Columns In Fire Research Articles

Crestbond shear connector for load transfer on concrete filled composite columns in fire

ABSTRACT: This paper presents a numerical study of the Crestbond shear connector, characterized by a steel plate with regular cuttings, when used as a load transfer element in concrete filled composite columns in fire. The developed numerical model was calibrated with experimental results of composite columns in fire and later the load transfer devices were inserted. Numerical analyzes were performed with the software Abaqus and comprised the variation of the composite column diameter and of the loading levels, as well as the comparison with the results obtained when is used a through steel plate without cuttings (Shear Flat) as a load transfer device. With the analyzes performed, it was observed that the Crestbond shear connector and the Shear Flat present very similar thermomechanical performance in relation to the load transfer capacity. Thus, the Crestbond shear connector has the potential to be applied alternatively to the Shear Flat as a load transfer device in concrete filled composite columns, with the advantage of the possibility of associate use of longitudinal and manly transverse reinforcement.

Simplified design procedures for the structural analysis of reinforced concrete columns in fire

Simple analytical procedures are described to predict the structural behavior (axial displacements and failure) of reinforced concrete (RC) columns exposed to fire. The analyses assume a concentric load (no initial eccentricity) or an equivalent axial load to take into account first and second order effects. The main objective is to provide reliable and straightforward tools for a preliminary assessment of the structural integrity of RC columns. The procedures compute the total displacements and the failure (time and type) of RC columns with different concrete strengths (normal to ultra-high performance), shapes, fire incidences, load levels and initial eccentricities. The results of 24 experimental tests from different authors are predicted and compared with the analytical solution. The displacements are estimated by the introduction of EN 1992-1-2 (2004) stress–strain curves and a simplified bilinear constitutive law that takes into account the Load Induced Thermal Strain (LITS) as a reciprocal to the modulus of elasticity. In general, a good agreement with the experimental time vs. displacement curves was obtained. Failure is predicted with the adoption of the nominal stiffness method (EN 1992-1-2, 2004) to take into account the second order effects. Sectional analyses (axial-bending equilibrium) and simplified analytical procedures assuming an equivalent axial load replacing the axial-bending force (force equilibrium) are carried out. The results of both analyses were very close, indicating the reliability of the simplified procedure. The prediction of the fire resistance time demonstrated a good approximation, on the safe side, with the experimental data. The results are compared with the EN 1992-1-2 (2004) prescriptive formula.

Behaviour and design of duplex stainless steel CHS stub columns at elevated temperature

AbstractThis paper investigates the behaviour and design of duplex stainless steel stub columns with a circular hollow cross‐section (CHS) at elevated temperature. A numerical model is developed to supplement the limited number of test results available in the literature. Following validation, the numerical approach is employed to gain an understanding of the critical behavioural characteristics which have not previously been studied. In addition, the paper considers and extends the continuous strength method (CSM) to include duplex stainless steel for CHS stub columns in fire. This approach employs a base curve linking the cross‐sectional resistance to the deformation capacity and is implemented herein using an elastic, linear hardening material model. The cross‐sectional resistances obtained from the CSM are compared with those from the numerical analysis, as well as with the standardised procedures in the European, American and Australia/New Zealand design standards. It is demonstrated that the proposed design method can lead to more accurate and less scattered strength predictions compared with current design codes.

Structural Behaviour and Fire Design of Duplex and Ferritic Stainless Steel CHS Stub Columns

This paper investigates the structural behaviour and design of duplex and ferritic stainless steel stub columns with a circular hollow cross-section (CHS) at elevated temperature. A numerical model is developed to supplement the limited test results on stainless steel CHS stub columns in the literature. Following validation, the numerical approach is employed to gain an understanding of the critical behavioural characteristics which have not previously been studied. In addition, the paper considers and extends the continuous strength method (CSM) to include duplex and ferritic stainless steel for CHS stub columns in fire. The CSM employs a base curve linking the cross-section resistance to its deformation capacity and implements an elastic, linear hardening material model. The cross-sectional resistances obtained from the proposed CSM are compared with those from the numerical analysis, as well as with the standardised procedures in the European, American and Australia/New Zealand design standards. It is demonstrated that CSM can lead to more accurate and less scattered strength predictions than current design codes.

