Columns In Fire Research Articles

Buckling and Critical Temperature Prediction Formulas of Restrained H-Section Steel Columns in Fire

Buckling and Critical Temperature Prediction Formulas of Restrained H-Section Steel Columns in Fire

GMNIA with beam elements and strain limits based fire design approach for steel beams and beam–columns

The use of advanced analysis techniques harnessing the capabilities of the currently available computational resources can lead to considerably more accurate fire design of steel structures relative to the currently adopted simple fire design methods in practice. In this paper, a structural steel fire design method by Geometrically and Materially Nonlinear Analysis with Imperfections (GMNIA) and using strain limits is put forward for the fire design of steel beams and beam–columns. In the proposed fire design method, the GMNIA of a steel member or structure is performed using computationally efficient beam finite elements and strain limits are adopted to account for the detrimental effects of local buckling on the ultimate capacities. To verify the accuracy of the proposed method, extensive numerical parametric studies are conducted through shell finite element modelling. The results indicate that the proposed approach provides accurate and safe ultimate resistance and limit temperature estimations for steel beams and beam–columns in fire, with a considerably higher level of accuracy and reliability relative to the provisions of the European structural steel fire design standard EN 1993-1-2.

Cross-sectional behaviour of QN1803 high-strength stainless steel I-section stub columns in fire

QN1803 is a novel high-strength stainless steel with low nickel. Compared to traditional stainless steel, QN1803 high-strength stainless steel has higher yield strength and lower cost without compromising anti-corrosion properties, thus leading to a wide application prospect. This paper focuses on the cross-sectional behaviour of QN1803 high-strength stainless steel I-section stub columns in fire. A total of eight stub column tests were finished, of which six were elevated temperature tests and two were room temperature control tests. Room temperature and elevated temperature tensile coupon tests were conducted to obtain the material properties of the QN1803 high-strength stainless steel. The design methods of European fire design standard EN 1993-1-2 and the latest fire design methods proposed by Xing et al., which will be incorporated into the upcoming version of EN 1993-1-2, were evaluated in terms of accuracy and safety for the fire design of QN1803 high-strength stainless steel I-section stub columns. It was found that most ultimate cross-sectional resistances of QN1803 high-strength stainless steel I-section stub columns predicted by the design rules of European fire design standard EN 1993-1-2 were unsafe, while the ultimate cross-sectional resistances predicted by the design methods of Xing et al. were safe and accurate for the tested QN1803 high-strength stainless steel I-section stub columns.

Stability and design of stainless steel hollow section columns at elevated temperatures

This paper investigates the stability and design of stainless steel circular, elliptical, square and rectangular hollow section (CHS, EHS, SHS and RHS) columns at elevated temperatures. Nonlinear shell finite element models are employed to conduct comprehensive parametric studies whereby extensive benchmark structural performance data on the behaviour and resistance of stainless steel hollow section columns at elevated temperatures is generated. In total, 26,760 cold-formed and hot-rolled austenitic, duplex and ferritic stainless steel CHS, EHS, SHS and RHS columns at elevated temperatures are taken into account, considering various member slendernesses, cross-section geometries, cross-section slendernesses and elevated temperature levels. New flexural buckling design rules are put forward for stainless steel hollow section columns in fire, which consistently considers the elevated temperature strength at 2% total strain f2,θ as the reference material strength for all cross-sections classes. The accuracy, safety and reliability of the proposed new design rules are assessed for a wide range of cases. Comparisons are also made against the results obtained through the column fire design rules of the European structural steel fire design standard EN 1993-1-2 [1]. It is demonstrated that the proposed new design rules furnish more accurate, safe-sided and reliable flexural buckling resistance predictions for stainless steel hollow section columns at elevated temperatures relative to the column fire design provisions of EN 1993-1-2 [1].

