What is the purpose of the "R" factor in seismic design calculations?

The "R" factor, or response modification factor, plays a crucial role in seismic design calculations by reflecting a building’s ability to withstand seismic forces based on its ductility. It essentially quantifies how much the design forces can be reduced due to the expected behavior of the structure during an earthquake. Buildings designed with certain materials and systems possess energy-absorbing characteristics that allow them to deform without collapsing. This ductility is essential because it enables the structure to dissipate energy during seismic events, thus providing stability and reducing the overall design forces needed to ensure safety.

By using the "R" factor in calculations, engineers can design structures that are both cost-effective and safe, as it allows them to reduce the seismic loads in their calculations, accounting for the inherent performance capabilities of the materials and design of the structure.