G550高强度冷成型钢高温力学性能瞬态试验研究

冷成型钢高温材料特性是进行冷成型钢结构抗火设计及数值模拟的重要参数。现有的钢材高温材性数据大多基于稳态试验方法得到,而瞬态试验方法显然较前者更接近实际火灾情况。本文利用MTS810试验系统对1mmG550冷成型钢进行了详细高温瞬态试验研究,并将试验结果与已完成的同批次G550冷成型钢高温稳态试验进行了细致对比,结果表明:①G550冷成型钢瞬态试验极限强度折减系数在525℃以下时与稳态试验结果比较接近,但在550℃以上时明显高于稳态试验结果;②相同温度、应力水平下,G550冷成型钢瞬态试验应变值高于稳态试验结果,尤其在应力-应变曲线非弹性段二者差异明显。因此,瞬态试验方法与稳态试验方法对G550冷成型钢并不等效。然后,基于统计平均方法得到瞬态试验高温弹性模量以及屈服强度,同样与稳态试验结果进行了比较;最后,通过数值拟合提出了G550冷成钢高温材性折减系数及本构关系表达式,表达式与试验结果基本吻合,满足工程精度要求。     

Q345冷成型钢高温力学性能试验研究

冷成型钢高温材料特征指标是进行冷成型钢结构抗火设计及数值模拟的重要参数。现有的钢材高温材性数据大多基于稳态试验方法得到,而瞬态试验方法较前者更接近实际火灾情况。利用MTS810试验系统对1.5mm厚Q345冷成型钢进行了高温力学性能试验研究,将瞬态、稳态试验结果进行对比分析。试验结果表明： Q345冷成型钢瞬态试验抗拉强度折减系数在430～700 ℃时普遍高于稳态试验结果,二者相对误差27%～57%;超过100 ℃,Q345冷成型钢瞬态试验高温弹性模量明显低于稳态试验结果,相对误差17%～156%;450～550 ℃时,相同温度、应变水平下,Q345冷成型钢瞬态试验应力-应变曲线弹塑性阶段应力值明显高于稳态试验应力值,导致瞬态试验高温屈服强度高于稳态试验结果,相对误差28%～40%。通过数值拟合给出Q345冷成型钢高温材性折减系数及本构关系表达式,表达式与试验结果基本吻合。     

屈服强度550MPa高强冷弯薄壁型钢结构轴压构件承载力计算模式研究

本文收集了目前国内外屈服强度550MPa高强钢材冷弯薄壁型钢结构轴心受压构件的试验数据,参考现行的《冷弯薄壁型钢结构技术规范》(GB 50018—2002),对其承载力的合理计算模式进行了系统研究。结果表明,现行规范考虑板组约束计算截面有效宽厚比的方法对屈服强度550MPa高强钢材冷弯薄壁型钢结构轴心受压构件极限承载力的计算是可靠的。     

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。     

G550级高强薄板钢材的材性及应用

近年来,壁厚不大于1 mm、屈服强度达550 MPa的高强冷弯薄壁型钢已在国外实际工程中开始应用.为了促进高强冷弯薄壁型钢结构在中国的推广应用,验证高强钢材对中国冷弯薄壁型钢结构技术规范相关设计规定的适应性,本项目开展了G550级冷弯薄壁型钢的材性和轴压构件受力性能的系列试验研究,以期提出适用G550级钢材构件的设计计算建议.首先对t为0.48,0.60,0.75,1.00 mm 4种厚度G550级冷轧镀层薄板进行了材性试验研究,考察和分析了其材性特征,讨论了在中国推广和应用高强冷弯薄壁型钢时可能存在的主要问题及相应对策.材性试验结果可用于构件的试验研究和分析.     

