1997 UBC SEISMIC CODE 1

[spike]

In formula (32)the base shear is computed using a ratio of hx/hr. hx is defined as the attachment elevation with respect to grade of the element or component while hr is defined as the structure roof elevation above grade.

What does this mean? If the unit is a hvac unit five feet above grade that is supported by steel beams, is hx equal to the roof elevation plus five feet?

 

[whyun]

Design seismic force is amplified at higher elevations in a building. Best way to describe it is "whipping effect".

If your building is, say, five stories with roof height at 60 feet and you have a wall mount unit at 5 feet above seismic base, your hr = 60 feet and hx = 5 feet. "roof elevation plus five feet" is incorrect.

If a unit is mounted, say on top of a penthouse roof which is higher than the main roof, hr need not exceed the main roof height.