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Title: How to Model Post-Cracking Torsional Stiffness and Why It Matters

Author(s): Edvard P.G. Bruun, Allan Kuan, and Evan C. Bentz

Publication: Symposium Paper

Volume: 344


Appears on pages(s): 49-63

Keywords: reinforced concrete; torsion; post-cracking stiffness; serviceability; spandrel beam; redistribution

DOI: 10.14359/51728290

Date: 10/1/2020

Post-cracking stiffness is an important parameter in determining the proper distribution of forces in the analysis of statically indeterminate reinforced concrete structures. While the ACI 318-19 code specifies typical values to use in modelling flexural cracking, the same guidance is not available when calculating post-cracking torsional stiffness. This paper presents a summary of the academic literature on the topic as the basis for developing a novel stiffness-based design procedure, which is then implemented in the design case study of a spandrel beam supporting a cantilevered roof slab. This example demonstrates a situation where a specific torsional stiffness is required to satisfy serviceability requirements. The design method is general and, therefore, applicable to any situation where an accurate measure of torsional stiffness or moment redistribution is required – this removes the need to iteratively model and design to capture post-cracking effects in structural members.