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. (C) 2011 Elsevier Ltd. All rights reserved

Creep-free fire analysis of steel structures with Eurocode 3 material model

Purpose – This paper aims to propose a methodology to remove inherent implicit creep from the Eurocode 3 material model for steel and to present a creep-free analysis on simply supported steel members. Design/methodology/approach – Most of the available material models of steel are based on transient coupon tests, which inherently include creep strain associated with particular heating rates and load ratios. Findings – The creep-free analysis aims to reveal the influence of implicit creep by investigating the behaviour of simply supported steel beams and columns exposed to various heating regimes. The paper further evaluates the implicit consideration of creep in the Eurocode 3 steel material model. Originality/value – A modified Eurocode 3 carbon steel material model for creep-free analysis is proposed for general structural fire engineering analysis

Tensile Membrane Action of Thin Slabs Exposed to Thermal Gradients

A number of simplified design methods have been developed to predict composite slab capacities in fire. Most of these extend ambient-temperature large-deflection slab behavior to the elevated-temperature phase by reducing the strengths of fire-exposed concrete and reinforcement while neglecting the effects of thermal expansion and thermal bowing of the slab. Experiments have shown that there are significant differences between the predictions from these methods and the actual behavior and failure modes of ambient- and elevated-temperature concrete slabs in tensile membrane action. Therefore, this paper describes the development of a new analytical method that incorporates both thermal and mechanical effects into the prediction of slab behavior in fire conditions. It uses the variational Rayleigh-Ritz approach to classical large-deflection plate theory. The method is found to produce accurate predictions of deflections and membrane tractions; however, it requires further refinement for accuracy of stresses. The results are compared with numerical modeling using VULCAN, a specialist finite-element (FE) program for structural fire engineering

Effect of transient strain on strength of concrete and CFT columns in fire – Part 2: Simplified and numerical modelling

This paper presents finite element analysis on columns of concrete and concrete-filled tubular section. This study has been conducted with Vulcan, a specialist structural fire engineering FE program, which has been further developed to incorporate the transient strain of concrete. The implementation of transient strain in Vulcan has been validated against a Shanley-like simplified model. A further extension of the simplified model has been carried out in order to provide direct comparability with the FE modelling. The effects of transient strain, considering thermal gradients through the column cross-section, have been evaluated with both the simplified and FE models. Finally, parametric studies on concrete-filled tubular columns, considering the effects of slenderness ratio, reinforcement, the steel casing and thermal gradient within the cross-section, have been conducted using Vulcan

A structural fire engineering prediction for the Veselí fire tests, 2011

Fire hazards and full-scale structural tests have provided evidence that the beam-column connections of building frames are the weakest structural elements, which are vulnerable to fracture in fire. Connection fractures may lead to extensive damage or even progressive collapse. However, current design methods for connections are solely based on ambient-temperature behaviour, the additional forces and rotations generated in fire are not taken into account. The Structural Fire Engineering Research Group of the University of Sheffield is involved in a European-collaborative project which concerns the behaviour and robustness in fire of practical connections to composite columns. This includes two natural fire tests in a full-scale composite structure in Veselí, the Czech Republic. The Sheffield team was responsible for predicting the structural behaviour in the tests before they were conducted. This assessment was conducted using the specialist structural fire engineering FEA program Vulcan. This paper reports the results of this predictive analysis

High-temperature tests on joints to steel and partially-encased H-section columns

This paper reports on a series of tests at elevated temperatures on connections between steel beams and H-section columns, both unfilled and partially-concrete-encased. Reverse-channel connections to both types of column, as well as flush endplate connections to partially-encased H-section columns, were studied. The experiments aimed to investigate the behaviour of beam-to-column connections subject to significant tying forces and large rotations in fire situations, and to provide test data for development and validation of simplified component-based connection models. It has been found that reverse-channel connections provide not only high strength, but also the high ductility which is required to reduce the possibility of connection fracture and to improve the robustness of buildings in fire

Behaviour of steel grade S275JR columns under the influence of high-temperature creep

This paper presents a test study focused on determining the influence of high-temperature creep on the buckling of grade S275JR columns. Both capacity and stationary-creep column tests were performed within the study. The results obtained demonstrate that high-temperature creep influences a column’s load-bearing capacity, even at 400°C if the column’s load ratio is above 87%. The reduction of load capacity due to creep was determined experimentally up to 600°C, for which the failure due to creep occurred at load ratios above 57% of the capacity at 600°C. The failure times for all the column creep tests occurred within 240 min of loading at constant temperature. Numerical modelling of the creep tests, including their constant-temperature capacity, was performed with the help of the research software Vulcan, by explicitly including a previously developed creep model for grade S275JR steel in the total strain formulation

The effect of high-temperature creep on buckling behaviour of aluminium grade EN6082AW T6 columns

The paper presents an experimental study that investigates the influence of high-temperature creep on reduction of the buckling load capacity of aluminium grade EN6082AW T6 columns. The study was performed by performing constant-temperature capacity and creep tests on 17 column specimens of approximately 2.6 m length. A total of eight capacity tests and nine creep tests were carried out. Results obtained within the study have revealed a critical temperature interval of 160–260 °C within which high-temperature creep significantly influences the columns' buckling load capacity. The load level at which high-temperature creep influences the reduction of columns' buckling-load capacity, by exhibiting low short-term creep resistance, is above 90% of the column's axial load capacity. The study provides relevant thermo-mechanical criteria for the assessment of creep-induced buckling of the tested aluminium alloy, in the form of the load level intervals at prescribed temperatures, which induce column buckling within the 240 min interval that is generally considered as a relevant fire resistance period for building structures over 18 m in height

Tensile Behaviour of Galvanised Grade 8.8 Bolt Assemblies in Fire

In structural fire engineering, the importance of bolt assemblies is often overlooked. Connection design uses the temperature-dependent bolt strength-reduction factors prescribed in Eurocode 3, despite the existence of two distinct failure modes under tension; necking of the bolt shank, and thread-stripping. While literature exists to predict failure modes at ambient temperature, there is no method for failure mode prediction for elevated temperatures where ductility is critical to avoid collapse. Galvanised M20 structural bolt assemblies and bolt material from a single batch have been tested under tension at a range of temperatures and strain-rates typical of those experienced in fire. Turned-down bolt test data produced stress-strain curves characteristic of different microstructures at ambient temperature, despite a tempered-martensitic microstructure being specified in the standards. The failure modes of bolt assemblies were found to be dependent on the as-received microstructure at ambient temperature. At elevated temperatures, however, only thread-stripping was observed