[Univ of Cambridge][Dept of Engineering]
The structural behaviour of a composite stub-girder
incorporating an asymmetric slim floor beam
by M.D. Weerasinghe

A comprehensive study carried out on a new type of composite floor framing system mainly suitable for multi-storey high-rise buildings is presented in this thesis.

The new floor arrangement, referred to as the ASB Castellated Stub-Girder system, has been developed by integrating the commercial Slimdek system into the conventional stub-girder floor arrangement.  Slimdek is the registered trade mark of Corus (formerly British Steel), for a composite floor system which utilises a steel asymmetric slim-floor beam (known as an ASB) and a special deep decking type, and has a minimum depth of construction for its span.  In the new arrangement, the Slimdek system replaces the composite deck-slab used in stub-girder assemblies.  The bottom chord and the stubs are created using a single standard UB section by adopting a suitable castellated cutting pattern.  The new system is aimed at achieving clear spans in the 15-19 m range and a totally unpropped construction sequence.

A full-scale specimen of the ASB castellated stub-girder with a clear span of 12.6 m was constructed and successfully load tested to failure.  The work carried out on this test specimen included the preliminary analysis, its construction and assembly, and a series of tests performed during both the non-composite and the composite stages.  The structural behaviour of the specimen was extensively investigated with detailed measures of strains (concrete and steel), displacements, loads etc.

The non composite stage tests revealed the significance of providing sufficient lateral support to prevent the lateral torsional buckling under construction stage laoding.  The main composite stage static load tests demonstrated the ability of the composite ASB castellated stub-girder to carry the design ultimate load.  The specimen displayed a near perfect linear-elastic behaviour up to 1.8 times the serviceability load level.  The maximum deflection observed under the serviceablity load level was well within the limits specified by standard codes of practice.  The structure displayed a ductile failure mode while sustaining a load 2.7 times the serviceability load.  The failure was mainly due to yielding of the steel in the bottom chord of the assembly.

An advanced three-dimensional finite-element model of the test specimen was develoed using the finite-element software ABAQUSA (version 5.8).  The ability of this model to provide an accurate detailed prediction of the complex behaviour of the ASB castellated stub-girder has been demonstrated by the good agreement shown with the experimental results.  The suitability of other exisiting methods commonly used to analyse stub-girders, i.e. the layered beam method, the Vierendeel type frame method and the non-prismatic beam method was also investigated.  Comparison of the results from these methods with the experiments lead to the conclusion that the elastic layered beam method produced better results within the elastic range of the structure and hence could well be useful as a hand-calculation method in analysing this type of stub-girder.



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