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 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.     

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。     

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.     

Compression tests of cold-formed plain and dimpled steel columns

This paper presents compression tests of cold-formed plain and dimpled steel columns. A series of compression and tensile tests were conducted on plain and dimpled steel of different geometries. For each group of geometries the source of material for both the plain and dimpled steel columns was taken from a single coil. Within each group the sections were fabricated either by press-braking or cold-rolled forming. The buckling and ultimate strength of the columns was investigated. The change in strength of the dimpled columns resulting from the cold working associated with the dimpling process was also considered. This paper contains the results obtained when comparing the test strengths of short plain and dimpled steel columns using a compression test. In outlining the work the test setup and testing procedure will be described. Enhancements in buckling and ultimate strengths were observed in the dimpled steel columns-caused by the cold-work of the material during the dimpling process. The results showed that the buckling and ultimate strengths of dimpled steel columns were up to 33% and 26% greater than plain steel columns, respectively.     

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.     

Nonlinear analysis of concrete-filled square stainless steel stub columns under axial compression

Concrete-filled stainless steel tubes (CFSST) can be considered as a new and innovative kind of composite construction technique, and have the potential to be used extensively in civil engineering. This paper employs a nonlinear analysis of square CFSST stub columns under axial compression. A three-dimensional nonlinear finite element (FE) model is developed using ABAQUS, where nonlinear material behaviour, enhanced strength corner properties of steel, and initial geometric imperfections are included. Close agreement is achieved between the test and FE results in terms of load-deformation response and ultimate strength. In light of the numerical results, the behaviour of stainless steel composite columns is compared with that of carbon steel composite columns. A simple model is proposed to calculate the ultimate strength of square CFSST stub columns.     

Research on cold-formed steel columns

The paper summarises research on cold-formed steel columns performed by the author. Cold-formed steel members are either cold-rolled or brake-pressed into structural shapes. As a result, cold-formed steel open sections are usually singly-point- or non-symmetric. The most common types of singly-symmetric sections are channel and angle. The research focused on cold-formed steel open sections, such as plain and lipped channels, channels with simple and complex edge stiffeners as well as plain and lipped angles, and unequal angles. In addition, cold-formed steel built-up closed sections with intermediate stiffeners were investigated. Both experimental and numerical investigations into the strength and behaviour of cold-formed steel columns were conducted. The column strengths obtained from these investigations were compared with the design strengths obtained using various international standards for cold-formed steel structures. Furthermore, the behaviour and design of cold-formed steel lipped channel columns at elevated temperatures were also investigated. The paper also summarises the design recommendations for cold-formed steel columns.     

Experimental investigation of concrete-filled cold-formed high strength stainless steel tube columns

This paper presents an experimental investigation of concrete-filled cold-formed high strength stainless steel tube columns. The high strength stainless steel tubes had a yield stress and tensile strength up to 536 and 961 MPa, respectively. The behaviour of the columns was investigated using different concrete cylinder strengths varied from 40 to 80 MPa. A series of tests was performed to investigate the effects of the shape of the stainless steel tube, plate thickness and concrete strength on the behaviour and strength of concrete-filled high strength stainless steel tube columns. The high strength stainless steel tubes were cold-rolled into square and rectangular hollow sections. The depth-to-plate thickness ratio of the tube sections varied from 25.7 for compact sections to 55.8 for relatively slender sections. The columns had different lengths so the length-to-depth ratio generally remained at a constant value of 3. The concrete-filled high strength stainless steel tube specimens were subjected to uniform axial compression. The column strengths, load-axial strain relationships and failure modes of the columns were presented. The test strengths were compared with the design strengths calculated using the American specifications and Australian/New Zealand standards that consider the effect of local buckling using an effective width concept in the calculation of the stainless steel tube column strengths. Based on the test results, design recommendations were proposed for concrete-filled high strength stainless steel tube columns.     

Tests of cold-formed high strength stainless steel compression members

The paper describes a test program on cold-formed high strength stainless steel compression members. The duplex stainless steel having the yield stress and tensile strength up to 750 and 850 MPa, respectively, was investigated. The material properties of the test specimens were obtained from tensile coupon and stub column tests. The test specimens were cold-rolled into square and rectangular hollow sections. The specimens were compressed between fixed ends at different column lengths. The initial overall geometric imperfections of the column specimens were measured. The strength and behaviour of cold-formed high strength stainless steel columns were investigated. The test strengths were compared with the design strengths predicted using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures. Generally, it is shown that the design strengths predicted by the three specifications are conservative for the cold-formed high strength stainless steel columns. In addition, reliability analysis was performed to evaluate the current design rules.     

Universal design charts for insulation of steel members in fire

Charts for insulation design of steel members in fire are usually provided by the manufacturers of the specified fire protection materials. These design charts may not be readily available, particularly for fire protection materials which may not have proprietary rights. This paper describes the development of a set of universal charts for the insulation design of steel members in fire for a wide range of fire protection materials of known physical and thermal properties. The main advantage of using these charts for insulation design is that they apply to all types of fire protection material without referring to the manufacturers’ design charts. In addition, high density fire protection materials, such as concrete, for steel members have also been included in these universal design charts.     

Elevated temperature material properties of stainless steel alloys

Appropriate assessment of the fire resistance of structures depends largely on the ability to accurately predict the material response at elevated temperature. The material characteristics of stainless steel differ from those of carbon steel due to the high alloy content. These differences have been explored in some detail at room temperature, whilst those at elevated temperature have been less closely scrutinised. This paper presents an overview and reappraisal of previous pertinent research, together with an evaluation of existing elevated temperature stainless steel stress-strain test data and previously proposed material models. On the basis of examination of all available test data, much of which have been recently generated, revised strength and stiffness reduction factors at elevated temperatures for a range of grades of stainless steel have been proposed, including four grades not previously covered by existing structural fire design guidance. A total of eight sets of strength reduction factors are currently provided for different grades of stainless steel in EN 1993-1-2 and the Euro Inox/SCI Design Manual for Structural Stainless Steel, compared to a single set for carbon steel. A number of sets of reduction factors is appropriate for stainless steel since the elevated temperature properties can vary markedly between different grades, but this has to be justified with sufficient test data and balanced against ease of design — it has been proposed herein that the eight sets of reduction factors be rationalised on the basis of grouping grades that exhibit similar elevated temperature properties. In addition to more accurate prediction of discrete features of the elevated temperature material stress-strain response of stainless steel (i.e. strength and stiffness reduction factors), a material model for the continuous prediction of the stress-strain response by means of a modified compound Ramberg-Osgood formulation has also been proposed. The proposed model is less complex than the current provisions of EN 1993-1-2, more accurate when compared to test results, and the model parameters have a clear physical significance.