Fire testing of austenitic stainless steel I-section beam–columns

With the increasing use of stainless steel elements in construction, the need for comprehensive rules to enable their efficient structural design is clear. To date, the fire behaviour of stainless steel I-section beam–columns has been the subject of relatively little research. In particular, there is an absence of experimental data. To address this gap in knowledge, full-scale anisothermal fire tests on six grade 1.4301 austenitic stainless steel I-section beam–columns have been carried out; the test procedure and results are reported herein. The test specimens were subjected to eccentric axial compression with two eccentricity values so as to achieve different combinations of axial compression and uniform minor axis bending. Complementary initial local and global geometric imperfection measurements, room temperature tensile coupon tests and room temperature beam–column tests were also carried out. Based on the obtained experimental results, together with additional numerical results from a previous study, the existing design rules in the European structural steel fire design standard EN 1993-1-2 and the new design method of Kucukler et al. (2021) for stainless steel beam–columns in fire, which will be incorporated into the next version of EN 1993-1-2, are assessed.

Vedic Pravargya, a Thermodynamic Process, That Boosts Immunity, Reduces Pollution, and Mitigate COVID-19 Like Viruses

Pravargya is Vedic and scientific process that generates very high Sun surface like temperatures in the range of 5,000 K to 9,225 K by producing a fireball (fire column) for shorter periods (1 to 3 seconds) that can produce UV (ultraviolet) rays to mitigate viruses/bacteria.

Effect of rotational restraint conditions on performance of steel columns in fire

This paper presents a parametric analysis about the effect of rotational restraint conditions on the dynamic performance of steel columns in fire. A fine finite element model of restrained steel columns, which adopts an explicit dynamic solver, was developed and validated by test data in literatures. Nine values of rotational restraint stiffness ratio (βR) from 0.0001 to 10.0 were considered. Other considered parameters include the axial restraint stiffness ratio, load ratio and column slenderness ratio. Based on the results, a quantitative criterion of identifying dynamic and quasi-static failures of steel columns in fire was proposed. It is found that βR has a significant effect on the performance of steel columns in fire. With the increase of βR, three failure scenarios about the failure types of steel columns in fire may appear, namely, all the failures are dynamic failures (Scenario I), failures turn from dynamic failure into quasi-static failure (Scenario II), and all the failures are quasi-static failures (Scenario III). In failure scenario I, there exists a βR with which the column shows the minimal dynamic effects. Besides, when βR reaches a critical value (namely βR,cr), the effect of further increasing the value of βR on the response of steel columns becomes slight for all the above scenarios. βR,cr is affected by the axial restraint stiffness ratio, load ratio and column slenderness ratio, and is generally less than 1.5. In fire-resistant design, the steel columns with a βR greater than βR,cr can be considered as rotationally fixed at both ends. Moreover, with the increase of βR, the buckling and critical temperatures of steel columns both increase, and the increase in critical temperature is significantly larger than that in buckling temperature.

Simple analytical models for restrained steel columns in fire

In structural engineering, two distinct types of computational strategies can be used to analyse a frame under the action of forces. The first type applies to any general structure and incorporates all involved complexities in the mathematical model. It aims to represent as accurately as possible all involved phenomena and leads usually to very complex numerical models, such as the Finite Elements ones. In contradistinction to this, the alternative strategies adopt simple but representative mathematical models that focus on a specific phenomenon. Despite their limited range of application, simple mathematical models enable an easier analysis of the key phenomena without forfeiting accuracy. They also provide a good depiction of the true physical response. Within this latter context, the present work presents two simplified structural models for the analysis of restrained elasto-plastic steel columns under fire conditions: i) an adapted Shanley model and ii) a two rigid rods frame joined by a plastic hinge. Only uniform heating is considered, as reference, and all nonlinear properties are taken into account, namely the degradation of the steel’s properties with heating, the nonlinear constitutive behaviour and the instability effects arising from the restraining of the thermal elogation. These two models enable the computation of the critical temperature, which defines the column’s strength in case of fire, with minimum effort and within an acceptable accuracy.

