ROBUSTNESS OF CONNECTIONS TO CONCRETE-FILLED STEEL TUBULAR COLUMNS UNDER FIRE DURING HEATING AND COOLING

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Abstract

ABSTRACTJoint behaviour in fire is currently one of the most important topics of research in struc-tural fire resistance. The collapse of World Trade Center buildings and the results of the Cardington full-scale eight storey steel framed building fire tests in the UK have dem-onstrated that steel joints are particularly vulnerable during the heating and cooling phases of fire. The main purpose of this research is to develop robust joints to CFT col-umns that are capable of providing very high rotational and tying resistances to make it possible for the connected beam to fully develop catenary action during the heating phase of fire attack and to retain integrity during the cooling phase of fire attack.This research employed the general finite element software ABAQUS to numerically model the behaviour of restrained structural subassemblies of steel beam to concrete filled tubular (CFT) columns and their joints in fire. For validation, this research com-pared the simulation and test results for 10 fire tests previously conducted at the Univer-sity of Manchester. It was envisaged that catenary action in the connected beams at very large deflections would play an important role in ensuring robustness of steel framed structures in fire. Therefore, it was vital that the numerical simulations could accurately predict the structural behaviour at very large deflections. In particular, the transitional behaviour of the beam from compression to catenary action presented tremendous diffi-culties in numerical simulations due to the extremely high rate of deflection increase. This thesis will explain the methodology of a suitable simulation method, by introduc-ing a pseudo damping factor. The comparison between the FE and the experimental re-sults demonstrates that the 3-D finite element model is able to successfully simulate the fire tests. The validated ABAQUS model was then applied to conduct a thorough set of numerical studies to investigate methods of improving the survival temperatures under heating in fire of steel beams to concrete filled tubular (CFT) columns using reverse channel con-nection. This study investigated five different joint types of reverse channel connection: extended endplate, flush endplate, flexible endplate, hybrid flush/flexible endplate and hybrid extended/flexible endplate. The connection details investigated include reverse channel web thickness, bolt diameter and grade, using fire-resistant (FR) steel for dif-ferent joint components (reverse channel, end plate and bolts) and joint temperature control. The effects of changing the applied beam and column loads were also consid-ered. It is concluded that by adopting some of the joint details to improve the joint ten-sile strength and deformation capacity, it is possible for the beams to develop substan-tial catenary action to survive very high temperatures. This thesis also explains the implications on fire resistant design of the connected columns in order to resist the addi-tional catenary force in the beam.The validated numerical model was also used to perform extensive parametric studies on steel framed structures using concrete filled tubular (CFT) columns with flexible re-verse channel connection and fin plate connection to find means of reducing the risk of structural failure during cooling. The results lead to the suggestion that in order to avoid connection fracture during cooling, the most effective and simplest method would be to reduce the limiting temperature of the connected beam by less than 50°C from the limit-ing temperature calculated without considering any axial force in the beam.

Keyword(s)

concrete filled tubes, reverse channel connection, cooling, robustness ; connection, fire resistance, catenary action, numerical modelling,

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