Numerical modelling of light gauge cold-formed steel compression members subjected to distortional buckling at elevated temperatures

Fire safety design of building structures has received greater attention in recent times due to continuing loss of properties and lives during fires. However, fire performance of light gauge cold-formed steel structures is not well understood despite its increased usage in buildings. Cold-formed steel compression members are susceptible to various buckling modes such as local and distortional buckling and their ultimate strength behaviour is governed by these buckling modes. Therefore a research project based on experimental and numerical studies was undertaken to investigate the distortional buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. Lipped channel sections with and without additional lips were selected with three thicknesses of 0.6, 0.8, and 0.95 mm and both low and high strength steels (G250 and G550 steels). More than 150 compression tests were undertaken first at ambient and elevated temperatures. Finite element models of the tested compression members were then developed by including the degradation of mechanical properties with increasing temperatures. Comparison of finite element analysis and experimental results showed that the developed finite element models were capable of simulating the distortional buckling and strength behaviour at ambient and elevated temperatures up to 800 °C. The validated model was used to determine the effects of mechanical properties, geometric imperfections and residual stresses on the distortional buckling behaviour and strength of cold-formed steel columns. This paper presents the details of the numerical study and the results. It demonstrated the importance of using accurate mechanical properties at elevated temperatures in order to obtain reliable strength characteristics of cold-formed steel columns under fire conditions.     

Experimental study of the mechanical properties of light gauge cold-formed steels at elevated temperatures

In recent times, light gauge cold-formed steel sections have been used extensively as primary load-bearing structural members in many applications in the building industry. Fire safety design of structures using such sections has, therefore, become more important. Deterioration of mechanical properties of yield stress and elasticity modulus is considered the most important factor affecting the performance of steel structures in fires. Hence, there is a need to fully understand the mechanical properties of light gauge cold-formed steels at elevated temperatures. A research project based on experimental studies was, therefore, undertaken to investigate the deterioration of mechanical properties of light gauge cold-formed steels. Tensile coupon tests were undertaken to determine the mechanical properties of these steels made of both low- and high-strength steels and thicknesses of 0.60, 0.80 and 0.95 mm at temperatures ranging from 20 to 800 °C. Test results showed that the currently available reduction factors are unsafe to use in the fire safety design of cold-formed steel structures. Therefore, new predictive equations were developed for the mechanical properties of yield strength and elasticity modulus at elevated temperatures. This paper presents the details of the experimental study, and the results including the developed equations. It also includes details of a stress–strain model for light gauge cold-formed steels at elevated temperatures.     

The effect of cement dosage on mechanical properties of concrete exposed to high temperatures

Although concrete is a non-combustible material, it is found that when exposed to high temperatures, such as fire, the physical, chemical and mechanical properties of concrete can drastically change. Thus, it becomes important to assess the relative properties of concrete under high temperatures in order to evaluate and predict the post-fire response of reinforced concrete (RC) buildings and structures. This paper assesses the effects of elevated temperatures and cement dosages on the mechanical properties of concrete. Two concrete mix designs were considered in this research in an attempt to study the effects of cement dosage (250 and 350 kg/m³) on the post-fire response of concrete. Once cast, the test samples were first exposed to elevated temperatures ranging from 100 to 800 °C, and then allowed to cool down slowly to ambient room temperature of 20 °C before being tested to failure. Several tests were then carried out to determine the mechanical properties of the cooled concrete specimens. The test results indicated that at temperature above 400 °C, concrete undergoes significant strength loss when compared to the strength of non-heated concrete. In addition this strength reduction was found to be unaffected by the cement dosages. The experimental results were also compared with current European standard (BS EN 1992-1-2:2004 standard) strength equations and American Concrete Institute standard (ACI 216.1).     

