Time dependent analysis for composite steel-concrete beams with innovative deep trapezoidal decks

![CDATA[Composite steel concrete beams are formed by connecting concrete slabs to a supporting structural steel beam. In the early 1900’s, composite beams were considered favourable for bridge design but in recent decades, composite steel and concrete structures are employed extensively in modern high rise buildings. The flexural strength of composite beams is greatly influenced by the strength and ductility of the shear connectors between the structural steel beam and the concrete slab. The behaviour of the shear connectors is important in understanding the shear force transmitted and the degree of slip which occurs at the interface of the steel and concrete. Composite steel-concrete beams are becoming increasingly popular in multistorey buildings due to their higher span/depth ratio, reduced deflections and increased stiffness value. However, their performance is highly dependent on the load-slip characteristics of the shear connectors. More recently, trapezoidal profiled slabs are becoming increasingly more popular for high-rise buildings when compared with solid slabs because they can achieve large spans with little or no propping and they require less concrete and plywood formwork. However, the profiles used to achieve these savings can have a detrimental effect on the shear connector behaviour. For composite steel-concrete structures, concrete not only provides the compressive strength, but also the fire resistance to the floor surface, whilst the steel predominantly provides the tensile strength. When acting compositely through a shear stud, the composite steel-concrete member is stiffer and stronger. When concrete creep and shrinkage are considered, the deformation will increase with time. When the creep and shrinkage behaviour of the concrete is considered, the factors affecting the shear connection are the stiffness and strength of shear connectors and the stiffness and strength of the surrounding concrete. The strength of concrete is reduced according to time due to creep and shrinkage. It has been found that the flexural strength of composite beam is greatly controlled by the ductility and strength of headed stud shear connectors. In addition, the ductility and strength of headed stud shear connectors are also influenced by time-dependent characteristics of composite concrete-steel beam members which herein mainly related to creep and shrinkage of concrete. However, this long-term behaviour is often treated in an oversimplified manner in most of standard codes. Therefore, this paper herein investigated the behaviour of headed stud shear connectors in the composite beam which takes consideration of creep and shrinkage of concrete. This paper conducted two series of push test experimental studies and they were compared with Finite Element Analysis using commercial software known as ABAQUS. The experimental studies consisted the testing of eight push test specimens. The first series of experiment were the short-term behaviour of headed stud shear connectors in solid and profile steel sheeting slab. The second series of experiments were similar to first series but long-term behaviours were taking into account for both solid and profiled steel sheeting slab. In order to prove that an accurate finite element model has been developed to investigate the behaviour of the shear connection in composite steel-concrete beams for both solid and profiled slabs when creep is taken into account, the finite element models were compared with existing push test experimental studies. In general, there are three differences in behaviours between solid slab and profiled steel sheeting. The first difference is the failure mode. For solid slab, the failure mode is related to shear connector failure. For profiled steel sheeting slab, the failure mode is related to concrete failure. Secondly, the increasing rate of strain in headed stud shear connectors of solid slab are higher than in profiled steel sheeting slab. The strain is directly proportional with stress. Therefore, in other word, stresses in headed stud shear connectors in solid slab are higher than the headed studs in profiled steel sheeting slab. After 100 days, the slip of headed studs for solid slab increased by 50% compared with only 10% in profiled steel sheeting slab. Finally, the ultimate load of headed stud in solid slab is generally higher than in profiled slab. It can be concluded that the interface between steel and concrete plays a major role in the strength of the headed stud shear connectors. If profiled steel sheeting is placed between the steel and concrete, the strength of headed stud shear connectors is about 54% of the strength of studs in a solid slab specimen without profiled steel sheeting. This decrease in strength can explained due to the reduction of friction at the interface of steel and concrete. The results from experiments are also compared with Australian, Eurocode 4 and American Standards. It can be seen that the Australian and Eurocode 4 Standards have good agreements with the experimental results and finite element results while American Standard seems to be overestimate for strength of headed studs in solid slabs. Therefore, the designer should consider this issue carefully to prevent from exceeding the failure mode. From the finite element analyses, when creep and shrinkage are considered, the reduction in stiffness, ultimate shear capacity and slip capacity for both the solid and profiled slabs was observed. From the finite element analyses, the solid slab demonstrated that the failure mode is dominated by shear yielding failure, whilst failure in the profiled slabs can be attributed to concrete failure. Stresses in the shear connector and concrete are lower compared with those in the solid slab, due to the addition of the steel profile. It also can be observed that the solid slab generally has a higher ultimate load compared with that of the slab with profiled steel sheeting.]]