Experimental and Numerical Study on Progressive Collapse of the Midcolumn in a Glulam Timber Frame Exposed to Fire

This paper presents experimental and numerical investigations on the progressive collapse behavior of a two-story glulam frame when the middle column is exposed to ISO834 standard fire. The stress characteristics, temperature distributions, and deformation relationship curves of the bottom middle column after a fire are obtained. The collapse resistance performance and the mechanism of the glulam structure under local fire are studied in combination with the failure mode of the frame. The experimental results show that the failure time of the middle column in fire is 16.8 min, and the time to determine the progressive collapse of the structure is 17.17 min when the rotation angle exceeds the limit. The structure can remain stable for some time after the failure of the fire column, but as the fire continues to spread, the structure will collapse further. In addition, a temperature-field analysis model with the grid element type of DC3D8 and a structural field analysis model with the grid element type of C3D8R was established by using ABAQUS software under the experimental conditions. The numerical simulation is carried out by means of thermal–mechanical sequential coupling. The numerical simulation results show that the temperature of the beam end rose to more than 300 °C in about 15 min although it was not directly exposed to fire. The temperature of the whole connector at the pillar foot in the bottom column reached more than 700 °C when the fire time reached 60 min. The numerical simulation results agree well with the experimental results. The analysis results show that, in the failure process of the fire column, the junction of beam ends begins to produce stretching force to form a catenary effect. As the fire time increases, the catenary effect eventually fails. The stretching force at the beam end rapidly decreases and withdraws from the work, causing the progressive collapse of the frame.

Buckling and critical temperatures of restrained H-section steel columns in fire considering dynamic effect

In this paper, a parametric analysis on the buckling and critical temperatures of restrained H-section steel columns in fire, considering the dynamic effect, is presented. The developed finite element model of steel columns was verified by test results. Then, the buckling and critical temperatures of steel columns, obtained by static and explicit dynamic analyses, were compared. Moreover, the effects of rotational and axial restraint stiffness ratios, as well as load ratios, were studied. It was found that as the rotational restraint stiffness ratio increased, the buckling and critical temperatures both rose, and the rotational restraint stiffness ratio affected the critical temperature more than the buckling temperature. Besides, when the axial restraint stiffness ratio and load ratio rose, the buckling and critical temperatures dropped significantly. Finally, by combining the findings of the research and existing calculation formulas in literatures, new simplified formulas for buckling and critical temperatures were proposed, respectively. The prediction results of the proposed calculation formulas agreed well with the parametric analysis results. The proposed formulas were further verified by test data. According to the comparative results, the proposed formulas could well predict the buckling and critical temperatures of steel columns in fire.

Nonlinear stability and design of prestressed stayed columns in fire considering non-uniform temperature distribution

Prestressed stayed columns are vulnerable to fire due to material weakening. The fire behaviour of the stayed columns is investigated with non-uniform temperature distributions considered using steady state analysis within ABAQUS. This study aims to reveal the load-carrying capacity reduction of stayed columns in fire and propose a design method. Results show that ultimate loads decrease with temperature increasing, and the stayed columns approximate failure at the temperatures higher than 600°C. The failure deflected mode maintains similar to the dominant linear buckling mode for the uniform temperature distribution, and the sensitivity of ultimate loads to prestressing levels becomes less significant at higher temperatures. Moreover, compared to buckling and ultimate behaviour, initial post-buckling behaviour shows more significant sensitivity to temperature distributions. The non-uniform distribution can lead to asymmetric deflected mode due to stiffness re-distribution from the material weakening at elevated temperatures, and the post-buckling behaviour of the cases with rotational symmetric mode dominance is sensitive to prestressing levels. Finally, a fire design method is proposed based on the ultimate load reduction in fire.

Resistance and behaviour of carbon steel and stainless steel columns with axial and rotational end‐restraints in fire

AbstractDue to the presence of neighbouring structural elements, carbon steel and stainless steel columns are typically axially and/or rotationally restrained at their ends in fire. Despite the significant influence of the axial and rotational end‐restraints on the behaviour, the fire design rules for steel columns provided in the European structural steel fire design standard EN 1993‐1‐2 were originally developed considering the structural response of axially unrestrained steel columns with pin‐ended support conditions. Thus, EN 1993‐1‐2 may provide inaccurate ultimate resistance predictions for axially and/or rotationally restrained steel and stainless steel columns in fire. Considering this, this paper investigates the behaviour and design of carbon steel and stainless steel columns with axial and/or rotational end‐restraints at elevated temperatures. A number of carbon steel and stainless steel I‐section columns are considered, taking into account different axial and rotational end‐restraint stiffnesses, room temperature loading ratios, member and cross‐section slendernesses. A new design method for the fire design of axially and/or rotationally restrained steel and stainless steel columns is put forward. The accuracy of the proposed new design method is extensively verified against the results from nonlinear shell finite element modelling.

