Macro modeling of post-punching behavior of flat slabs in progressive collapse

Document Type : Original Research

Authors
V. Ghiasi 1
E. Mousapoor 2
R. Madandoust 3
1 Vahed GhiasiAssistant Professor of Geotechnical EngineeringDepartment of Civil EngineeringFaculty of Civil and Architecture EngineeringMalayer University - IranH/P: (0098)912-537-4295Second email: v_gh10@yahoo.com
2 PhD CANDIDATE OF CIVIL ENGINEERING,MALAYER UNIVERSITY
3 Associate Professor, Department of Civil Engineering, Guilan University, Rasht, Iran
Abstract
Reinforced concrete flat slabs are simply a plate of uniform thickness placed on columns without the help of beams or capitals or drop panels. Due to the direct transfer of slab loads to the supporting column, the column tends to punch through the slab. Flat slabs without shear reinforcements often have a shear failure with very little ductility and no sign of warning. Most studies of flat plate performance were attended to punching shear failure, and very little research was conducted on the flat plate behavior after punching failure and its subsequent progressive failure. Consequently, the literature on the behavioral characteristics of flat slabs following punching failure is very restricted.

Over the past years, researchers have proposed different models of grid model and shell element model for 3D modeling of flat slabs. In the grid model, the slab is simulated by a grid of beam elements, Because the load-bearing process in the slabs is somewhat similar to the load-bearing process in the beams. This method can be used to analyze the progressive collapse but requires much effort in modeling the slabs. The use of multilayer shell element for modeling slabs can be used with less effort and higher accuracy.

In the present study, two improved methods of macro modeling were proposed to predict the post-punching behavior of the slab-column connections. These modeling techniques can be used to analyze the progressive collapse of reinforced concrete flat slab buildings. Liu et al. (2015) proposed a macro model to analyze the progressive collapse of flat plate buildings. In this macro model, the slab-column joint region is simulated by the inflexible shell element. The critical section of the punching shear around the joint region is considered at distance half slab effective depth from the edge of the column. To simulate the slab away from the punch environment, a multilayered shell element consisting of concrete and rebar with nonlinear material properties is used. The junction area between the critical punch section and the edge of the column is modeled with two beam elements for each column face. Then flexural, shear, torsional, and axial behaviors are defined with six degrees of freedom for the connector beam elements. This model can be used to evaluate the potential for progressive collapse of flat slab buildings, but this model ignores the post-punching resistance of flat slabs.

The post-punching resistance of flat slabs without transverse reinforcement, without taking into account the interaction of aggregates, the sum of the shear transfers through tensile reinforcements and integrity reinforcements. In the present study, the model presented by Liu et al. (2015) was modified to assess the post-punching response of the slab-column connections. In the proposed model, constant residual shear strength is assumed after the punching shear failure for the connector beam element to consider to the post-punching shear transfer through the flexural reinforcements. The remaining shear strength ( ) improves by increasing the diameter of the integrity reinforcement. The remaining shear strength ​​for integrity bars with diameters of 8 to 14 mm is recommended about 30% to 40% of the punching strength, respectively.

To evaluate the post-punching resistance of flat slabs due to the integrity reinforcements, two methods of modeling, rebar model and link element model were presented. In the modeling with the rebar, the integrity reinforcement of the specified length and with an initial distance of the strain is placed vicinity to the connector beam element. The length of the rebar and the initial distance of the strain through the calibration with the test results were and , respectively. In the second method of modeling, a link element is placed vicinity to the connector beam element. The link element is activated after the punch. A mechanical model was presented for the contribution of integrity reinforcements in the post-punching shear transfer. Comparison of the final results of the two modelings mentioned above with the test results shows that both methods of modeling have acceptable accuracy in predicting post-punching strength, post-punching stiffness, and deformation capacity. To improve the proposed models, further studies are needed on the modeling of the exterior slab-column connections of the flat slab structure and the shape of the cross-sectional area.

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Modares Civil Engineering journal
Volume 19, Issue 6
November and December 2019
Pages 201-218