Deformation capacity evaluation for flat slab seismic design

A Muttoni,M Lamperti Tornaghi,P Bamonte,I R Pascu,B Isufi,L Martinelli,A Pinho Ramos,D Coronelli,A Setiawan,G Tsionis

doi:10.1007/s10518-021-01302-x

Abstract

In flat-slab frames, which are typically designed as secondary seismic structures, the shear failure of the slab around the column (punching failure) is typically the governing failure mode which limits the deformation capacity and can potentially lead to a progressive collapse of the structure. Existing rules to predict the capacity of flat slab frames to resist imposed lateral displacements without losing the capability to bear gravity loads have been derived empirically from the results of cyclic tests on thin members. These rules account explicitly only for the ratio between acting gravity loads and resistance against concentric punching shear (so-called Gravity Shear Ratio). Recent rational models to estimate the deformation capacity of flat slabs show that other parameters can play a major role and predict a significant size effect (reduced deformation for thick slabs). In this paper, a closed-form expression to predict the deformation capacity of internal slab-column connections as a function of the main parameters is derived from the same model that has been used to develop the punching shear formulae for the second generation of Eurocode 2 for concrete structures. This expression is compared to an existing database of isolated internal slab-column connections showing fine accuracy and allowing to resolve the shortcomings of existing rules. In addition, the results of a testing programme on a full-scale flat-slab frame with two stories and 12 columns are described. The differences between measured interstorey drifts and local slab rotations influencing their capacity to resist shear forces are presented and discussed. With respect to the observed deformation capacities, similar values are obtained as in the isolated specimens and the predictions are confirmed for the internal columns, but significant differences are observed between internal, edge and corner slab-column connections. The effects of punching shear reinforcement and of integrity reinforcement (required according to Eurocode 2 to prevent progressive collapse after punching) are also discussed.

Highlights

Flat slab buildings for commercial, office and residential use are a common solution in many countries
This paper presents the development of a closed-form expression derived from mechanical models to predict the deformation capacity of internal slab-column connections without shear reinforcement subjected to imposed horizontal displacements as well as the deformation capacities measured on a full-scale flat slab frame test setup consisting of two floors with and without shear reinforcement and 12 columns (4 internal connections, 12 edge connections and 8 corner connections)
The slab rotation is typically smaller that the interstorey drift ratio, but in some cases the slab rotation can exceed the interstorey drift ratio

Summary

Introduction

Flat slab buildings for commercial, office and residential use are a common solution in many countries Extensive studies of their behaviour under gravity and seismic actions have been carried out in North America starting from the 1970s whereas European research developed more recently. EC8:2004 (CEN 2004a) explicitly states that primary flat plate frames are “not fully covered” (clause 5.1.1(2)) Following these definitions, Fardis (2009) proposes the design of flat plate frames as secondary systems, based on resistance verifications for the structure bearing gravity loads at the deformations imposed by the design earthquake. This research has led to the development of code provisions (ACI 318 2019) and design rules (Joint ACI-ASCE Committee 421 2010) encompassing performance-based prescriptions for the ultimate rotation capacity of slab-column connections (Hueste et al 2007; Zhou and Hueste 2017). Design codes to date in Europe have been reluctant to include detailed deformation capacity criteria for slab-column connections, in contrast to the North American codes

Results

Discussion

Conclusion