Mechanical properties of cold-formed steels at elevated temperatures

Mechanical properties have an important role in the fire safety design of cold-formed steel structures due to the rapid reduction in mechanical properties such as yield strength and elastic modulus under fire conditions and associated reduction to the load carrying capacities. Hence there is a need to fully understand the deterioration characteristics of yield strength and elastic modulus of cold-formed steels at elevated temperatures. Although past research has produced useful experimental data on the mechanical properties of cold-formed steels at elevated temperatures, such data do not yet cover different cold-formed steel grades and thicknesses. Therefore, an experimental study was undertaken to investigate the elevated temperature mechanical properties of two low and high strength steels with two thicknesses that are commonly used in Australia. Tensile coupon tests were undertaken using a steady state test method for temperatures in the range 20–700 °C. Test results were compared with the currently available reduction factors for yield strength and elastic modulus, and stress–strain curves, based on which further improvements were made. For this purpose, test results of many other cold-formed steels were also used based on other similar studies undertaken at the Queensland University of Technology. Improved equations were developed to predict the yield strength and elastic modulus reduction factors and stress–strain curves of a range of cold-formed steel grades and thicknesses used in Australia. This paper presents the results of this experimental study, comparisons with the results of past research and steel design standards, and the new predictive equations.     

超高强钢S960的火灾后性能

目前,高强钢和超高强钢都被用于一些标志性建筑的结构部件中并能充分利用其强度。然而,现有的文献中关于高强钢和超高强钢,特别是超高强钢的结构性能和材料性能的研究有限。超高强钢的潜在优势和较低的研究水平使其需要更多的关注和研究。建筑中使用超高强钢所做的钢构件有时会不可避免地遇到火灾危险,而火灾后是否能对其再度利用则需要进行可靠的评估。为了研究超高强钢S960火灾后的材料性能,进行一项试验评估S960所构成部件的结构的火灾后性能。对从1000℃冷却下来的S960样本进行拉伸试验,获得火灾后的弹性系数、屈服强度、极限强度和应力-应变曲线,并发现火灾后结构钢的性能很大程度上取决于钢的等级。提出一些独特的预测方程来评估火灾后S960钢的力学性能。     

高温下冷弯薄壁型钢的力学性能分析

由于火灾中冷弯薄壁型钢结构的力学性能参数(如:屈服强度和弹性模量)的急剧下降,引起承载力的下降,使得力学性能成为冷弯薄壁型钢结构设计的重要指标。因此需要掌握高温下冷弯薄壁型钢的力学性能(屈服强度和弹性模量)。尽管有高温下冷弯薄壁型钢力学性能的相关试验数据,但是该数据不能涵盖各种不同等级和厚度的冷弯薄壁型钢。针对在澳大利亚经常使用的两种厚度的高强和低强冷弯薄壁型钢进行高温试验研究。拉伸试验的温度控制在20～700℃。将试验结果与已知的降低屈服强度、弹性模量和应力-应变曲线的因素进行比较,以确定哪个因素可以被提高。在昆士兰工科大学,很多不同的冷弯型薄壁型钢的试验结果用于同样类似的研究中。改进后的方程用于分析一定范围内不同等级和厚度的冷弯型钢屈服强度和弹性模量降低的因素及应力-应变曲线。给出试验结果及与之前研究结果的比较、钢的设计标准和新的计算方程。     

The effect of elevated temperatures on corroded and uncorroded reinforcement embedded in mortar

In this study, the alterations at the reinforcement steels embedded in corroded and not corroded reinforcement mortar specimens after high temperatures were investigated. These reinforced mortar specimens with 3 cm and 5 cm covers were first exposed to 20, 100, 200, 300, 500 and 800 °C temperatures. Subsequently, to determine the mechanical properties tensile tests were applied on the steels taken out of these mortar specimens. With the tensile tests performed stress–strain curves of the steel bars exposed to several temperatures were obtained. Besides, yield and ultimate strengths of the steel bars were also determined. It is obtained that, high temperatures affected the mechanical properties of reinforcement steel considerably negatively. Results of the study have shown that the larger the covers are the better the steel bars are protected against high temperatures.     

Distortional buckling tests of cold-formed steel compression members at elevated temperatures

In recent times, light gauge cold-formed steel sections have been used extensively since they have a very high strength to weight ratio compared with thicker hot-rolled steel sections. However, they are susceptible to various buckling modes including a distortional mode and hence show complex behaviour under fire conditions. Therefore, a research project based on detailed experimental studies was undertaken to investigate the distortional buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. More than 150 axial compression tests were undertaken at uniform ambient and elevated temperatures. Two types of cross sections were selected with nominal thicknesses of 0.60, 0.80, and 0.95 mm. Both low (G250) and high (G550) strength steels were used. Distortional buckling tests were conducted at six different temperatures in the range of 20-800 ^°C. The ultimate loads of compression members subject to distortional buckling were then used to review the adequacy of the current design rules at ambient and elevated temperatures. This paper presents the details of this experimental study and the results.     

