Cold-formed steel columns made with open cross-sections subjected to fire

An experimental study on the fire behaviour of cold-formed steel lipped channel (C) and built-up I (2C) slender columns with restrained thermal elongation is presented. The studied parameters were the stiffness of the surrounding structure, type of cross-section, end support conditions and initial applied load level on the columns. The results showed that increasing the stiffness of the surrounding structure and initial applied load level for the semi-rigid support conditions and both cross-sections, lead to a significant reduction of the critical temperature whereas for the pin-ended support conditions the reduction is supposed to be smaller.     

Experimental investigation on ferritic stainless steel columns in fire

This paper presents the performance of EN 1.4003 ferritic stainless steel hollow section columns when exposed to fire loading. Experimental analysis of the behaviour of ferritic stainless steel structural members in fire represents a novelty. In detail, three column tests were carried out in the framework of the Research Fund of Coal and Steel (RFCS) project named Structural Applications of Ferritic Stainless Steels (SAFSS, RFSR-CT-2010-00026). Tubular thin-walled members were considered in this study because structural applications of ferritic stainless steels generally incorporate such profiles. Three columns were tested: two square hollow sections (SHS) and a rectangular hollow section (RHS) of different length. Fire loading was applied under a constant concentrically compressive load. Identical column tests at room temperature are also reported.     

Class 4 stainless steel I beams subjected to fire

Structural elements composed of Class 4 sections are common in stainless steel buildings structures. These thin walled profiles are more susceptible to the occurrence of local buckling. Additionally, in beams the lateral-torsional buckling is also a common failure mode. These instability phenomena are intensified at high temperatures. This work has the main objective of presenting a numerical study on the fire behavior of beams with Class 4 stainless steel sections when subjected to pure bending and high temperatures. The influence of several parameters, as geometrical imperfections and residual stresses, on the ultimate load will be evaluated and comparisons between the numerical results and the Eurocode 3 rules will also be made.     

A new design method for stainless steel columns subjected to flexural buckling

The objective of this paper is to develop a design method for stainless steel columns subjected to flexural buckling. In this method, the strength curve for every type of stainless steel in common use is generated by using the base strength curves and a slenderness conversion formula. A validated finite element model is employed to generate the strength curves for different types of stainless steels. According to the strain hardening exponent n in the Ramberg-Osgood material model, cluster analysis is conducted to separate these strength curves into two groups. In each group, one type of material parameter is selected as the base material and the corresponding strength curve is expressed in the form of a modified Perry formula that serves as the base strength curve. A slenderness conversion formula is derived for any two stainless steel columns with the same section type and boundary conditions, but with different types of stainless steel. Based on the base strength curves and the slenderness conversion formula, the strength curve for every type of stainless steel in common use can be generated. The predictions of the proposed method show a good agreement with the results of the finite element analysis.     

Lateral-torsional buckling of stainless steel I-beams in case of fire

This work presents a numerical study of the behaviour of stainless steel I-beams subjected to lateral-torsional buckling in case of fire and compares the obtained results with the beam design curves of Eurocode 3.New formulae for lateral-torsional buckling, that approximate better the real behaviour of stainless steel structural elements in case of fire are proposed. These new formulae were based on numerical simulations using the program SAFIR, which was modified to take into account the material properties of the stainless steel.     

圆钢管及钢管混凝土柱的抗火设计

分别对圆钢管、钢管混凝土、中空夹层钢管混凝土柱进行了抗火设计,并对结果进行比较分析。结果表明,在较高荷载比下柱的耐火极限不能满足实际要求,必须进行防火保护。在相同条件下,耐火极限从大到小排序为:圆钢管混凝土、中空夹层钢管混凝土、钢管柱。在一级耐火等级下,钢管混凝土柱和中空夹层钢管混凝土柱需要厚涂型钢结构防火涂料的厚度可比钢管柱分别少55%和18%以上。随着荷载比的减小或截面尺寸的增加,柱的耐火极限提高,需要的保护层厚度减小。对于钢管混凝土柱,若采用水泥砂浆保护层,其厚度是防火涂料的3倍及以上。     

