by Wendel M. Sebastian
In this study, the behaviour of a composite space truss bridge under highway loading has been investigated, and the applicability of existing design codes to this bridge form has been assessed. Three novel ideas have been proposed and tested, namely: the use of the composite space truss form in bridges to provide fast erection schemes in motorway widening situations; the use of a 150mm thick ribbed concrete slab on profiled steel sheeting for the bridge deck, and the structural use of a Glass Reinforced Plastic (GRP) enclosure, initially intended only as rust protection for the steel truss.
A space truss test specimen incorporation the above features, in particular a full scale 150mm thick concrete slab on ALPHALOK profiled sheeting and (stud) shear connected to the steel truss top chords, has been designed to existing structural codes, constructed, and tested to failure under simulated wheel loads for a complete HB vehicle.
From the tests conducted, it was found that the GRP enclosure significantly enhanced the behaviour of the composite structure in global bending and torsion. The short-term response of the slab to wheel loading was excellent, with no observed cracking or crushing of the concrete up to Ultimate HB loading, and the profiled steel sheeting successfully served as unpropped formwork to the wet concrete, and as bottom lateral reinforcement to the slab. Also, the (limited) shear connection provided was adequate for the required concrete slab-steel top chord composite action to develop up to Ultimate HB loading. Generally, the composite structure performed well under HB loading, and failed in a ductile, stable manner by yielding of the steel bottom chords.
The code approach was found to be very conservative as applied to the design of the concrete slab and shear stud connection of the space truss bridge test specimen. In addition, the punching shear capacity of the slab was found to be much greater than the code-predicted value, and well in excess of Ultimate HB wheel loading, probably due to enhancing membrane effects in the slab, which the codes do not recognise.
A Non-Linear Finite Element computer program has been written for the analysis of structural steel - reinforced concrete composite structures. Advanced features of the program include layered elements to follow plastification of the concrete slab and steel beams, an element for shear study action, and models for concrete behaviour after cracking. Results from the program compared well with published test and other theoretical data, and with data from the present tests. Also, a frame model, with equivalent bar elements to represent the in-plane shear stiffnesses of the concrete slab and GRP panels, has given good predictions of the composite specimen's torsional behaviour.
From the work carried out in this study, it can be concluded that composite space truss bridges with profiled steel sheeting and GRP enclosures can provide safe, economic solutions to the problem of rapid bridge replacements in motorway widening situations.
[Cambridge University | CUED | Structures Group | Geotechnical Group]
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