Backstay Effect in Multistory Buildings

Abstract

Multistory buildings comprising podiums, basements, and setbacks can experience the so-called backstay effect when subjected to lateral loading. Backstay effects arise when there is a discontinuity in one of the lateral force-resisting systems in a building. This discontinuity results in the migration of the shear force carried by one system to another via a transfer diaphragm. Such effects are commonly observed in tall buildings with basements surrounded by perimeter walls.

In this context, the perimeter basement walls act as unintended structural backstays, resisting lateral forces and transferring them down to the foundation. The backstay effect is particularly problematic during seismic events, where sudden dynamic loads can amplify force migration. Engineers mitigate this by analyzing and detailing the transfer diaphragm, with tools like the stringer panel method.