Abstracts:The design of concrete walls or columns reinforced by several encased steel profiles, also called hybrid walls, is similar to the one of classical reinforced concrete, although specific features require adequate design approaches. Experimental research and numerical models demonstrated the feasibility and validity of such structural components, but simple and practical design methods are still lacking regarding their shear resistance. The evaluation of longitudinal shear action effects at the steel profile–concrete interface is a key aspect: research results have been achieved in a more or less recent past for different types of connection but without leading to design conclusions. In this paper, the classical equivalent truss model for reinforced concrete subjected to shear is extended to take into account the contribution of the encased profiles to the shear stiffness and strength. Resulting action effects in the steel profiles, in the concrete and at the steel profile–concrete interfaces are established which allows performing design checks for those three components. In particular, it is evidenced that friction is one of the main component of the resistance to longitudinal shear at the steel profile-concrete interface. It can be directly checked since the proposed method clearly identifies the compression stresses at that location. The validity of the method is assessed by referring to tests results from experimental campaigns in China and in Europe. Some of these tests were carried out without shear connectors welded to the encased steel profiles allowing however achieving the full bending resistance of the element without any apparent problem related to longitudinal shear, like slippage between concrete and steel profile. For some other tests, failure was observed as a consequence of an insufficient shear connection. A detailed assessment of these results shows that the new design proposal is perfectly consistent with all the experimental observations.

Abstracts:A severely corroded end cross-girder is found occasionally, while the girder is expected to play an important role under seismic loading. The prevention of the end cross-girder against corrosion is therefore crucial in the bridge maintenance. To this end, the present research aims at improving its inspectability by installing inspection holes in the end cross-girder. The influences of the holes on the load-carrying capacity are then studied, to be specific. It is revealed that the inspection holes would reduce the load-carrying capacity considerably; the degree of the influence varies with the shape, the position and the size of the hole. Six reinforcement methods are therefore considered. Full recovery of the capacity turns out to be possible if the inspection hole is the same size as that of the standard opening (manhole) in a steel bridge structure, while it is not an easy task when the inspection hole is larger.

Abstracts:Composite special moment frames (C-SMFs) usually consist of concrete-filled steel tube (CFT) columns, wide flange (WF) steel beams, and rigid beam-to-column connections. The current International Building Code (IBC 2015) prevents the use of high strength materials (steel yield stress F y ≥450MPa and concrete compressive strength f' c ≥70MPa) for CFT columns in C-SMFs. This is due to the lack of knowledge of the overall seismic behavior of high strength C-SMFs. This paper evaluates the seismic performance of high strength C-SMFs by conducting nonlinear static pushover (NSP) and nonlinear time-history (NTH) analyses using analytical models developed and benchmarked by the authors. The NSP analyses indicate that high strength C-SMFs have good lateral load-deformation behavior, and the effects of local degradation on the lateral response are negligible when the roof drift angle is less than an upper bound limit (approximately 1.5%). The NTH analyses indicate that high strength C-SMFs satisfy the acceptance criteria (including the interstory drift angle) specified in FEMA-350 and ASCE 7-10 for Immediate Occupancy, Life Safety, and Collapse Prevention performance levels when subjected to the frequent occurrence earthquake (FOE), design basis earthquake (DBE), and maximum considered earthquake (MCE), respectively. Higher mode effects dominate the seismic performance of taller (12-story) high strength C-SMFs, but dynamic instability is not an issue when the maximum roof drift angle is less than the plateau drift angle (∆ p ), which corresponds to the onset of negative stiffness in the lateral load-displacement response from the NSP analyses.

Abstracts:A new type of composite shallow floor beam has been developed to minimise the overall structural depth and provide service integration. The new composite shallow floor beam consists of an asymmetric steel section with regularly spaced web openings and a concrete slab incorporated between the top and bottom flanges. The web openings are filled with in-situ concrete when the floors are cast. The longitudinal shear force is transferred by the concrete plugs passing through the web openings, acting with or without reinforcing tie-bars. This paper presents an experimental investigation of the shear transferring mechanisms in a flexural bending test carried out on a full-scale composite shallow cellular floor beam prototype. The flexural test provided information on the slip behaviour and shear performance of the shear transferring mechanisms under a loading profile similar to uniformly distributed loading. The flexural test demonstrated significant composite action and that the flexural failure of the beam specimen was due to failure of the concrete plugs.

Abstracts:Steel-concrete composite structures are commonly used in buildings and bridges because it takes advantage of tensile strength of steel and compressive strength of concrete. The two components are often secured by shear connectors such as headed studs to prevent slippage and to maintain composite action. In spite of its popularity, very little research was conducted on steel-concrete composites particularly on headed stud shear connectors in regards to its post-fire behaviour. This research investigates the post-fire behaviour of innovative shear connectors for composite steel and concrete. Three types of connectors were investigated. They are conventional headed stud shear connectors, Blind Bolt 1 and Blind Bolt 2 blind bolts. Push-out test experimental studies were conducted to look at the behaviour and failure modes for each connector. Eighteen push tests were conducted according to Eurocode 4. The push test specimens were tested under ambient temperatures and post-fire condition of 200°C, 400°C and 600°C. The results in ambient temperature are used to derive the residual strength of shear connectors after exposing to fire. This research showed that the headed studs performed well compared to Blind Bolts 1 and 2 at ambient and target temperatures. The stress concentrations around the casing of Blind Bolt 1 were found to cause a reduction in strength of the specimens. Findings from this research will provide fundamental background in designing steel-concrete composites where there is danger of fire exposure.