Study on nominal values of mechanical properties of high strength steel at elevated temperature and after fire exposure

高强钢高温下和高温后的力学性能是进行高强钢结构抗火设计和火灾后评估的重要基础。我国GB 51249—2017《建筑钢结构防火技术规范》和欧洲规范EC3中针对普通低碳钢提出了高温下屈服强度和弹性模量计算公式，但其不适用于高强钢。国内外学者对高温下和高温后高强钢力学性能已开展了一系列试验研究，但由于钢材强度等级、试验设备、加热速率和加载制度等影响，导致试验结果离散性较大，不能应用于实际工程中。同时不同学者提出的力学性能指标计算式各不相同，均不具有普遍适用性。采用数理统计中t分布与置信区间的方法对高强钢高温下和高温后力学性能试验数据进行统计分析，得到不同温度下力学性能指标具有95%保证率的标准值，拟合出高强钢高温下和高温后力学性能指标的计算式，并与GB 51249—2017和欧洲规范EC3预测结果进行对比。结果表明：自然冷却和浸水冷却条件下，高强钢高温后屈服强度发生明显下降的转折点分别是600℃和 500℃；高温下高强钢的屈服强度折减系数低于普通结构钢；高强钢弹性模量折减系数在作用温度小于600℃时低于普通结构钢的，而在温度大于600℃时高于普通结构钢的。     

高强钢高温下和高温后力学性能指标的标准值研究

高强钢高温下和高温后的力学性能是进行高强钢结构抗火设计和火灾后评估的重要基础。我国GB 51249—2017《建筑钢结构防火技术规范》和欧洲规范EC3中针对普通低碳钢提出了高温下屈服强度和弹性模量计算公式,但其不适用于高强钢。国内外学者对高温下和高温后高强钢力学性能已开展了一系列试验研究,但由于钢材强度等级、试验设备、加热速率和加载制度等影响,导致试验结果离散性较大,不能应用于实际工程中。同时不同学者提出的力学性能指标计算式各不相同,均不具有普遍适用性。采用数理统计中t分布与置信区间的方法对高强钢高温下和高温后力学性能试验数据进行统计分析,得到不同温度下力学性能指标具有95%保证率的标准值,拟合出高强钢高温下和高温后力学性能指标的计算式,并与GB 51249—2017和欧洲规范EC3预测结果进行对比。结果表明：自然冷却和浸水冷却条件下,高强钢高温后屈服强度发生明显下降的转折点分别是600℃和 500℃;高温下高强钢的屈服强度折减系数低于普通结构钢;高强钢弹性模量折减系数在作用温度小于600℃时低于普通结构钢的,而在温度大于600℃时高于普通结构钢的。     

Temperature development in structural stainless steel sections exposed to fire

The initial material cost of structural stainless steel is about four times that of structural carbon steel, due largely to the expense of the alloying elements and the relatively low volume of production. Given broadly similar structural performance, additional areas of benefit need to be identified and exploited in order to establish stainless steel as a viable alternative material for construction. In addition to the familiar benefits of corrosion resistance, low maintenance, high residual value and aesthetics, one such area is fire resistance. The mechanical and thermal properties of stainless steel differ from those of carbon steel due to variation in chemical composition between the materials. A comparison of these properties for austenitic stainless steel with those for structural carbon steel is presented herein, and implications of the differences explored.Accurate and efficient determination of the temperature development within a structural member upon subjection to fire is paramount. In this paper, comparisons of temperature development in structural stainless steel sections are made between existing test results, numerical simulations and the simple calculation model of Eurocode 3: Part 1.2. Based on these comparisons, revised values for the heat transfer coefficient and emissivity of structural stainless steel members exposed to fire are proposed. In the temperature development calculation model of EN 1993-1-2, it is proposed that emissivity be taken as 0.2 (in place of the currently adopted value of 0.4) and the heat transfer coefficient be taken as 35 W/m² K (in place of the currently adopted value of 25 W/m² K). The significance of such revisions to the fire resistance and critical temperature is assessed. Application of the revised values in the predictive models for member resistances at elevated temperatures in Eurocode 3: Part 1.2 also reveals improved agreement with test results on axially loaded stainless steel columns in fire, and average enhancements in fire resistance of 10%.     

