Mechanical Model for flexural behaviour of slab-column connections under seismically induced deformations

Reinforced concrete (RC) flat slabs supported on columns are one of the most widely used structural systems for office and industrial buildings. In regions of medium to high seismic risk RC walls are typically added as lateral force resisting system and to increase the lateral stiffness and strength. Although slab-column systems are not expected to contribute to the lateral resistance of the structure due to their low stiffness, the slab-column connection have to have the capacity to follow the seismically induced lateral displacements of the building while maintaining the capacity to transfer the vertical loads from the slab to the columns. Otherwise, brittle punching failure of the slab occurs and the deformation capacity of the entire building is limited by the deformation capacity of the connection. The present paper presents a model for predicting the flexural behaviour of slab-column connections without transverse reinforcement when subjected to earthquake-induced deformations, considering both the load and the deformation of the slab. The model is based on the Critical Shear Crack Theory (CSCT) and presents a rational approach for predicting the transferred moment-rotation relationship of slab-column connections as well as the contribution of all resistance-providing mechanisms respecting equilibrium principles in both local and global level. The model proved to be accurate enough when compared with tests (monotonic and cyclic) found in the literature.

Mechanical model for flexural behaviour of slab-column connections under seismically induced deformations

Katrin Beyer, Ioannis Sokratis Drakatos, Aurelio Muttoni

2014

Seismic behaviour of slab-column connections without transverse reinforcement

Ioannis Sokratis Drakatos

Reinforced concrete (RC) flat slabs supported by slender columns are often used as gravity load resisting system for buildings in regions of moderate and high seismicity. Such buildings are typically braced by RC walls, which carry the largest portion of the horizontal loads generated during earthquakes. Therefore, the slab-column system does not contribute significantly to the lateral stiffness and strength of the structure, but each slab-column connection must be able to accommodate the seismically induced drifts of the building while maintaining its capacity to transfer vertical loads from the slab to the columns. Otherwise, brittle punching failure of the slab occurs and the deformation capacity of the entire building is limited by the deformation capacity of the slab-column connection if the building is not designed to resist progressive collapse. The first part of this work presents an experimental investigation on 13 full-scale internal slab-column connections without transverse reinforcement. The objective of the test campaign is to assess the influence of the loading history (monotonic vs. reversed cyclic) for different gravity loads and reinforcement ratios. The study shows that cyclic loading leads, in particular for slabs subjected to low gravity loads, to significant moment strength and deformation capacity reduction when compared to results obtained from monotonic loading tests. The effect of cyclic loading is more pronounced for slabs with low reinforcement content. In the second part, a mechanical model is presented for predicting the moment-rotation relationship of interior slab-column connections without transverse reinforcement when subjected to seismically induced drifts. The model accounts explicitly for the three load transfer mechanisms between slab and column contributing to the unbalanced moment resistance, i.e., eccentric shear, flexure and torsion. The moment resistance and deformation capacity are deduced from the intersection of the moment-rotation curve with a failure criterion that is based on the Critical Shear Crack Theory and distinguishes between monotonic and cyclic loading conditions. The model predicts well the moment strength and the deformation capacity of slabs tested within this research and reported in the literature. The third part of this thesis proposes an extension of the mechanical model for the moment-rotation relationship presented earlier to account for the hysteretic behaviour and cumulative damage effects on slab-column connections subjected to cyclic loading. A hysteretic moment-curvature relationship is proposed for the radial direction, based on local deformation measurements from the cyclic tests. Cyclic damage is considered by adopting a damage index proposed by a previous study. The extended model predicts more accurately the response of cyclic tests than the simplified approach based on the monotonic model. Finally, based on the theoretical investigation of the two previous parts, two methods are proposed for the numerical analysis of flat slab buildings to simulate the column deformation and the slab deformation until midspan. First, an Effective Beam Width method is presented and compared to test results of flat slab buildings with over-hangs. Then, a simplified method is proposed for the analysis of slab-column connections not part of the lateral force-resisting system. This method allows estimating the contribution of column and slab deformation to the interstorey drift.

EPFL2016

Lateral force resisting mechanisms in slab-column connections: An analytical approach

Katrin Beyer, Ioannis Sokratis Drakatos, Aurelio Muttoni

In many countries reinforced concrete (RC) flat slabs supported on columns is one of the most commonly used structural systems for office and industrial buildings. To increase the lateral stiffness and strength of the structure, RC walls are typically added and carry the largest portion of the horizontal loads generated during earthquakes. While the slab-column system is typically not relevant with regard to the lateral stiffness and strength of the structure, each slab-column connection has to have the capacity to follow the seismically induced lateral displacements of the building while maintaining its capacity to transfer vertical loads from the slab to the columns. If this is not the case, brittle punching failure of the slab occurs and the deformation capacity of the entire building is limited by the deformation capacity of the slab-column connection. This article presents an analytical approach for predicting the moment resistance of all mechanisms that contribute to the strength of the slab-column connection when subjected to earthquake-induced drifts. The approach is based on the Critical Shear Crack Theory (CSCT). The performance of the model is verified comparing the analytical predictions with experiments from the literature. The influence of gravity induced loads on the flexural behaviour of slab-column connections under seismic loading as well as the contribution of the various resistance-providing mechanisms for increasing drifts are discussed.

2014

Seismic behaviour of slab-column connections without transverse reinforcement

Ioannis Sokratis Drakatos

EPFL2016