Abrupt change in elevational profile, such as in a building with tower and podium that creates a notch effect. A setback exists if the horizontal dimension of the seismic force-resisting system in any story is more than 130% of that in an adjacent story. Note that the Setback irregularity is a special albeit common case of Change in Vertical Structure, and therefore it has its own definition.

SET_diagram_Charleson_1

Simplified examples of typical building shapes that have setbacks (A. Charleson, Seismic Design for Architects, Architectural Press 2008, p154 fig. 9.21).

An example of a building with several setbacks (FEMA 2002)

An example of a building with several setbacks (FEMA 154, 2002)

SET_Canada_Brzev_1

A building with a setback, Vancouver, Canada (S. Brzev)

SET_Florida_USA_Brzev_1

A building with a setback, Fort Lauderdale, Florida, USA (S. Brzev)