An experimental study of loaded full-scale cold-formed thin-walled steel structural panels under fire conditions

This paper presents the results of eight tests on loaded full-scale cold-formed thin-walled steel structural panels, two tests at ambient temperature and six exposed to the standard fire condition on one side. The test panels used two types of lipped channels, 100×54×15×1.2 mm or 100×54×15×2 mm, each channel having two service holes, one near the top and one near the bottom. Three different load levels, being 0.2, 0.4 and 0.7 times the load carrying capacity of the same panel tested at ambient temperature, were applied during the six fire tests. At ambient temperature, failure was local buckling around the top service hole. Under fire condition, the main failure mode was overall flexural–torsional buckling about the major axis, with the lateral deformations of the test panels being mainly caused by thermal bowing due to temperature gradients. The interior insulation in some tests was burnt through and this had noticeable influence on temperature developments in the test steel channels. In two fire tests using the thinner channels, the test panels failed before 30 min that is considered to be the minimum standard fire rating of this type of construction.     

Corner properties of cold-formed steel sections at elevated temperatures

This paper presents the mechanical properties of the corner parts of cold-formed steel sections at elevated temperatures. Light-gauge structural members are cold-formed which results the mechanical properties of the corner parts being different from the flat parts. However, previous research has focused on the investigation of the corner parts of cold-formed steel sections at normal room temperature and the performance of the corner parts at elevated temperatures is unknown. An appropriate model for fire resistant design of steel structures necessitates a correct representation of mechanical properties of structural steel at elevated temperatures. Therefore, experimental investigation on corner coupon specimens at different temperatures ranged from approximately 20 to 1000 °C was conducted to study the behaviour of the corner parts of cold-formed steel sections at elevated temperatures. Two kinds of corner coupon specimens, namely the inner corner coupon specimens and outer corner coupon specimens having the steel grade of G500 (nominal 0.2% proof stress of 500 MPa) and nominal thickness of 1.9 mm were tested. The test results were compared with the flat coupon specimens taken from the same cold-formed steel sections as the corner coupon specimens. A unified equation to predict the yield strength (0.2% proof stress), elastic modulus, ultimate strength and ultimate strain of the corner parts of cold-formed steel sections at elevated temperatures is thus proposed in this paper. Generally, it is shown that the proposed equation adequately predicts the test results of the corner coupon specimens. Furthermore, stress–strain curves at different temperatures are plotted and a stress–strain model is also proposed for the corner parts of cold-formed steel sections.     

Post-fire residual mechanical properties of concrete made with recycled rubber aggregate

This study investigates the effects of elevated temperatures on the residual mechanical performance of concrete produced with recycled rubber aggregate (RRA). Four different concrete compositions were prepared: a reference concrete (RC) made with natural coarse aggregate and three concrete mixes with replacement rates of 5%, 10% and 15% of natural fine and coarse aggregate by RRA from used tyres. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the compressive strength and the splitting tensile strength were evaluated and compared with reference values obtained prior to fire exposure. For the replacement rates used in the present experiments, the obtained results show that concrete made with recycled rubber aggregate (CRRA) present a thermal response that is roughly similar to that of RC; in addition, although residual mechanical properties of CRRA are noticeably more affected than those of RC, particularly for higher exposure temperatures, the relative reduction should not prevent it from being used in structural applications.     

Residual stresses in welded flame-cut high strength steel H-sections

The presence of residual stress in members can significantly compromise the stiffness and fatigue life of steel structural components. Researches in this area are well documented for structural members of mild carbon steels. Nevertheless, due to the difference of stress–strain relations and material properties under ambient and high temperatures, the residual stress distribution in a high strength steel member is physically different from those fabricated from mild carbon steel. It is imperative to study the residual stress distribution for structural members fabricated from high strength steel. In this paper, the residual stresses of three welded flame-cut H-section columns with a nominal yield strength of 460 MPa but different cross-section dimensions were investigated. Both sectioning and hole-drilling methods were used in the measurement and the obtained residual stresses were compared between the two methods. The magnitudes and distributions of the measured residual stresses are identical with those of carbon steel, however in relatively smaller residual stress ratios. Finally, based on the measurements, a simplified residual stress distribution for 460 MPa high strength steel members with welded flame-cut H-section is proposed.     

