no burn-through. Furthermore, the tensile strength of protected composite coupons was screened and showed a retention of 79 % original tensile strength after suffering 120s of heat/fire attack. This investigation resulted in a proposed protective strategy and mechanism offering a meaningful approach to enhancing the fire safety and fire structural integrity of polymer composites.

Properties of polyoxymethylene fibre-reinforced seawater sea sand concrete exposed to high temperatures

Using seawater and sea sand to prepare concrete can reduce the adverse effects from the resource shortage and environmental pollution caused by the exploitation of freshwater and river sand. In addition, polyoxymethylene (POM) fibres have excellent alkali resistance and durability, which may considerably improve the mechanical properties of seawater sea sand concrete (SWSSC). Fires are a common structural disaster. Thus, the post-fire mechanical properties of POM fibre-reinforced SWSSC (POM-SWSSC) were experimentally studied herein to evaluate the residual bearing capacity of the POM-SWSSC structure after a fire. After the heating and cooling procedures, the apparent morphology and mass loss of POM-SWSSC were recorded, and the compressive and tensile properties of the POM-SWSSC were investigated. The test results indicate that the colour of the SWSSC changed from cyan to yellow with an increase in exposure temperature. When the temperature exceeded 400 °C, the mechanical properties of the POM-SWSSC deteriorated sharply. Compared with compressive strength, high temperature had a more significant effect on the tensile strength of SWSSC. The addition of POM fibres could alleviate this issue. Compared with other fibre-reinforced concretes, POM-SWSSC had a higher cube compressive strength and splitting tensile strength after exposure to 400 °C. A prediction model was proposed to quantify the effect of different high temperatures on the mechanical properties of NF-SWSSC and POM-SWSSC.

Experimental study on post-fire mechanical properties and fracture behavior of Q690 steel

Fire is one of the most dangerous disasters for high-strength steel structures. After a fire event, the deterioration of mechanical properties of high-strength steel together with the complex stress state generated by external loading may lead to an unexpected failure or collapse of high-strength steel structures. Therefore, it is very critical to evaluate the post-fire behavior of high-strength steel considering stress state effects. This study conducted a series of tests on the Q690 steel specimens after being exposed to high temperatures ranging from 200 to 1000°C with different cooling methods including air-cooling and water-cooling methods. The tested specimens including smooth round specimens, notched round specimens, and grooved plate specimens were designed to consider the failure mode in axisymmetric stress state and plane strain state. To validate the accuracy of the test results, the mechanical properties such as yield strength, ultimate strength, elastic modulus, and ultimate strain were first compared with other test results reported in the literature. The post-fire ultimate strength under different stress states was obtained directly from the test results. Then, the post-fire equivalent plastic strain at fracture of different specimens was obtained from test data together with the finite element analysis results and analyzed under different stress triaxialities and Lode angle parameters. According to this study, an increase in stress triaxiality can result in an increase in ultimate strength but a decrease in the equivalent plastic strain at fracture for Q690 steel after fire. When the exposed temperature was not higher than 800°C, the post-fire strength of the plane strain specimen (Grooved Plane Specimen) was higher than that of the axisymmetric tension specimen (Notched Round Specimen) under a similar stress triaxiality; however, for their post-fire equivalent plastic strain at fracture, the results were opposite.

Effects of Cooling Procedures on Postfire Mechanical Properties and Fracture Resistance of G20Mn5QT Cast Steel