Structural performance of cold-formed lean duplex stainless steel columns

The structural performance of cold-formed lean duplex stainless steel columns was investigated. A wide range of finite element analysis on square and rectangular hollow sections and other available data, with a total number of 259 specimens, were considered. An accurate finite element model has been created to simulate the pin-ended cold-formed lean duplex stainless steel columns. Extensive parametric study was carried out using the validated finite element model. The column strengths predicted from the parametric study together with the available data are compared with the design strengths calculated from various existing design rules for cold-formed stainless steel structures. It is shown that the existing design rules, except for the ASCE Specification as well as the stub column and full area approach, are conservative. Modifications are proposed for the AS/NZS Standard, EC3 Code, and direct strength method. Reliability analysis was performed to assess the existing and modified design rules. It is also shown that the modified design rules are able to provide a more accurate and reliable predictions for lean duplex stainless steel columns. In this study, it is suggested that the modified design rules in the AS/NZS Standard and the modified direct strength method to be used in designing cold-formed lean duplex stainless steel columns.     

Structural design of stainless steel concrete filled columns

This paper presents the behaviour and design of axially loaded concrete filled stainless steel circular and square hollow sections. The experimental investigation was conducted using different concrete cube strengths varied from 30 to 100 MPa. The column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with existing design methods for composite carbon steel sections — Eurocode 4 and ACI. It was found that existing design guidance for carbon steel may generally be safely applied to concrete filled stainless steel tubes, though it tends to be over-conservative. A continuous strength method is proposed and it is found to provide the most accurate and consistent prediction of the axial capacity of the composite concrete filled stainless steel hollow sections due largely to the more precise assessment of the contribution of the stainless steel tube to the composite resistance.     

Structural performance of cold-formed high strength stainless steel columns

This paper describes an accurate finite element model for the structural performance of cold-formed high strength stainless steel columns. The finite element analysis was conducted on duplex stainless steel columns having square and rectangular hollow sections. The columns were compressed between fixed ends at different column lengths. The effects of initial local and overall geometric imperfections have been taken into consideration in the finite element model. The material nonlinearity of the flat and corner portions of the high strength stainless steel sections were carefully incorporated in the model. The column strengths and failure modes as well as the load-shortening curves of the columns were obtained using the finite element model. Furthermore, the effect of residual stresses in the columns was studied. The nonlinear finite element model was verified against experimental results. An extensive parametric study was carried out using the verified finite element model to study the effects of cross-section geometries on the strength and behaviour of cold-formed high strength stainless steel columns. The column strengths predicted from the parametric study were compared with the design strengths calculated using the American Specification, Australian/New Zealand Standard and European Code for cold-formed stainless steel structures. The results of the parametric study showed that the design rules specified in the American, Australian/New Zealand and European specifications are generally conservative for cold-formed high strength stainless steel square and rectangular hollow section columns, but unconservative for some of the short columns.     

On the buckling of axially restrained steel columns in fire

This paper describes the behaviour of restrained steel columns in fire. It follows the introduction of extra load into the column through the axial restraint of the surrounding cooler structure and the consequential buckling. Key to this understanding is the post-failure behaviour and re-stabilisation of the column, which is discussed with reference to a finite element model and an analytical model. Through bi-directional control of the temperature, the finite element model allows the snap-back behaviour to be modelled in detail and the effects of varying slenderness and load ratio are investigated. The analytical model employs structural mechanics to describe the behaviour of a heated strut, and is capable of explaining both elastic and fully plastic post-buckling behaviour.Through this detailed explanation of what happens when a heated column buckles, the consequences for steel-framed building design are discussed. In particular, the need to provide robustness is highlighted, in order to ensure that alternative load paths are available once a column has buckled and re-stabilised. Without this robustness, the dynamic shedding of load onto surrounding structures may well spread failure from a fire’s origin and lead to progressive collapse.     