Experimental investigation of cold-formed steel material at elevated temperatures

This paper presents the mechanical properties data for cold-formed steel at elevated temperatures. The deterioration of the mechanical properties of yield strength (0.2% proof stress) and elastic modulus are the primary properties in the design and analysis of cold-formed steel structures under fire. However, values of these properties at different temperatures are not well reported. Therefore, both steady and transient tensile coupon tests were conducted at different temperatures ranged approximately from 20 to 1000 °C for obtaining the mechanical properties of cold-formed steel structural material. This study included cold-formed steel grades G550 and G450 with plate thickness of 1.0 and 1.9 mm, respectively. Curves of elastic modulus, yield strength obtained at different strain levels, ultimate strength, ultimate strain and thermal elongation versus different temperatures are plotted and compared with the results obtained from the Australian, British, European standards and the test results predicted by other researchers. A unified equation for yield strength, elastic modulus, ultimate strength and ultimate strain of cold-formed steel at elevated temperatures is proposed in this paper. A full strain range expression up to the ultimate tensile strain for the stress–strain curves of cold-formed carbon steel at elevated temperatures is also proposed in this paper. It is shown that the proposed equation accurately predicted the test results.     

An approach for evaluating the residual strength of fire exposed bridge girders

This paper presents an approach for evaluating residual strength of fire exposed steel bridge girders. The approach involves three stages of analysis that is to be carried out at ambient conditions, during exposure to fire, and after cooling of the fire exposed bridge girder. In the first stage, load carrying capacity of the girder is evaluated at room temperature. In the second stage of analysis, thermal and structural response of the bridge girder is traced under specified fire exposure and loading conditions. In the third stage (after the bridge girder has cooled down), residual capacity of the girder is evaluated by incrementing load on the girder till failure occurs. The proposed approach is applied to carry out a set of numerical studies on a typical steel girder using finite element computer program ANSYS. Results from numerical studies indicate that the maximum fire temperature (and associated temperature in steel) is the most critical factor that influences residual strength of a fire exposed bridge girder. A girder exposed to typical “external” fire conditions, with maximum fire temperatures reaching to 600–700 °C, retains about 70 to 80% of its strength on cooling. On the other hand, a steel bridge girder exposed to hydrocarbon fire, with a maximum temperature of about 1100 °C, looses most of its strength during heating phase of the fire and experiences failure.     

Behavior of steel-sheathed shear walls subjected to seismic and fire loads

A series of tests was conducted on six 2.7 m×3.7 m shear wall specimens consisting of cold-formed steel framing sheathed on one side with sheet steel adhered to gypsum board and on the opposite side with plain gypsum board. The specimens were subjected to various sequences of simulated seismic shear deformation and fire exposure to study the influence of multi-hazard interactions on the lateral load resistance of the walls. The test program was designed to complement a parallel effort at the University of California, San Diego to investigate a six-story building subjected to earthquakes and fires. The test results reported here indicate that the fire exposure caused a shift in the failure mode of the walls from local buckling of the sheet steel in cases without fire exposure, to global buckling of the sheet steel with an accompanying 35% reduction in lateral load capacity after the wall had been exposed to fire. This behavior appears to be predictable, which is encouraging from the standpoint of residual lateral load capacity under these severe multi-hazard actions.     

Concrete cover effect on reinforced concrete bars exposed to high temperatures

The mechanical properties of structural reinforcement steel have been investigated after the exposure to high temperatures. Plain steel, reinforcing steel bars embedded into mortar and plain mortar specimens were prepared and exposed to 20, 100, 200, 300, 500, 800 and 950 °C temperature for 3 h individually. The S420 deformed steel bars with diameters of ∅10, ∅16 and ∅20 were used. The mortar was prepared with CEM I 42.5 N cement and fly ash. The tension tests on reinforcements taken from cooled specimens were performed, and the variations in yield strength, ultimate strength and in resilience of three different dimensioned reinforcements were determined. A cover of 25 mm provides protection against high temperatures up to 400 °C. The high temperature exposed plain steel and the steel with 25-mm cover has the same characteristics when the reinforcing steel is exposed to a temperature 250 °C above the exposure temperature of plain steel.