Structural fire design of SHS, RHS and CHS high strength steel columns

Despite substantial progress in recent years to improve the design guidance for high strength steel (HSS) structural elements, this has mainly been for ambient conditions with their fire response still in need of further research. Accordingly, this paper reports on an investigation into the structural performance of unprotected HSS hollow section columns in fire. Finite element models of columns made from square, circular and rectangular hollow sections are developed and are validated against test data at ambient and elevated temperature. The validated models are employed to perform parametric studies to assess the influence of a range of variables such as the grades of HSS, levels of temperature exposure and cross-sectional geometry. The structural fire design resistance method for a column given in the Eurocode is assessed based on the FE results. Consequently, new buckling curves are proposed, which provide a more accurate prediction of the real capacity and reliability analysis is also performed on the new proposed design formulations.

Numerical Modeling and Design of CFRP-Strengthened Short Steel Tubular Columns in Fire

AbstractThis paper presents the details of a numerical study aimed at investigating the fire performance of carbon-fiber-reinforced polymer (CFRP)-strengthened short square hollow section (SHS) ste...

Effects of nonlinear elevated temperature stress-strain characteristics on the global buckling capacities of cold-formed steel columns

Losses caused by the recent fire incidents are demanding proper fire safety regulations and fire protection measures around the globe while construction cost and call for environmental protection are increasing. The expansion of cold-formed steel (CFS) construction to mid-rise buildings in recent times has therefore identified the need for the accurate prediction of the load bearing capacities of CFS columns in fire. Despite several useful studies on the compression capacities of CFS columns at elevated temperatures, effects of nonlinear elevated temperature stress-strain characteristics on the global buckling capacities of CFS columns have not been investigated yet. In this research, the effects of nonlinearity between proportional limit stress and yield strength, strain hardening between yield and ultimate strengths and varying yield strength to Young's modulus ratio on the capacities of pinned and fixed ended columns subject to flexural-torsional and flexural buckling were investigated using a detailed numerical parametric study involving 1320 finite element analysis results. The accuracy of current CFS design standards, such as AS/NZS 4600, AS 4100 and EC3 Part 1–2, in predicting the elevated temperature global buckling capacities was investigated, and new design rules were proposed for Australian and European standards.

Testing of stainless steel I-section columns in fire

Although there have been significant developments in the testing, simulation and design of stainless steel structural elements at room temperature, the structural response of stainless steel members in fire has received significantly less attention. In particular, full-scale fire tests on stainless steel I-section members, which are becoming increasingly widely used in structural engineering applications to meet growing load-carrying capacity demands, are currently scarce. In this paper, the results of eight full-scale anisothermal fire tests on grade 1.4301 laser-welded austenitic stainless steel I-section columns are reported. Complementary initial local and global geometric imperfection measurements, room temperature tensile coupon tests and room temperature column buckling tests are also described. On the basis of the findings from the fire experiments, the accuracy and safety of the European fire design standard EN 1993-1-2 and the recent design recommendations of Kucukler et al. [1] for stainless steel columns in fire, which will be incorporated into the upcoming version of EN 1993-1-2, are assessed. It is observed that, relative to the existing column fire design rules in EN 1993-1-2, the design method of Kucukler et al. [1] provides more reliable ultimate strength predictions for austenitic stainless steel I-section columns in fire.

ISO 834 standard fire test and mechanism analysis of square tubed-reinforced-concrete columns

The tubed-reinforced-concrete (TRC) column is an innovative steel-concrete composite column and its steel tube is terminated at beam-to-column connections to mainly work as hoop reinforcements without sustaining axial load directly. Fire performance of TRC columns differs from that of concrete-filled steel tubular (CFST) columns since the axial deformation behaviour of the TRC columns would mainly depend on the inner reinforced concrete and local buckling of steel tube is minimised. However, no research has been reported on the behaviour of square TRC columns under fire exposure. Five slender square TRC columns subjected to standard fire and axial loading were tested in this study and the effects of load ratio and load eccentricity were investigated. Failure mode of the test specimens was dominated by global flexural buckling, whereas tube local buckling was also observed. The experimental results show that load ratio has a significant influence on the fire resistance of test specimens while the influence of load eccentricity is marginal. A sequentially-coupled thermo-mechanical finite element analysis (FEA) model was developed using ABAQUS. This FEA model was validated well against the test results when using the measured column end rotations as realistic boundary conditions. Different from the case of a CFST column, the axial load applied to a TRC column in fire is mainly sustained by the concrete and reinforcing bars and the high-temperature capacity contribution of steel tube is neglectable. With the increase of exposure time, the applied load gradually transfers from concrete to reinforcements until the yielding of re-bars.

