ASCE 7-22 Table 1.5-1 & 1.5-2 | Risk Categories and Importance Factors
Not all buildings are equally critical. A grain silo (Category I) carries lower risk than a standard office building (Category II), an auditorium or school (Category III), or an essential hospital / fire station (Category IV). Essential structures must be designed for higher design loads so they remain operational after disasters.
Governing Formula
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Notation & Variables
Worked Example
Classify structure
A regional emergency operations center and 911 dispatch facility is designated as Risk Category IV (Essential Facility).
Look up Table 1.5-2
From Table 1.5-2, the seismic importance factor is Ie = 1.50 and snow importance factor is Is = 1.20.
Apply to seismic base shear
Base shear coefficient Cs = (SDS / (R / Ie)) scales up proportionally by 1.50, increasing design lateral force by 50%.
When does this govern?
Risk Category classification is the fundamental starting point of all structural design criteria under the International Building Code (IBC).
Common Mistake
Classifying an occupied assembly space with > 300 occupants as Category II instead of Category III, which violates IBC §1604.5.