Numerical Analysis of Restrained Continuous Steel Columns under Standard Fire

The steel column performance in realistic structures during a fire has yet to be fully understood because existing research emphasizes single-story performance, thereby disregarding the influence of continuous steel columns in multi-story configurations devoid of fire. This paper presents a numerical study to comprehend the overall structural fire performance of continuous steel columns, considering the effect of loading ratios, restraint ratios, column continuity, and single-sided lateral moments. An advanced numerical model was initially developed using ABAQUS and validated against experimental tests. The validated numerical model was subsequently employed to investigate the effects of several parameters, including axial restraint ratios (α = 0.05–0.35) and axial load ratios (n = 0.3–0.8). The study findings indicated that the restraint ratios within the designed range have a slightly beneficial impact on the fire resistance of continuous steel columns. The column continuity did not exert a significant impact on the performance of steel columns in fire. Additionally, the comparison showed that the current design approach in EN 1993-1-2 was conservative for predicting the limiting temperature of internal and edge columns.

Testing of high strength aluminium alloy SHS and RHS stub columns in fire

With the aim of investigating the local buckling behaviour of high strength aluminium alloy columns in fire, six room-temperature tests and eighteen steady-state tests at elevated temperatures on 7A04-T6 aluminium alloy square hollow section (SHS) and rectangular hollow section (RHS) stub columns were conducted in this study. Initial geometric imperfections of each specimen were first measured by the 3D laser scanner. Then, the test setup, procedure and instrumentation are fully reported. Test results, including failure modes, ultimate resistances and load-end shortening curves were obtained and utilised to evaluate the accuracy of current design methods for aluminium alloy stub columns in fire. The evaluation indicated that European and Chinese codes provided rather conservative predictions at 400 °C, while most of the resistances predicted by the American specification and methods proposed by Maljaars et al. were on the unsafe side. New design approaches proposed by the authors obtained more accurate and less scattered resistance predictions. Furthermore, the finite element (FE) models were also established and achieved good agreements with the test results.

Computationally Efficient Method for Steel Column Buckling in Fire

The stability of axially loaded steel columns with compact rectangular hollow sections at elevated temperatures is studied in this paper. The current Eurocode model for checking the buckling resistance of columns in fire was developed on a similar basis to that for ambient conditions. Due to the effect of the complex non-linear behaviour of steel in fire, the standard design model is not always fully appropriate, and certain parameter ranges may give unsafe results. In this work, an analytical method to determine the buckling resistance of steel columns at elevated temperatures is proposed, accounting for variable non-linear stiffness properties which have significant effects on the flexural buckling resistance of steel columns in fire. A finite element model was developed, and an extensive numerical study was performed to explore the effects of different parameters on the behaviours of steel columns at elevated temperatures. The proposed method is validated by comparing the performance with the results of the numerical model. Its improved accuracy with respect to the current Eurocode method is verified. The advantage of the new technique is its computational efficiency, which is valuable in reliability evaluations or data-based design procedures demanding numerous calculation cycles. The potential of the method for probability-based analysis is supported by the format, which enables us to explicitly handle the uncertainties of essential parameters. The proposed framework is suitable for extension to incorporate different material models and section types.

Experimental investigation of austenitic stainless steel I-section stub columns at elevated temperatures

Due to the absence of fundamental fire test data on the cross-section behaviour of stainless steel columns, an experimental investigation of stainless steel stub columns has been conducted at elevated temperatures to fill this gap. A total of six austenitic stainless steel welded I-section stub columns were isothermally tested at three different elevated temperature levels. Two stub columns were tested at room temperature as the reference group. Complementary material tests and local geometric imperfection measurements were conducted and presented herein. On the basis of the obtained experimental results, the accuracy and reliability of the design methods of the existing European fire design standard (EN 1993-1-2) and the design method of Xing et al. for cross-sectional behaviour of stainless steel columns in fire, which has been incorporated into the upcoming version of EN 1993-1-2, are evaluated. It is concluded that, compared to the existing design rules of EN 1993-1-2, the design method of Xing et al. can provide safe and reliable cross-sectional resistance predictions for the tested austenitic stainless steel I-section stub columns.