Thermal bowing on steel columns embedded on walls under fire conditions

The contact of steel columns with building walls is responsible for huge thermal gradients within its cross-section during fire. Current regulatory codes for fire design of steel members provide a formulation to assess the load-bearing capacity of these members assuming uniform temperature through the cross-section; however, this is not what happens in the major part of the cases in real structures where the columns are embedded on walls. The walls on one hand will provide a temperature reduction on the columns, which is somehow favourable in terms of its fire resistance, on the other hand the differential heating on the columns cross-section may lead to unfavourable stresses (bending moments) responsible for instability (thermal bowing). Considering that the structural behaviour of columns is strongly dependent of the second order effects this is an important phenomenon which may lead to a significant reduction on its fire resistance. This paper presents the results of a numerical study to assess the influence of the differential heating on the fire design of steel columns. New interaction axial force–bending moment diagrams for non-uniformly heated H steel columns are proposed.     

高温下钢管自密实混凝土柱抗火性能试验研究

进行了圆钢管自密实混凝土柱的高温抗火性能试验研究,主要测量钢管表面的温度场、构件轴向变形和跨中侧向位移随时间的变化关系;分析了各种因素对耐火极限和轴向变形、跨中侧向位移曲线的影响规律。     

Numerical verification of stainless steel overall buckling curves

The introduction of new material grades or fabrication methods in engineering structures always raises concerns about the validity of current design rules, especially those based on earlier material or member testing. One of the fundamental structural checks is the overall stability of beams and columns. In this paper, a series of virtual buckling tests is calculated and compared to the Eurocode buckling curves. The main focus is on the applicability of ferritic steels that generally have different stress–strain behaviour than austenitic or duplex grades.     

Heat transfer in concrete-filled carbon and stainless steel tubes exposed to fire

Nowadays, performance-based design methods are increasingly used for fire resistance assessment of structures. To implement these methods, it is paramount to determine the temperature development within a structural member exposed to fire as accurately and efficiently as possible. Numerical models are developed in this paper to simulate the temperature development in concrete-filled carbon and stainless steel tubes. It was found that the influence of the moisture content in concrete and the thermal contact conductance at the steel–concrete interface is significant. New models for thermal conductivity of concrete and thermal contact conductance at the interface are proposed in this paper. Comparisons of temperature development are made between numerical simulations and extensive experimental results. Improved agreement with test results is achieved when the proposed models are used in the heat transfer analysis.     

Axial restraint effects on the fire resistance of composite columns encasing I-section steel

This paper presents an experimental study of the axial restraint effect on fire resistance of four unprotected encased I-section composite columns. Axial restraints were applied to simulate thermal restraints from adjoining cool structures onto a heated composite column in a compartment. These real-sized 3.54 m long columns were subjected to concentric axial force at a load ratio of 0.7 at normal ambient temperature. Different degrees of axial restraint are investigated. An electric furnace was used to apply four-face heating condition on the columns for approximating a realistic fire scenario. All columns failed in flexural buckling mode. In the later part of the paper, finite element simulations were conducted to compare with test results. Numerical predictions of both temperature distribution and structural response during heating agree reasonably well with experimental data. Both test results and numerical analyses show that axial restraint significantly reduces the column fire resistance. Moreover, it was also observed that during heating all specimens underwent concrete spalling at mid-height, which noticeably decreased the fire resistance. Column critical times are also predicted according to Eurocode 4 Part 1.2, which are consistently shorter than the numerical predictions.     

Behaviour of short and slender concrete-filled stainless steel tubular columns

In this paper, a series of tests were carried out on short and slender concrete-filled stainless steel tubular columns to explore their performance under axial compression or combined actions of axial force and bending moment. Empty short steel hollow sections were also tested for comparison. The test results showed that the performance of the composite columns was quite good and have the potential to be used extensively as structural members. Comparisons of the test results were also made with several existing design methods for conventional concrete-filled carbon steel tubular columns as presented in Australian standard AS 5100 (2004), American code AISC (2005), Chinese code DBJ/T 13-51-2010 (2010), and Eurocode 4 (2004), which indicates that all the codes are somewhat conservative in predicting the load-carrying capacities of both short and slender columns.     