冷弯中厚壁矩形管屈服强度设计公式研究

为研究冷弯中厚壁矩形管冷弯效应对钢材屈服强度及其他力学性能的影响,对国内现有冷弯中厚壁矩形管的板件试样材性试验和短柱试验数据按照不同截面形式、钢材等级、厂家、厚度及成型方式等进行汇总与分析。提出了适合我国钢材特点与考虑冷弯效应的冷弯中厚壁矩形管屈服强度设计建议公式,将国内外相关规范的考虑冷弯效应的屈服强度计算公式和本文建议公式的计算结果与冷弯中厚壁方(矩)形管短柱试验数据进行对比分析,以验证本文建议公式的适用性。结果表明:按北美规范AISI S100-2007、欧洲规范EN 1993-1-3:2006和加拿大规范CAN3-S136-M84:1984得到的强度计算值相比于个别短柱试验值偏大,我国GB 50018—2002《冷弯薄壁型钢结构技术规范》中的屈服强度计算值相对于采用宝钢和Q345钢材的某些短柱试验值偏大。本文建议公式计算值相对于短柱试验值均偏小且安全,可作为冷弯中厚壁矩形管考虑冷弯效应的强度设计公式。     

G550高强度冷成型钢高温力学性能稳态试验研究

冷成型钢高温材料特性是进行冷成型钢结构抗火设计及数值模拟的基本参数。现有的冷成型钢高温材性试验可能限于试验设备条件及材料差异等原因导致结果差异明显,不够理想。本文利用MTS810试验系统对1mmG550冷成型钢平板及转角部位进行详细的高温稳态试验研究。将G550冷成型钢平板与转角部位试验结果进行细致对比,表明:G550冷成型钢转角冷弯效应在常温下可能引起不利影响;高温情况下,转角部位材性折减系数普遍低于平板部位,可能导致冷成型钢构件提早发生高温局部、畸变屈曲。将G550冷成型钢弹性模量、屈服强度、极限强度折减系数与现行欧洲规范、澳大利亚规范、英国规范及其他学者的试验结果进行比较,并采用统一表达式进行数值拟合。拟合公式与试验结果符合良好,满足工程精度要求。基于Ramberg-Osgood模型,提出G550冷成型钢高温本构关系表达式,表达式与试验曲线吻合良好,满足冷成型钢数值模拟要求。     

G550高强冷弯薄壁卷边槽钢受弯构件承载力直接强度法研究

直接强度法在冷弯薄壁型钢构件的设计中得到越来越多的应用,然而用于校准原有受弯构件直接强度法设计公式的试验数据均来自于屈服强度较低的纯弯构件,对于屈服强度较高的受弯构件尚无试验数据可供参考,限制了直接强度法设计公式的使用范围。采用有限元程序ANSYS对已有G550高强冷弯薄壁卷边槽钢受弯构件试验进行了模拟,验证了有限元分析的可行性。在此基础上,对卷边形式为直卷边、斜卷边和复杂卷边的G550高强冷弯薄壁槽钢构件分别进行纯弯和非纯弯作用下的有限元参数分析,以原有直接强度法设计公式为基准,修正出适用于不同卷边形式的G550高强冷弯薄壁槽钢受弯构件在纯弯和非纯弯作用下的直接强度法设计公式,为今后的规范修订和工程应用提供必要的参考。     

国际期刊导读

高温下冷弯薄壁型钢的力学性能分析 摘要：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。     

Q345冷成型钢高温力学性能试验制度参数的影响研究

钢材的火灾全过程高温本构是开展冷成型钢结构抗火研究的重要输入数据。为此,开展国内常用Q345冷成型钢高温力学性能试验研究,定量考察峰值温度保温时间、降温速率、温度历程以及稳态与瞬态试验方法等试验制度参数对其力学性能的影响,结果表明： 高温稳态试验中,峰值温度保温时间和降温速率对钢材高温力学性能影响不明显,相对百分偏差均介于-10%~10%之间;若温度历程中各次升温过程峰值温度中的最高温度和拉伸温度均相同,则钢材在多次升降温过程下的高温力学性能与其在一次升降温过程降温段的高温材性相互接近; 高温瞬态试验中,温度历程对钢材高温试验应变影响显著;相同拉伸温度下,升温与降温段的应变相对偏差最高可达12904%; 稳态与瞬态试验方法对考虑温度历程的冷成型钢高温应变影响亦非常明显,试验参数范围内,相同拉伸温度下应变相对百分偏差最大可达14851%。总之,Q345冷成型钢的火灾全过程高温本构需考虑温度历程中各次升温过程峰值温度中的最高温度和拉伸温度的影响,且稳态与瞬态试验方法所构建的高温本构模型并不等效。     

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.     

