If I have a steel tower or masonry shaft supporting an egress stair, do the structure needs to be designed for importance factor 1.5? I do not see anywhere in the code that explicitly defines this.
Your question implies the exterior egress stairway that are not an integral part of the building structure .
In this case, Clause 13.1.3 is applicable . The below excerpt , Copy and paste from the standard.
13.1.3 Component Importance Factor.
All components shall be assigned a component Importance Factor as indicated in this section. The component Importance Factor, Ip, shall be taken as 1.5 if any of the following conditions apply:
1. The component is required to function for life-safety
purposes after an earthquake, including fire protection
sprinkler systems and egress stairways.
.....
Use it up, wear it out;
Make it do, or do without.