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Title: A practical approach to modelling integrity failure of composite floors in fire
Author: Javaheriafif, Mohammadali
ISNI:       0000 0004 6060 7576
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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The connections in a steel-framed building are subjected to a complex set of forces in fire conditions. Large axial forces (compressive forces that later become tension as the steel softens from rising temperature) will affect the beams and their connections, in addition to the shear forces and bending moments caused by gravity loading. Therefore, the performance of a joint in response to such loads plays a key role in overall behaviour of the frame. Large deflections of composite slabs contribute significantly to the robustness of composite steel-framed buildings in fire. The composite slab contributes to the rotational stiffness of a joint through the tensile resistance of its reinforcement acting at a large lever arm from the centroid of the steel beam, adding to the hogging moment capacity of the connection. This clearly results in considerably higher rotational stiffness, but for the purposes of structural fire engineering design there should also be enough ductility in the reinforcement to provide sufficient rotation capacity to the joint. Adequate ductility in composite slabs is a requirement for the robustness of composite buildings in fire. Finite element analysis of steel frames in fire often assumes the slab to be continuous and the inevitable cracking that takes place is accounted for using smeared cracking approaches. At beam-to-column connections the presence of the slab increases the stiffness and strength of the joint, but existing analytical techniques do not adequately address the effect of cracking in the slab at these locations. In order to investigate the influence of the concrete slab on the joint performance, a method to allow for the development of discrete cracks in the concrete slab, largely as a result of the hogging bending moments over supporting steel beams and connections, has been developed. In order to avoid the complexities of generalized discrete cracking analysis, fracture at key locations is represented by the use of 'break-elements'. The new break-element represents the crack development in the composite slab, mainly across internal beams on the column grid where it is assumed that cracks will initially occur. The model results in a localisation of the yield and ultimate strains in the rebar, enabling the crack width at which it fractures to be represented in terms of the local bond characteristics beyond the crack faces. The approach is being implemented in the Vulcan software, which is capable of modelling geometrical non-linearity, also considering non-linear material behaviour at elevated temperature. The software has the advantage of combined static and dynamic solvers, which makes it possible to trace the structural behavior of a single member or a whole frame from initial static response, through local failure or instability, to stable post-buckling behaviour. The composite joint is modelled using the existing two-dimensional component- based model for bare steel connections, acting compositely with the 3D slab shell element through link elements representing shear studs. The newly developed break elements can be located at the perimeter nodes of every slab element across the entire floor slab area. This will enable a more accurate investigation of the crack development within a slab panel in fire scenarios. Once the break element fractures the dynamic solver can temporarily be activated to search for the next re-stabilization. After re-stabilizing, the analysis continues using the static solver. Three series of previous experimental tests on composite joints at ambient temperature with different bare steel connections have been modelled in Vulcan software in order to validate the newly developed break element. The model is capable of predicting the occurrence of the initial cracks, tracing the behaviour of the mesh reinforcement represented at each break element and the sequence of failure of the reinforcement in the composite slab. Furthermore, the numerical model can represent the rotational response of composite joints with a reasonable level of accuracy, subject to the limitations of the current version of the software in modelling component-based connection elements. A series of parametric studies was conducted in order to investigate the influence of reinforcement ratio, reinforcement material properties (characteristic yield strength), concrete material properties (characteristic compressive strength), composite slab thickness and the different aspect ratio on the overall performance of the composite panel. The outcome of the analysis has been presented in terms of the slab vertical deflection, rotational displacement of the connection and the horizontal movement of the slab edges (crack propagation). The calculated result from the updated version was compared with the result from the original software and appropriate discussion has been drawn.
Supervisor: Davison, John Buick ; Burgess, Ian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available