Study on the Sensitivity of Mesh Size and Properties in the Analysis of Semirigid Diaphragms

This paper was prepared for and presented at the 2020 Structural Engineers Association of California Convention. It was also presented at the 21st Association of Structural Engineers of the Philippines International Convention.

The behavior of a building structure in response to an earthquake or wind, including the distribution of the lateral forces to the lateral force resisting elements such as moment frames, braced frames or shear walls, is heavily dependent upon the nature, extent and stiffness of the floor and roof diaphragms. It is important that the building model used in the analysis and design of a building accurately – or adequately – captures the effects of these diaphragms.

Building Codes have extensive requirements related to diaphragms, and increasingly, structures are required to be analyzed with the diaphragms modeled as semirigid diaphragms. Determination of the properties to assign to these diaphragms in the analysis can be difficult, tedious, and time-consuming, with low confidence in their suitability.

A study was performed by repeatedly analyzing a structure modeled with a semirigid diaphragm, varying the analytical diaphragm mesh sizes and varying the diaphragm material properties, notably the diaphragm effective modulus of elasticity, E. This was done for a wide range of mesh sizes and effective E values. The resulting frame story shears, diaphragm forces, and drifts from each of these scenarios were compared, to determine the sensitivity of the analysis to these diaphragm parameters. The results of this study are presented in this paper, with useful insights and practical recommendations based on the results.