Postfire mechanical properties and fracture resistance of G20MnQT cast steels were investigated experimentally and effects of different cooling procedures were studied. G20Mn5QT cast steels were heated to 600°C, 700°C, 800°C, and 900°C and cooled to ambient temperature in air or in water. Metallographic examination found significant phase transformations only in G20Mn5QT cast steels cooled quickly in water from elevated temperatures of 800°C (800W) and 900°C (900W). Original ferrite and pearlite transformed to harder phases, such as martensite and bainite, in these two postfire materials. Vickers hardness tests, Charpy impact tests, and tension coupon tests were conducted on original and postfire G20Mn5QT cast steels. All test results confirmed the findings about phase transformation in metallographic examination. The martensite and bainite generated in phase transformation caused significant hardening of 800W and 900W. With no significant phase transformation, most mechanical properties of 600A, 600W, 700A, 700W, 800A, and 900A varied within ±15% compared with those of the original material, and the postfire Charpy impact energies increased moderately due to improvement of material homogeneity. Smooth notched tensile specimens were tested and material parameters in the void growth model were calibrated to evaluate the postfire fracture resistance of G20Mn5QT cast steels at high stress triaxialities. It was found that the fracture of 600A, 600W, 700A, 700W, 800A, and 900A was almost as ductile as that of the original G20Mn5QT cast steels, whereas 800W and 900W fractured brittlely, and had fracture resistance 87.6% and 78.4% lower than that of the original material.

POST-FIRE DAMAGE ASSESSMENT OF SLIP-CRITICAL CONNECTIONS

Steel buildings may encounter fire events during their life cycle. There is a necessity of exploring the ways of assessing the remaining capacity of steel structures after exposure to high temperature. In this study, the post-fire mechanical properties of slip-critical connections and the adopted high strength bolts are experimentally studied using an electronic furnace, an electronic torsion wrench, strain gauges and a universal testing machine. Each of the slip-critical connections is composed of a single JIS F10T high strength bolt and an A36 steel PL-70x70x18 (mm) with a standard hole in the center. The test parameters are elevated temperature (400°C, 600°C and 800°C) and exposure time (60 min, 90 min and 120 min). The test items include: (1) torque strength, (2) torque coefficient, and (3) bolt pretension. The tests are made before and after the connections are exposed to high temperature. The results obtained from the above tests can be used as an important reference for judging whether a steel structure continues to be used after fire damage.

Experimental Investigation on Post-Fire Mechanical Properties of Glass Fiber-Reinforced Polymer Rebars

Glass fiber-reinforced polymer (GFRP) rebars are commonly used as an alternative to conventional steel reinforcement in a variety of structural applications due to their superior low cost, strength-to-weight ratio, and durability. However, their mechanical properties after exposure to elevated temperatures, particularly in fire-prone environments, remain a significant concern. To address this concern, the present study focuses on investigating the residual tensile behavior, specifically the tensile strength and elastic modulus, of GFRP rebars exposed to high temperatures that are realistically encountered during fire incidents. The temperature range considered in this analysis spans from 100 °C to 400 °C, with a heating rate of 20 °C/min. The fire duration of 1 h is used. This comprehensive analysis is essential for enhancing our understanding of the performance and applicability of GFRP rebars in fire-prone environments. Based on their actual application in the construction industry, five specimens of three different rebar sizes (16, 20, and 25 mm) were examined for the effect of rebar size on tensile behavior after fire exposure. In addition, the effects were investigated of air- and water-cooling methods on residual tensile behavior. The nominal tensile strength, elastic modulus, and ultimate strain of GFRP rebars at ambient temperature are 930 MPa, 50.2 GPa and 1.85%, respectively. The test results indicated that as the temperature increased to 400 °C, the ultimate tensile strength of the GFRP bars decreased by up to 55%, while the ultimate strain increased by up to 44%, regardless of the cooling method. In addition, when rebars of sizes 16–25 mm were subjected to a 400 °C fire treatment, the smaller the rebar, the greater the percentage of ultimate tensile and strain reduction. These findings hold great significance for the utilization of GFRP bars within the construction industry. This study offers valuable insights into the design of fire-resilient structures, emphasizing the importance of considering rebar size and cooling methods due to their impact on the post-fire tensile strength and strain of GFRP rebars.

