Characterizing transition zone between steel‐concrete composite and reinforced concrete members

Abstract

Several structural situations can benefit from the presence of embedded steel profiles within a classical reinforced concrete (RC) member (see Fig. 1) to achieve a localized higher performance such as great spans above large free spaces, beams above auditoriums or hotel lobbies or a greater free height in columns, due to architectural reasons , which leads to local higher slenderness. Multiple research studies have therefore been conducted in the past century to understand and subsequently characterize the behaviour of such composite sections (mostly columns). However, in order to use them in practice, a suitable connection between the composite columns and different structural elements, e.g. concrete slabs, RC beam or composite beams, is also necessary. Although the available literature and the current standards provide validated guidelines to design such composite sections and members with one embedded steel profile, a knowledge gap is prevalent regarding the transition zone in a column that would consist of a composite part and a pure RC part along its height. The research project "INERD" (INnovations for Earthquake-Resistant Design) [1] investigated such transition zones in order to mitigate soft storey failures of RC moment resisting frames submitted to earthquakes. In the case-studies, the columns were made to be locally stronger and more ductile by embedding a hot rolled steel profile in their critical zones (at the first storey of the building i.e. the soft-storey prone level). Therefore, the transition zones were designed to be stronger than the rest of the columns. However, the embedded profile was not considered in the structural design and was only applied as a safety belt triggered during seismic ground motions. Design guidelines were proposed [2] for the connection between both parts of the column-composite part and RC part. Appropriate values were also provided for the shear and the bending forces to be transferred, for cases where beams are connected to the columns in the transition zone. However, an efficient force transfer mechanism between the embedded steel profile and the surrounding concrete was not considered in the local transition zones. Further in-depth investigations were therefore needed to characterize the local force-transfer mechanism and was duly carried out in the research project SmartCoCo (Smart ORIGINAL ARTICLE Abstract Steel profiles are often embedded in reinforced concrete (RC) members (specially columns) to achieve higher performance in a localised manner. Although the RC and steel-concrete composite zones of such members can be respectively designed according to the Eurocodes 2 and 4, the transition zone between the steel profile and its surrounding concrete needs to be adequately designed to ensure an effective transmission of the axial force, shear and bending moments carried by the interrupted steel component. This is currently not covered by the design standards and is only partially covered by the current literature. This article proposes a design procedure for such transition zones relying on a 2-steps approach: (1) Definition of the transverse load to be transferred from the steel part of the composite zone to the RC part, based on a reasonable distribution of the contact pressure between the profile and the surrounding concrete; and (2) Evaluation of a suitable "strut & tie" mechanism to ensure the appropriate transfer of this load and consecutive design of the transverse reinforcement. In order to evaluate the efficiency of the design approach, the transition zone between a composite and a pure RC part of a locally composite column was investigated experimentally. Four test specimens were designed and tested in order to study the influence of certain parameters, e.g. length of the assumed transfer zone, presence of one or two lateral beams at the level of the transfer zone, compression level in the column, etc.