Effect of Soil Flexibility Modelling on Seismic Drift

Soil flexibility can have a significant effect on the behavior of a structure. ASCE 7-16-12.7.1 (Foundation Modeling) states, “For purposes of determining seismic loads, it is permitted to consider the structure to be fixed at the base. Alternatively, where foundation ?exibility is considered, it shall be in accordance with Section 12.13.3 or Chapter 19.” Therefore, structural engineers must decide the most appropriate analytical assumptions for the structure considering its construction details. The Figure illustrates four types of base restraint conditions that may be considered.

Higher ?exibility (pinned base) lengthens the period of the building, resulting in a smaller calculated base shear but larger calculated story drifts.

One drawback to the pinned-base condition is that the story drift of the frame, especially the story drift in the lowest story, is difficult to keep within allowable code limits. If the story drift of the structure exceeds acceptable limits, rotational restraint can be increased at the foundation by a variety of methods, as illustrated in the Figure.

For more information about ASCE 7-16 Provisions for seismic Drift Determination, refer to the article published on Structure Magazine through this link:

https://www.structuremag.org/?p=15358

The article can be also download through this link: 

https://www.structuremag.org/wp-content/uploads/2019/11/261912-C-CodesandStandards-Alsheikh-1.pdf

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