Fire Resistance Design of Concrete-Filled Steel Tube Stub Columns

This paper introduces simplified design methods for calculating the load bearing capacity of axially loaded concrete-filled carbon steel tubular (CFST) stub columns and concrete-filled stainless steel tubular (CFSST) stub columns. Refined finite element (FE) models are developed for capturing the fire performance of CFST and CFSST stub columns under ISO 834 fire. The accuracy of proposed FE models is validated by comparing with existing test data. By using the new developed FE models, the design equations suggested in Eurocode 4 for calculating the axial load bearing capacity of CFST and CFSST stub column in fire are evaluated. It is found that the Eurocode 4 generally provide an unsafe prediction due to the influence of unsuitable consideration on thermal expansion and adopted material strength values. In order to improve the equations in Eurocode 4, correction factors kc and ks for the concrete core and steel tube are defined and calibrated based on comprehensive parametric analysis, and refined equations for predicting the axial load bearing capacity of CFST and CFSST stub columns in fire are suggested. Finally, a design example based on the proposed simplified design methods is demonstrated in this paper.

Research on Fire-Resistance Capacities of Steel Columns with Partial-Damaged Fire Protection Coating

The protection coating is possible to be damaged during its service life, which may induce a more complicated temperature distribution of columns in fire. In order to find out the fire-resistance capacities of the steel columns with partial-damaged fire protection coating, a detailed study on the effect of partial-damaged fire protection coating on steel columns was conducted with the finite element method. The fire-resistance capacity of I-section steel columns with partial-damaged fire protection coating was studied for the different cases, and the heating condition was the international standard fire curve. It is verified that the partial-damaged fire protection coating can cause local longitudinal non-uniform temperature distribution for the whole column and sectional temperature difference for a segment of the column at the same time. The parametric analysis including the locations of defects on protection coating, the slenderness of columns and the load ratios were considered. The results show that the relatively small areas losses of the protection coating can significantly reduce the fire-resistance capacity of the steel columns. The degradation in fire-resistance capacity is more significant when the losing areas on the upper part of the column, and the cases with high load ratio (≥0.7) are more sensitive to the negative effect of the losses of the protection coating. The residual fire-resistance capacity of the column decreases with the development of the defect length on the protection coating. The results can provide the basic assessments on fire-resistance capacities of steel columns with partial-damaged fire protection coating.

Compressive resistance of high-strength and normal-strength steel CHS members at elevated temperatures

The compressive behaviour of hot-finished circular hollow section (CHS) steel members in fire is investigated in this paper through numerical modelling. CHS members with high-strength steel grades of S690 and S460 are taken into consideration in addition to those made up of normal-strength grades S355, S275 and S235. Numerical models of CHS structural steel members able to replicate their response in fire are validated. Using the validated finite element models, extensive parametric studies are carried out for the purpose of exploring a broad range of factors influencing the cross-section and member buckling response of CHS steel members under axial compression at elevated temperatures. The accuracy and safety of the design recommendations provided in the European structural steel fire design standard EN 1993-1-2 for the determination of the axial compression resistances of CHS steel members in fire are assessed. New design methods able to provide accurate and safe estimations of the cross-section axial compression resistances and flexural buckling resistances of CHS steel members at elevated temperatures are proposed. The higher accuracy, reliability and safety of the proposed design methods relative to the existing design provisions in EN 1993-1-2 are illustrated. • Behaviour and design of high-strength and normal-strength steel CHS members under compression in fire are investigated. • FE models able to mimic response of CHS steel columns in fire are developed and validated using results from experiments. • New design rules for cross-section and flexural buckling resistance of CHS steel columns in fire are established. • Proposed design rules lead to more accurate results relative to the current design rules in EN 1993-1-2. • Accuracy, safety and reliability of proposed design rules are verified against extensive numerical data.