Abstracts:Concrete filled double skinned steel tubes (CFDST) are proved to have good structural performance in terms of strength, stiffness, ductility and fire resistance. Long CFSDT columns find application in elevated corridors, bridge piers and also in buildings. However, the behaviour of CFDST long columns is still not fully understood and there is limited research in this area. In this paper, axial capacity equations for long column CFDST sections are proposed based on strength super-position method of design. Column capacity computed using the proposed equation is validated through experimental studies conducted by the authors (for columns having L/D ratio of 20) as well as additional tests reported in literature. Tests were conducted on CFDST, Concrete Filled Steel Tube (CFST) and Concrete Filled Hollow Single skinned Steel Tube (CFHSST) cross-sections. Parameters considered in the test include (i) length of the column, (ii) shape of the inner tube, and (iii) absence of inner tube. Results from the test viz., (a) load carrying capacity, (b) load vs. axial deformation curves, and (c) load vs. lateral deflection curves, have been reported. Test result shows that the contribution of inner tube on the axial capacity of long column is less than the predicted value, as the column undergoes elastic buckling prior to yielding. A reduction factor is proposed to account for the reduced contribution of inner steel tube, and it is applied as a correction to the initially proposed equations. The results from proposed capacity equation are compared with experimental results and are found to be in good agreement. It is concluded that the long column axial capacity equation specified for CFST in AISC-360 and EC4 could be extended for CFDST sections after incorporating the new reduction factor.

Abstracts:This paper is concerned with the ultimate behaviour of composite floor slabs. Steel/concrete composite structures are increasingly common in the UK and worldwide, particularly for multi-storey construction. The popularity of this construction form is mainly due to the excellent efficiency offered in terms of structural behaviour, construction time and material usage all of which are particularly attractive given the ever-increasing demands for improved sustainability in construction. In this context, the engineering research community has focused considerable effort in recent years towards understanding the response of composite structures during extreme events, such as fires. In particular, the contribution made by the floor slab system is of crucial importance as its ability to undergo secondary load-carrying mechanisms (e.g. membrane action) once conventional strength limits have been reached may prevent overall collapse of the structure. Researchers have focused on developing the fundamental understanding of the complex behaviour of floor slabs and also improving the methods of analysis. Building on this work, the current paper describes the development and validation of a finite element model which can simulate the response of floor slab systems until failure, both at ambient and elevated temperature. The model can represent the complexities of the behaviour including the temperature-dependent material and geometric nonlinearities. It is first developed at ambient temperature and validated using a series of experiments on isolated slab elements. The most salient parameters are identified and studied. Thereafter, the model is extended to include the effects of elevated temperature so it can be employed to investigate the behaviour under these conditions. Comparisons with current design procedures are assessed and discussed.

Abstracts:The multi-scale model which combined the fiber beam element with the fine element was used to investigate the progressive collapse performance of steel beam to concrete-filled steel tubular (CFST) column connections. By using the nonlinear static analysis method and taking into account the influence of the adjacent framework of joints, the resistance of progressive collapse, the failure modes and the stress distribution revealed the resistance mechanism of these joints during the process of progressive collapse. And the vertical displacement time history curves of joints which displayed the progressive collapse resistance demands of these joints were obtained by using the nonlinear dynamic analysis method. The relationship between resistance capacity and resistance demand of these joints were obtained by analyzing the nonlinear static analysis results and the nonlinear dynamic analysis results. These analysis results showed that the frame structure with these joints which enabled to form the resistance mechanism and new alternate path of unbalanced loads can prevent the occurrence of progressive collapse after the failure of column connected to joints. And the adjacent framework can improve the ability of anti-progressive collapse of these joints.

Abstracts:This paper deals with an experimental study on seismic performance of a new type of exterior RCS connection, in which a steel profile embedded inside RC column is directly welded to the steel beam. A full scale exterior hybrid joint was built and tested under reversed-cyclic loading at the University of Transport and Communications of Vietnam. Seismic performance in term of load bearing capacity, story drift capacity, ductility, energy dissipation and stiffness degradation were evaluated. The test specimen showed a good response to cyclic load reversals. The experimental results indicated that the test specimen performed in a ductile manner and the stiffness degradation during the cycles performed was gradual. It was concluded that the studied RCS joint could be used as dissipative element in the structures of ductility class medium (DCM).

Abstracts:In order to qualify and quantify structural behaviour of stud shear connections with different configurations commonly adopted in building construction, a systematic experimental and numerical investigation into practical stud shear connections with both solid concrete slabs and composite slabs was carried out. This paper presents key findings of the investigation on these shear connections in both standard and modified push-out tests, and six test series with a total of 25 standard and modified push-out tests were conducted. It should be noted that while the shear connections in standard tests were under direct shear forces, the shear connections in modified tests were under co-existing shear and tension forces. For direct comparison, the measured load-slippage curves of these shear connections were rationalized to provide representative load-deformation characteristics of the connections which exhibited various degrees of ductility at both small and large deformations.