Abstract

This paper investigates the structural performance of hybrid members consisting of reinforced concrete flat slabs, with and without shear reinforcement, connected to steel columns by means of fully integrated shear-heads. A detailed account of the results from a series of six large scale tests on this form of hybrid structural system is provided. The test results offer a direct evaluation of the full load–deformation behaviour of the specimens as well as the ultimate punching shear strength attained prior to failure at the critical slab perimeter outside the shear-head region. The experimental findings enable the development of analytical models that depict the rotational response and flexural strength as a function of the shear-head embedment length, layout and section size. Additionally, the test results support the definition of a shear-head dependent control perimeter which is used in conjunction with the analytical slab models for full assessment of punching shear strength. The adequacy of strength predictions incorporated in current design methods for conventional reinforced concrete members are also examined in the paper. It is shown that existing design procedures either lack direct guidance for members provided with shear-heads, or lead to overly conservative strength predictions. Finally, in order to provide a reliable evaluation of the ultimate punching shear strength of hybrid elements, analytical design expressions which account for the characteristics of the shear-head system, are proposed. In comparison with conventional reinforced concrete design provisions, the suggested approach captures in a more realistic manner the influence of the embedded length of the shear-heads for such hybrid members with or without shear reinforcement.

Introduction

Hybrid systems consisting of steel columns with shear heads and reinforced concrete (RC) flat slabs have the potential for combining the structural behaviour synergies and practical construction merits of the constituent elements. Early interest in shear head configurations was shown with the development of high rise structures in the United States when the first systems to transfer loads between concrete columns and flat slabs were patented [1,2]. The use of shear-head systems against punching shear in conventional RC flat slabs was subsequently reported in various studies. Corley and Hawkins proposed a design procedure (implemented by the American Concrete Institute, ACI) as a result of a series of tests on cruciform shear-head systems fully embedded in the slab and consisting of two perpendicular I-sections placed between longitudinal reinforcement mats [3]. It was noted that shear-heads can increase the punching shear strength of RC flat slabs by up to 75%. Other intricate shear-head systems were reported in recent decades for RC flat slabs, including composite cruciform systems consisting of vertical plates acting as shearheads and provided with welded studs [4], fan-shaped systems made of wide tee pieces [5], and the Geilinger mushroom-head made of vertical plates bolted to the flanges of the column and surrounded by U-shaped edge beams [6]. Chana and Birjandi also carried out an extensive testing programme on typical cruciform systems having various arrangements of steel beams, including a closed-type system provided with edge beams [7].