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Flexible vs. Semi-Rigid vs. Rigid Diaphragms

Structural analysis of buildings has changed with the advancement of technology and the movement away from hand calculations. Now, most engineers use sophisticated software capable of handling the many shapes, varieties, and complexities of newer structures.  These software systems are able to handle the many degrees of freedom necessary to more accurately predict the behavior of diaphragms and their ability to distribute loads throughout the building frame.  Thus, engineers must now be careful to understand the types of diaphragms and how they behave within their analytical model to be sure it accurately portrays the elastic or inelastic response of the structure. 

In the May 2020 SE University session, Allen Adams, PE, SE, from Bentley Systems, Inc. / RAM, presented Practical Strategies for the Modeling and Analysis of Diaphragms.  Allen explained the differences between flexible, semi-rigid, and rigid diaphragms, and how these affect analytical results.  He reviewed code requirements pertaining to diaphragms and shared how sensitive analytical models are to the various components of semi-rigid diaphragm models.  Allen advised on balancing the necessary level of accuracy with business demands for speed and simplicity when modeling diaphragms.

Allen explained the basics of each type of structural diaphragm and how their use impacts analytical results.  Flexible diaphragms are basically ignored analytically.  The stiffness of the diaphragm is negligible compared to the structural frames, so the diaphragm does not facilitate any interaction between frames, and each frame in the building can translate independent of the other frames.  This is perhaps the simplest structural analysis because there is no element or restraints that represent the diaphragm included in the analysis.

Rigid diaphragms are the opposite.  Rigid diaphragms are so rigid compared to the frames that they can be considered infinitely rigid. Analytically, the diaphragm can translate and rotate but cannot deform.  The frames are rigidly connected to each other by the diaphragm.  Computationally, this reduces the degrees of freedom of the model, and the analysis time is much quicker.

Lastly, the semi-rigid diaphragm falls somewhere between the other two.  A semi-rigid diaphragm has some diaphragm stiffness, and the diaphragm is able to translate, rotate, and deform.  This deformation influences how the loads get distributed to the frames depending on their relative stiffness and the stiffness of the diaphragm.  This is the most complex of all the analyses due to the many degrees of freedom and complexity of the interaction of the elements.  Allen cautioned against confusing the use of the terms flexible and semi-rigid. Some use those terms interchangeably, but the codes identify them as distinctly different classes. While a semi-rigid diaphragm is “flexible”, we should not confuse those classifications when discussing diaphragms.

 After Allen explained the basics of each type of diaphragm, he reviewed the advantages and disadvantages in this short video:


Identifying the three types of diaphragms is necessary to determine what is required by code for your structural analysis.