ASCE 7 requires that egress stairways that are required to function for life-safety purposes after an earthquake be designed for an importance factor of 1.5.

ASCE 7 defines a Risk Category IV structure as follows:

• Buildings and other structures designated as essential facilities.
• Buildings and other structures, the failure of which could pose a substantial hazard to the community.
• Buildings and other structures (including, but not limited to, facilities that manufacture, process, handle, store, use, or dispose of such substances as hazardous fuels, hazardous chemicals, or hazardous waste) containing sufficient quantities of highly toxic substances where the quantity exceeds a threshold quantity established by the authority having jurisdiction to be dangerous to the public if released and is sufficient to pose a threat to the public if released.
• Buildings and other structures required to maintain the functionality of other Risk Category IV structures.

There is no upper limit to S_DS and S_D1 in Seismic Design Category D and higher. Use the Web App or the Java App to determine S_DS and S_D1 for your location.