Heating duration effects on post-fire structural steel mechanical properties

Post-fire mechanical assessment of steel structures is challenging due to the changes in mechanical properties from the heating and cooling phases of a fire. While the effects of cooling rate and temperature of exposure have been researched in depth, the effects of heating duration are not comprehensive. This paper investigates the effects of heating duration on post-fire A572 Gr. 50 steel mechanical properties over a temperature range of 500–800 °C for durations of 0.5 and 2.0 h. Specimens are subjected to tensile testing in accordance with ASTM A370, Vickers microhardness testing, and optical microscopy. The findings suggest that heating duration has an influence on the post-fire mechanical properties of steel at elevated temperatures (700, 800 °C). At these temperature exposures, retention of yield stress, retention of ultimate stress, and microhardness all decrease. Members subjected to 700 °C for 0.5 h have more retention in yield stress and microhardness compared to those exposed for 2.0 h, while the 800 °C conditions show little difference in mechanical properties between exposure times. The results of this study show that the duration of fire exposure is an important factor when estimating the mechanical properties of fire-exposed steels, in addition to the temperature and cooling rate.

Fire-induced damage assessment of cementless alkali-activated slag-based concrete

According to recent studies, concrete made with alkali activators and slag is a potential construction material. To ensure the safety and durability of concrete structures after a fire, it is crucial to assess the material properties and determine the extent of the damage. This paper investigated the effect of elevated temperatures (25 °C, 100 °C, 250 °C, 500 °C, 700 °C and 900 °C) on the properties of cementless alkali-activated slag-based concrete. In this regard, 216 cubic samples with eleven mixes were designed according to the literature and then prepared. Two categories of tests were utilized: nondestructive and destructive tests. The nondestructive evaluations included visual inspections, mass loss, and the measurement of ultrasonic pulse velocity (UPV) of the specimens. The destructive tests were conducted to determine the post-fire mechanical properties, including stress–strain curves, compressive strength, and strain energy. Microstructural analyses (EDS,SEM, and XRD) were performed to evaluate microstructural and chemical composition changes after exposure to high temperatures. According to the results, exposing slag-based concrete samples to less than 500 °C can enhance their compressive strength. The reduction in strength for slag-based concrete ranged from 22% to 59% at 700 °C, showing a considerable improvement compared to normal concrete. Afterward, regression-based models were proposed to bridge the nondestructive evaluations to the destructive measurements, yielding appropriate predictions with the highest R-factor of 0.96.

Predicting Seismic Collapse Safety of Post-Fire Steel Moment Frames

This paper summarizes a study focused on evaluating the post-fire performance of steel Intermediate Moment Frames (IMFs) following earthquakes. To this aim, archetypes comprising 3-bay IMFs with three different heights were seismically designed, and their two-dimensional finite element models were created in OpenSees software. The post-fire mechanical properties of steel were inserted into the models based on 64 different fire scenarios. The effects of different cooling methods are scrutinized at system level. To develop seismic fragility curves, Incremental Dynamic Analysis (IDA) was performed using 50 suites of far-field and near-field records, according to FEMA-P695. Then, the Collapse Margin Ratio (CMR) of each model was calculated based on the data from the fragility analysis. The results show that the seismic resistance of structures that experienced fire declines to some extent. In addition, the lowest safety level was observed when the structures were subjected to pulse-like near-field records.

Experimental study on post-fire mechanical properties of Grade 12.9 high-strength bolts

This paper aims to experimentally investigate the post-fire mechanical properties of Grade 12.9 high-strength bolts. To evaluate the effects of elevated temperatures and the bolt diameter on the post-fire mechanical properties, seven different diameters of the Grade 12.9 bolts were heated based on a standard temperature–time diagram with the ranges of 20 to 900 °C. Upon air-cooling and reaching room temperature (a heating and cooling cycle), 105 bolt specimens were subjected to the standard tensile test. The experimental results, including the post-fire stress–strain curve, yield strength, tensile strength, modulus of elasticity, and ductility, were presented. The results indicated that changes in mechanical properties were noticeable for temperatures higher than 400 °C. Although the post-fire modulus of elasticity did not significantly change, the post-fire yield and tensile strengths decreased substantially if the maximum temperature exceeded 400 °C. After heating up to 900 °C and then cooling down and reaching ambient temperature, Grade 12.9 steel bolts showed a 60% reduction in strength. The comparison of the results showed that the effect of diameter on the post-fire yield and tensile strengths of Grade 12.9 bolts was insignificant and could be neglected, but on elastic modulus and ductility, the diameter effect needs to be considered. In addition, the scanning electron microscope (SEM) images indicated that the elevated temperatures remarkably changed the microstructural characteristics of the Grade 12.9 bolts. Finally, based on the test results, the prediction equations of the post-fire mechanical properties of the Grade 12.9 high-strength bolts were proposed.