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.     

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.     

The application of BS5950: Part 8 of fire limit state design to the performance of ‘old’ structural mild steels

An investigation has been carried out to extend the guidance given in the new British Standard BS5950: Part 8, on fire limit state design, to the refurbishment and fire damage reinstatement of old steel framed, buildings.

Structural mild steel produced to BS15 approximately 50 years ago was found to be generally weaker at elevated temperatures than its modern counterpart—BS4360: Grade 43A (BS EN 10025: Grade 430A). However, providing in design calculations due recognition is given to the lower yield stress of old mild steel at ambient temperature, its performance in fire will be as good as that being currently produced. For the present time, it is therefore appropriate to adopt the same relationships between strength, loading and temperature for structural members given in the new Code, with no additional penalties on fire protection thickness should this be necessary.

Fire simulation treatments on steel manufactured to BS15 demonstrated that the degradation in strength properties is in agreement with work reported earlier on ‘weak’ mild steel—BS4360: Grade 43A. The results of a similar evaluation on mild steel produced since the 1986 revision of BS4360: Grade 43A are also in line with previous work.     

冷弯薄壁钢短柱在均匀高温下的性能

本篇章对常温及均匀高温条件下的冷成型薄壁短槽钢的轴向强度进行了初步研究。对常温及均匀高温下的一系列卷边的或非卷边的槽钢进行了试验研究。应用一系列的设计方法和商业化的有限元软件ABAQUS(1998)对试验进行了分析。本中涉及的设计方法包括：英国规范BS5950(1987)、欧洲规范Eurocode3Part1．3(CENl996)和美国规范AISI(1996)，在有限元分析中，考虑了几何非线性和材料非线性。高温下钢材的应力一应变曲线根据欧洲规范Eurocode3Part1．2(CENl995)和Qutinen(2001)采用。在某些试验中发现扭转屈曲，为了扩展针对这种破坏模式的设计方法，在这些规范中引用了Yong和Hancock(1992)考虑扭转屈曲的方法。对设计规范BS5950Part5(1987)、欧洲规范Eurocode3Part1．3(CENl996)和美国规范AISI(1996)中常温下薄壁柱的设计方法加以修改，来考虑钢材高温下的强度和刚度的变化。从试验结果、规范预期的结果和数值分析的比较可见，可很容易的通过上述修正当前规范的方法来分析薄壁型钢短柱高温下的性能。     

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.     

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.     

Axial strength of cold-formed thin-walled steel channels under non-uniform temperatures in fire

This paper presents the results of a numerical investigation into the axial strength of cold-formed thin-walled channel sections (columns) under non-uniform high temperatures in fire. The non-uniform temperature distributions are based on the results of a thermal analysis of thin-walled stud panels carried out by the authors. The general finite-element package ABAQUS is used to obtain strengths of columns with different lengths at different fire exposure times. To aid development of a hand calculation method of column strength in fire, the accuracy of using two ways of simplifying the non-uniform temperature distribution is investigated. The ambient temperature design method for cold-formed thin-walled columns in Eurocode 3 Part 1.3 (EN1993-1-3, Eurocode 3: design of steel structures, Part 1.3: general rules, supplementary rules for cold formed thin gauge members and sheeting, European Commission for Standardisation, Brussels, 2001) is modified to take into account the change in the strength and stiffness of steel at elevated temperatures and thermal-bowing effects. The results of this design method are compared to the ABAQUS simulation results.