Fire performance of unprotected and protected concrete filled steel hollow structural sections
Rush, David Ian
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Concrete filled steel hollow structural (CFS) sections are increasingly used to support large compressive loads in buildings, with the concrete infill and the steel tube working together to yield several benefits both at ambient temperature and during a fire. These members are now widely applied in the design of highly optimized multi-storey and high rise buildings where fire resistance ratings of two or more hours may be required. Whilst the response and design of these sections at ambient temperatures is reasonably well understood, their response in fire, and thus their fire resistance design, is less well established. Structural fire resistance design guidance is available but has been developed based on tests of predominantly short, concentrically-loaded, small-diameter columns in braced frames using normal strength concrete. The current prescriptive guidance is limited and the design of CFS columns is thus often based on a detailed performance based approach, which can be time consuming and expensive and which is generally not well supported by a deep understanding of CFS columns’ behaviour in real fires. This thesis aims to understand the fundamental thermal and mechanical factors at play within these sections so as to provide guidance on how to improve their design for fire resistance when applied either as unprotected or protected sections. A meta-analysis of available furnace test data is used to demonstrate that current guidance fails to capture the relevant mechanics and thus poorly predicts fire resistance. It is also demonstrated that the predictive abilities of the available design standards vary with physical characteristics of the CFS section such as shape and size. A factor which has been observed in furnace tests on CFS sections but which is not accounted for in available guidance is the formation of an air gap between the steel tube and the concrete core due to differential expansion; this affects their structural response in fire. The insulating effect of air gap formation has not previously been addressed in literature and an experimental program is presented to systematically assess the effects of a gap on the heat transfer through the section; showing that the presence of even a 1 mm gap is important. To explicitly assess the heat transfer response within both unprotected and fire protected (i.e. insulated) CFS sections, 34 large scale standard furnace tests were performed in partnership with an industry sponsor. Fourteen tests on large scale unloaded unprotected CFS sections are presented to assess current capability to predict the thermal response and to assess the effects of different sectional and material parameters on heating. New best practice thermal modelling guidance is suggested based on comparison between the models and observed temperatures from the tests. Twenty CFS specimens of varying size and shape, protected with different types and thicknesses of intumescent paint fire insulation, were also tested unloaded in a furnace to understand the thermal evolution within protected CFS sections and to develop design guidance to support application of intumescent coatings in performance based fire resistance design of CFS sections. These tests demonstrate that the intumescent coatings were far more effective than expected when applied to CFS sections, and that current methods of designing the coatings’ thickness are overly conservative. The reason for this appears to be that the calculation of effective section factor which is used in the prescription of intumescent coating thicknesses is based on the thermal response of unprotected CFS sections which display fundamentally different heating characteristics from protected sections due to the development of a thermal gradient in the concrete core. It is also demonstrated (by calculation supported by the testing presented herein) that the steel failure temperature (i.e. limiting temperature) of an unprotected CFS column in fire is significantly higher than one which is protected; procedures to determine the limiting temperature of protected sections are suggested. Finally, the residual strength of fire-exposed CFS columns is examined through structural testing of 19 of the 34 fire tested columns along with unheated control specimens. The results provide insights into the residual response of unprotected and protected CFS section exposed to fire, and demonstrate a reasonable ability to calculate their residual structural capacity. The work presented in this thesis has shed light on the ability of available guidance to rationally predict the thermal and structural response to fire of CFS columns, has improved the understanding of the thermal evolution within protected and unprotected CFS sections in fire, has provided best-practice guidance and material input parameters for both thermal and structural modelling of CFS sections, and has improved understanding of the residual capacity of CFS sections after a fire.