Behaviour and design of restrained steel column in fire, Part 3: Practical design method

This paper presents a practical design method for calculating the buckling and failure temperatures of restrained steel column under axial load or combined axial load and bending moment based on the results of extensive numerical parametric studies. Design equations for unrestrained columns in fire are adopted to calculate the buckling temperature of a restrained column by including the additional compression axial force due to restraint thermal elongation. The cross-section yield axial strength-plastic bending moment interaction curve is employed when calculating the failure temperature of restrained column. Results from the proposed method are compared with ABAQUS simulation for different cases. For the restrained column under axial load only, the buckling and failure temperatures calculated by the simplified method agree well with predictions by ABAQUS. For the restrained column under combined axial force and bending moment with realistic parameters, the buckling and failure temperatures predicted by the proposed method also agree well with ABAQUS predictions.     

Buckling of timber columns exposed to fire

A mathematical model for structural behavior of timber columns under fire has been proposed. The semi-analytical study has been carried out for evaluating the load-carrying capacity of timber columns exposed to fire. Particular emphasis has been given to critical buckling loads. For this purpose, a parametric study has been performed by which the influence of slenderness ratio, load level, and water content on critical buckling loads of timber columns have been investigated. The results of this preliminary study showed that the present semi-analytical method is conservative compared to the two simplified calculation methods offered by Eurocode 5 if the transfer of water is neglected, while, on the other hand, the results agree well for a water content of 12%. Moreover, for higher water contents, the present semi-analytical model is non-conservative compared to the Eurocode 5 methods.     

Capacity reduction and fire load factors for LRFD of steel columns exposed to fire

Capacity reduction and fire load factors corresponding to the load and resistance factor design (LRFD) format are developed for steel columns exposed to fire. A sample deterministic framework to determine fire and steel temperatures and the capacity of steel columns is adopted for this analysis to structure the methodology. A specific number of parameters that affect the structural response, including the fire load, ratio of floor area to the total area of the fire compartment, opening factor, thermal absorptivity of compartment boundaries, thickness, density and thermal conductivity of insulation, dead load, and live load are taken as random variables. Mechanical and sectional properties of steel (e.g., yield strength, cross-sectional area, etc.), are also considered to be random variables. The effect of active fire protection systems (e.g., sprinklers, smoke and heat detectors, fire brigade, etc.), in reducing the probability of occurrence of a severe fire is included. Given the choice of framework and based on detailed reliability analyses, it is shown that the capacity reduction and fire load factors should vary depending on the presence of active fire protection systems in a building.     

Residual stresses in cold-rolled stainless steel hollow sections

Stainless steel exhibits a pronounced response to cold-work and heat input. As a result, the behaviour of structural stainless steel sections, as influenced by strength, ductility and residual stress presence, is sensitive to the precise means by which the sections are produced. This paper explores the presence and influence of residual stresses in cold-rolled stainless steel box sections using experimental and numerical techniques. In previous studies, residual stress magnitudes have been inferred from surface strain measurements and an assumed through-thickness stress distribution. In the present study, through-thickness residual stresses in cold-rolled stainless steel box sections have been measured directly by means of X-ray diffraction and their effect on structural behaviour has been carefully assessed through detailed non-linear numerical modelling. Geometric imperfections, flat and corner material properties and the average compressive response of stainless steel box sections were also examined experimentally and the results have been fully reported. From the X-ray diffraction measurements, it was concluded that the influence of through-thickness (bending) residual stresses in cold-rolled stainless steel box sections could be effectively represented by a rectangular stress block distribution. The developed ABAQUS numerical models included features such as non-linear material stress-strain characteristics, initial geometric imperfections, residual stresses (membrane and bending) and enhanced strength corner properties. The residual stresses, together with the corresponding plastic strains, were included in the FE models by means of the SIGINI and HARDINI Fortran subroutines. Of the two residual stress components, the bending residual stresses were found to be larger in magnitude and of greater (often positive) influence on the structural behaviour of thin-walled cold-formed stainless steel sections.