ROBUSTNESS OF COMPOSITE FRAMED STRUCTURES IN FIRE

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Abstract

This thesis presents the results of a research study to investigate the behaviour of axially restrained composite beams at ambient and elevated temperatures, and how composite beams and their connections contribute to the robustness of composite framed structures in fire. The commercial finite element analysis package (ABAQUS, 2010) was used to develop the numerical simulation models. This research includes the following four main parts: (1) validation of the simulation model; (2) behaviour of axially restrained composite beams with partial shear interaction at ambient and elevated temperatures; (3) behaviour of composite beams with realistic connections at elevated temperatures and methods of increasing composite beam survival temperatures; and (4) response and robustness of composite frame structures with different extents of damage at elevated temperatures.Based on the results of composite beams, it was found that the survival of axially restrained beams is dominated by the development of catenary action. By utilising catenary action, it is possible for composite beams to develop load carrying capacity significantly above that based on bending resistance. During the development of catenary action, the compression force in the concrete flange of the composite beam decreases, thus reducing the forces in the shear connectors. As a result, the behaviour of shear connector failures ceases to be an issue during the catenary action stage. The results further show that, the load carrying capacities/survival temperatures of composite beams increase by increasing the level of axial restraint up to a certain limit and then decrease at higher levels. Typical realistic composite structures can provide composite beams with sufficient axial restraint to develop catenary action. For detailed composite beams with composite connections, three different beam sizes were investigated using flushed and extended end plate connections with different amounts of slab reinforcement, different load ratios and different bolt sizes. It has been found that the most important method to increase the survival time of composite beams is to use extended end plate connections with sufficient top and bottom reinforcement meshes in the concrete slab, i.e. increasing the amount of slab reinforcement is more beneficial than increasing the bolt size or the number of bolts.Based on the results of modelling a four bay (9 m each, two storey, 4 m high) composite frame with different extents of fire damage to different members, it was found that whenever any of the columns failed, progressive collapse of the frame would occur. Therefore, damages to columns should be prevented or the columns should be designed and constructed to allow for possible damage. If the beams are damaged, it is still possible for the damaged frame to achieve the reference fire resistance time of the undamaged structure (which is used as the criterion to accept that the damaged frame has sufficient robustness) by developing catenary action in the damaged beam. For this to happen, the columns should be designed to resist the catenary tensile force (tying force) in the beams, in addition to the compressive force.

Layman's Abstract

If the beams are damaged, it is still possible for the damaged frame to achieve the reference fire resistance time of the undamaged structure (which is used as the criterion to accept that the damaged frame has sufficient robustness) by developing catenary action in the damaged beam. For this to happen, the columns should be designed to resist the catenary tensile force (tying force) in the beams, in addition to the compressive force.

Keyword(s)

Robustness, Composite steel concrete structure, catenary action, Fire, Partial damage

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