Experimental Study of the Postfire Mechanical Properties of Grade 14.9 Superhigh-Tension Bolt

Fire hazard has been one of the most critical factors to threaten the safety of steel structures. Steel structures may not collapse when suffering fire and cooling down, but the serviceability of building after the fire needs to be evaluated before reusing. Hence, the postfire mechanical properties of steel are necessary. With the high- and ultrahigh-strength steel applied in the construction industry, superhigh-tension bolts (SHTB) are thus encouraged to apply in engineering. However, the current studies on the postfire mechanical properties mainly focus on the Grade 8.8s and 10.9s high-strength bolts. There is limited research work conducted to study the postfire mechanical properties of SHTB. In this study, coupon tests of Grade 14.9 SHTB were conducted, and the postfire performance was evaluated. Failure mode, stress-strain curve, elastic modulus, yield stress, tensile strength, tensile strain, percentage elongation after fracture, and percentage reduction of area were tested and analyzed. The postfire mechanical properties of different types of bolts were compared. The test results indicated that temperatures beyond 750°C cause decrease in the ductility of Grade 14.9 SHTB, regardless of the cooling condition. Accordingly, predictive formulas with good precision are proposed for the postfire mechanical properties of Grade 14.9 SHTB cooled down by air and water, respectively.

Post-Fire Mechanical Properties of Concrete Incorporating Waste EPS (Styrofoam) as Aggregate Replacement

Reusing waste is one of the most recent topics and one of the main contributors to sustainability. It is known that concrete is one of the most common materials to produce different types of construction members around the world. That is due to mainly its low cost, availability, long period of durability, and ability to withstand harsh environments. On the other hand, due to the rapid changes that have happened in the last few decades in the production of decorative materials, some material types of cladding are used for decoration purposes, such as Styrofoam (EPS) (trade name “Astropol”), which is developed from disposal materials. Discovering and implementing a method of reusing these wastes in concrete is beneficial for the environment to reduce waste around the globe. In the current study, Styrofoam (Astropol) waste was used as a replacement for fine aggregate since concrete structures contain this material in their composition. It is important to test these materials for fire resistance and expose them to an elevated temperature in order to discover the post-fire mechanical properties of the composite material. The experimental result showed that the post-fire compressive strength of concrete containing different ratios of EPS (Astropol) increases compared to conventional concrete. The compressive strengths were 19.94 MPa, 19.295 MPa, 16.806 MPa, and 17.66 MPa for 0%, 15%, 25%, and 50%, respectively, while the post-fire indirect tensile strength for all specimens containing EPS reduced as the fire duration and temperature increased.

Post-fire mechanical response of Q960E ultra-high-strength structural steel

The ultra-high-strength steel (UHSS) has gained significant attention in recent years owing to its prominent benefits in structural performance, economics and environmental aspect. The fire resistance of steel structures is always of great concern as the mechanical properties of steel are sensitive to high temperatures which could threaten the safety of the steel structures, and so do UHSS structures. The post-fire mechanical performance of steel is crucial for evaluating the residual capacity of the structure. Therefore, it is also necessary to evaluate the post-fire mechanical properties of UHSS. In this study, tensile tests were carried out to investigate the post-fire mechanical properties of Q960E UHSS with two types of cooling techniques, i.e. cooling in air or cooling in water. The residual elastic modulus, yield stress, tensile strength, tensile strain, rupture stress, and percentage reduction of area of Q960E UHSS subjected to various temperature levels were obtained from the post-fire stress-strain curves. The influence of the cooling techniques and temperature level on the post-fire mechanical properties of Q960E UHSS were discussed, and some empirical predictive equations were proposed for evaluating the post-fire mechanical properties. Finally, a novel stress-strain model was also proposed to predict the post-fire response of Q960E UHSS. The proposed model showed good consistency with laboratory experimental results. Therefore, it is concluded that the model can provide accurate predictions for the plastic strain-true stress curve for various types of steels.