Structures assigned a Seismic Design Category D, E, or F must provide structural observations if the structure height exceeds what limit?

For structures assigned a Seismic Design Category of D, E, or F, the requirement for structural observations is critical, particularly for taller buildings, due to increased risks associated with seismic activity in these categories. The threshold for requiring these observations is set at a height of 75 feet.

This requirement is implemented to ensure that the structural integrity of the building is closely monitored during construction, as buildings of this height in high seismic zones are more susceptible to structural issues under seismic loads. By mandating structural observations at this height, codes aim to ensure compliance with design specifications, construction quality, and safety measures which are essential components for mitigating earthquake impacts.

The options that suggest heights above or below this limit do not align with the established building codes aimed at enhancing safety during seismic events. The choice of 75 feet as a threshold effectively balances the risks involved with construction in high seismic areas and the need for practical oversight.