The Contribution of the Infill Walls to the Lateral Strength of Concrete Frames

Background:

Recent research studies conclude that the contribution of the infill walls to the overall lateral strength of frames is significant. The current state of the art includes two main approaches for the idealization of the behavior of the infill walls and their implementation in software. Micro modelling includes the use of the finite element method whereas the macro modelling, includes the use of one-dimensional compressive diagonal strut elements to replace the infill wall and provide the equivalent lateral stiffness.

Objective:

The aim of this study was to compare various methods for the simulation of the infill walls with the finite element method and propose an alternative approach which makes use of the rigid end offset which is a feature available in most of the finite element software.

Methods:

A reinforced concrete frame model with an infill wall was created. The model was modified to form combinations of infill wall thicknesses and values of Young’s modulus. The models were analyzed using the finite element method. The results were utilized to develop equations for the calculation of the length of rigid end offsets for the beams and columns of the frame. The rigid end offsets were then used in the analysis to numerically stiffen the frame and simulate an effective lateral strength contribution from the infill wall.

Results:

The results of the implementation of the rigid end offsets to simulate the contribution of the infill walls to the lateral stiffness of the frame were compared to the results of the results from the finite element analysis. Specifically, the results for the walls normally found in construction (less or equal to 3m in height and with thickness less or equal to 25cm) showed a very good agreement while the remaining results were very close.

Conclusion:

This work proposes equations for calculation of the length for the rigid end offsets which can be used in the analysis of frames with infill walls. The results show that the utilization of this feature from the structural analysis software in the analysis of frames, results in adequate stiffening of the overall frame, thus, providing an equivalent stiffness which accounts for the presence of the infill walls.

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