A voxels-based Rayleigh-Ritz method for the post-buckling elasto-plastic analysis of restrained steel columns in fire

The paper presents a novel numerical strategy to analyse steel columns in fire, based on the concept of voxel. The material obeys to an elasto-plastic constitutive law that degrades with temperature. The member's internal strain energy is computed through an original methodology: the column is considered to be a grid of small and perfectly packed parallelepipeds, denominated as voxels. Any quantity is constant along any voxel and equal to its value at the voxel's geometric centre. The coordinates of each voxel's geometric centre are inferred by the voxel's ordering in the global grid, and its constitutive law depends solely on the strains and on the temperature at the voxel's geometrical centre, and is independent of the neighbouring voxels. The total internal strain energy is given by the sum of all internal energies along the grid and depends solely on the temperature and of the degrees of freedom. The strategy is able to deal with complex behaviours, such as snap-back and spread of plasticity, and requires a smaller number of degrees of freedom to a given accuracy, when compared to alternative methodologies. It enables new and transformative insights on the behaviour of columns in fire and can be incorporated in other methodologies.

Buckling and post-buckling analysis of restrained non-uniform columns in fire

This paper investigates the buckling and post-buckling behavior of restrained non-uniform columns in fire where the temperature surrounding the column varies along the longitudinal direction. The material properties of the column are considered to be temperature-dependent (TD). First, according to the convective–radiative condition on the surfaces of the column, a dimensionally reduced nonlinear heat transfer equation is proposed and is solved by the differential quadrature (DQ) method. Then, considering the geometric imperfection, the nonlinear governing equation is derived based on the von-Kármán theory when the column is subjected to an axial compression load at the column end. An improved DQ method is applied to the buckling and post-buckling analysis of the non-uniform column under the fire. Considering the yield strength of material at elevated temperature, the ultimate load of the column is obtained. A tapered H-section steel column surrounded by localized fire is taken as an example. The present results are verified by comparing with those in the literature and those obtained from the finite element (FE) method. The parameter analysis reveals the temperature distribution, the buckling and post-buckling behavior of the tapered column in the localized fire.

Stainless steel I-section columns fire design considering the influence of loading history

The flexural buckling resistance of welded I-section stainless steel columns in fire considering the influence of the loading history is investigated in this paper. Finite element procedure used to replicate the response of stainless steel columns in fire has been developed and validated against existing experimental tests. Developed finite element procedure replicates real fire situation where column is firstly loaded and then heated until failure. Poor accuracy of the flexural buckling resistance according to the current Eurocode 3 design rules and recently proposed design rules was observed through an extensive parametric analysis, thus indicating that new contributing parameters need to be included in the design expressions. Two main contributing parameters were analyzed, ratio of the flange and web area Af/Aw and ratio of the flange and web thickness tf/tw. New flexural buckling rules were proposed and calibrated using the results from 3000 numerical simulations, according to the developed finite element procedure, considering a range of elevated temperature levels of practical importance, stainless steel grades, cross-sectional proportions and column slendernesses. The accuracy and reliability of new proposal were assessed against the results obtained through finite element procedure. Compared to the current Eurocode 3 design rules and recently proposed design rules, new proposal provides significantly more accurate prediction of flexural buckling resistance of stainless steel columns in fire.

Design of Steel Columns in Fire using Second‐order Inelastic Analysis with Strain Limits

AbstractIn this paper, a structural steel fire design approach using second‐order inelastic analysis with strain limits is proposed and applied to the design of steel columns in fire. The proposed design approach employs beam finite elements to conduct a second‐order inelastic analysis. Within the proposed method, strain limits are used to account for the effects of local buckling; while global instability, the loss of strength and stiffness and thermal effects are fully considered through second‐order inelastic analysis carried out using beam finite elements. The proposed method is assessed against validated shell FE results. It is observed that the proposed design approach provides consistently accurate and safe capacity predictions for steel columns in fire.