A reappraisal of the nominal curvature method in the fire design of reinforced concrete columns

A reappraisal of the method based on nominal curvature for designing reinforced concrete columns in fire is proposed, with the focus on the validation of an alternative and widely accepted way of applying the method, which differs from the current provisions of EN 1992-1-2. To this end, a systematic comparison between experimental and numerical results is carried out on a database consisting of 74 columns tested in different laboratories. The comparison shows that the method based on nominal curvature yields conservative estimates of the time to failure. Moreover, the numerical results obtained are reasonably close to those obtained by means of finite element analysis. By contrast, if the method is applied in combination with the simplified methods introduced in EN 1992-1-2, the numerical results are not conservative, with a significant overestimation of the bearing capacity of the columns, and of the time to failure, mainly because of the overestimation of the stiffness by the simplified methods.

Behaviour and design of stainless steel I-section columns in fire

The flexural buckling behaviour and design of stainless steel I-section columns at elevated temperatures are investigated in this paper. Finite element models able to accurately replicate the response of structural stainless steel columns in fire are created and validated. The models are then utilised to carry out extensive numerical parametric studies considering a broad range of stainless steel grades, cross-section geometries, slendernesses and elevated temperature levels. Using the results from the parametric studies, the safety and accuracy of existing design rules provided in the European structural steel fire design code EN 1993-1-2 for stainless steel columns in fire are assessed. It is observed that the existing design rules provide rather scattered and inaccurate ultimate strength predictions for stainless steel columns at elevated temperatures. For the purpose of establishing an accurate and practical means of designing stainless steel columns in fire, a new design approach, compatible with existing design rules in EN 1993-1-2, is proposed. The safety, accuracy and reliability of the proposed approach are illustrated for a wide range of cases against the results obtained through nonlinear finite element modelling. The proposed stainless steel column fire design rules are due to be incorporated into the upcoming version of the European steel fire design standard EN 1993-1-2.

Reliability based design method for buckling of steel columns in fire

Purpose This paper aims to reliability analysis of axially loaded steel columns at elevated temperatures considering the probabilistic features of fire. Design/methodology/approach The response function used in the reliability analysis is based on the non-linear FEM calculations. The stochastic variability of temperature is integrated with the procedure similar to the parameters of loading and material properties. Direct Monte Carlo simulations (MCSs) are implemented for probabilistic analysis. Computational costs are reduced by polynomial approximation of the response function of the column. Findings A design method for practical applications in the common Eurocode format is proposed. The proposed method can be used to estimate the failure probability of a steel column in fire conditions. If standard reliability criteria are applied, the results of the steel column buckling capacity in the fire according to the proposed procedure deviate from the Eurocode results in certain parameter ranges. Originality/value The proposed method for design calculations makes use of the advantages of MCS results, while the need for the tedious amount of calculations for the end user are avoided as the predefined factors are implemented in the procedure of Eurocode format. The proposed method allows better differentiation of the fire probability in the capacity assessment compared to the existing design methods.

Fire resistance design method for restrained stainless steel H-section columns under axial compression

Considering the material and geometric nonlinearity of restrained axially-compressed stainless steel H-section columns, this paper investigates their behavioural responses during fire exposure via the coupled thermo-mechanical analysis method and finite element models that were established by using ABAQUS. 7 restrained stainless steel H-section columns under axial compression were tested in the ISO-834 standard fire. A numerical simulation analysis was carried out, and the validity and feasibility of the finite element analysis model was proven by the test results. On this basis, a parameter-influencing analysis was performed to determine the main influencing factors of the fire resistance of a restrained axially-compressed stainless steel H-section column. The result shows that the load ratio n, slenderness ratio λ, axial restraint stiffness ratio βl and rotational restraint stiffness ratio βr are the key factors. Based on existing experimental studies and numerical simulations, an equivalent method of rotational restraint at the ends of a stainless steel column in fire is tentatively presented. A simplified formula, based on the effective length coefficient μθ of restrained axially-compressed stainless steel H-section columns, for calculating the critical temperature is proposed.