Evaluation of the effect of the size and shape of the opening on the punching shear capacity of the waffle panel

Document Type : Original Research

Authors
A. Yeganeh 1
B. Eftekhar 2
1 Master's Student, Department of Civil Engineering, Faculty of Engineering, Zand Institute of Higher Education
2 Assistant Professor, Department of Civil Engineering, Faculty of Engineering, Islamic Azad University of Larestan branch
10.22034/25.2.7
Abstract
An essential and decisive factor in the performance and design of various structures is their weight. One of the available solutions to reduce the weight of structures is using waffle panels. This slab consists of a grid of ribs regularly distributed in orthogonal directions and a thin concrete slab placed on the ribs. Using a flat waffle slab can put the roof under severe punching shear force. Also, placing the opening near the column can reduce the punching shear capacity of the slab. By reviewing the articles used in the codes, such as ACI 318-19 and also other valid articles about the punching shear capacity, it can be seen that most of the research done on this issue was done for simple concrete slabs and from such results the behavior of waffle panels are predicted. Past research has shown that the numerical methods of calculating the forces applied to concrete with the finite element method are very close to the results obtained from laboratory experiments. For this reason, in this research, to investigate the behavior of waffle panels against punching shear, nine samples of waffle panels have been modeled in ABAQUS software and analyzed using the finite element method. In both groups, for a fixed distance from the edge of the column to the opening, four different sizes of the opening on the slab were considered, and the changes made in the punching shear capacity and the ductility of the slab were calculated. The difference between the first and second group of slabs is in the opening geometry and how to increase their area. In the first group, only one dimension of the opening is increased, and the geometrical shape of the opening is rectangular. Considering the opening to this form, the b0 value introduced in the ACI Code remains constant. In this case, the punching shear capacity obtained from the equations of the code does not change. In the second group, the shape of the openings is square, but the area of the openings is equal to the first group.

A two-dimensional 2-node truss T3D2 finite element is used for the steel, and a three-dimensional 8-node hexahedral C3D8 finite element is used for the concrete. Also, to simulate the concrete compressive behavior, the Hognestad model is used in this research. The model results have been validated with the laboratory sample, and to provide a complete understanding of this issue, the modeling results have been compared with those obtained from the ACI 318-19 code. The punching capacity is noticeably reduced after placing an opening on the waffle slab. In case of using a rectangular opening at the edge of the column, the punching shear capacity decreases slowly with the increase of the opening area. This result is contrary to the assumption of the code. By using a square opening, with the increase of the area of the opening, the process of decreasing the punching shear capacity is almost uniform and linear. ACI code relations can have up to 25% error in this situation. These relations are more accurate for calculating the punching shear capacity in waffle panels with rectangular openings and slabs without openings. The reduction of punching shear capacity due to square openings is more than rectangular openings of the same area. This result is consistent with the relationships presented in ACI 318-19. Also, with the increase of the opening area, the difference in this value between groups 1 and 2 increases

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Modares Civil Engineering journal
Volume 25, Issue 2 - Serial Number 82
May and June 2025
Pages 7-19