Numerical Investigation on the Thermal Behaviour of Steel‐reinforced CFST Columns in Fire

AbstractIn previous research, the authors highlighted the limited fire resistance of slender concrete‐filled steel tubular (CFST) columns. A possible solution for enhancing the fire resistance of such columns consists of embedding an open steel profile within the concrete infill of the CFST section, generating the so‐called steel‐reinforced concrete‐filled steel tubular (SR‐CFST) section, where the inner steel profile results thermally protected by the surrounding concrete, thus delaying its degradation at elevated temperatures. This strategy may result beneficial in the fire situation as compared to other sectional configurations where the steel parts are directly exposed to the heat source.In this paper, a two‐dimensional finite element model will be developed for studying the thermal behaviour of SR‐CFST columns and validated by comparing the temperature distribution results with experimental tests available in the literature. The numerical model will be subsequently used to carry out parametric studies, in order to analyse the influence of the different parameters, such as the cross‐section shape, outer tube thickness or the inner steel profile sectional dimensions over the thermal response of these composite sections when subjected to fire.Finally, a simplified temperature distribution proposal will be developed, which may help practitioners for a fast evaluation of the cross‐sectional temperature field of SR‐CFST columns at a given fire resistance period by assigning an equivalent temperature to each component of the composite section (hollow steel tube, concrete encasement and inner steel profile). This simplified temperature proposal may result helpful when evaluating the sectional capacity of SR‐CFST columns in fire, by using a single strength and stiffness value for each component of the composite cross‐section corresponding to its temperature.

Simple Structural Models to Compute the Fire Resistance of Restrained Steel Columns using Energy Principles

AbstractA simple structural model to represent the behaviour of restrained steel columns in fire is presented. The heat transfer and the mechanical problems are split and the mechanical one is analysed in the temperature's domain, in order to allow the application of any appropriate fire curve. The structural model is composed by two rigid rods connected by plastic hinges. These respect the bending moments – axial forces interaction and degrade their resistance with heating. At the model's top cross section an axial spring is inserted, to represent the restraining of the surrounding frame to the column's axial elongation, and an axial load is applied in order to model the serviceability loading applied prior to the fire event. In the worst‐case scenario, fire occurs only in the neighbourhood of the analysed column while the surrounding frame remains unaffected. Therefore, both the serviceability load and the restraining spring stiffness remain constant throughout the fire event. Longitudinal temperatures gradients are accounted for. Both pinned‐pinned and clamped‐clamped boundary conditions are considered, to assess the influence of the column's statical indeterminacy on its fire resistance. Equilibrium is obtained by means of energy balance: when the temperature rises, the variation of the work produced by the serviceability load equals the variation of the internal strain energy stored at the restraining spring plus the variation of the energy dissipated at the plastic hinges. A set of illustrative examples closes the work.

Lateral impact behaviour of square CFST columns under fire condition

This paper reports a numerical investigation into the lateral impact behaviour of concrete-filled steel tubular (CFST) columns in fire. A coupled finite element (FE) model of square CFST columns under fire condition subjected to lateral impact is developed and verified. The full-range analysis on the impact behaviour of CFST columns under fire condition is carried out, namely the impact process, the development and distribution of the internal forces, the damage evolution, and the energy absorption. The impact responses of columns under different fire damage are also compared in detail. It shows that increasing the fire time causes a significant reduction in the impact resistance and energy absorption capacity of CFST columns that have the smaller plateau impact force and dynamic bending moment but larger mid-span deflection and impact duration. Various parameters affecting the impact responses of CFST columns under fire condition are further analyzed, and the simplified methods are finally proposed to evaluate the impact responses of CFST columns under fire condition, i.e., the dynamic flexural capacity, the maximum mid-span deflection and plateau impact force.

Numerical investigation of the behavior of HSC columns in fire

Numerical investigation of the behavior of HSC columns in fire