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ASCE 7-05 13.3.1 Seismic Design Force - z/h Question

ASCE 7-05 13.3.1 Seismic Design Force - z/h Question

ASCE 7-05 13.3.1 Seismic Design Force - z/h Question

I have a question regarding the z/h term in equation 13.3-1 for calculating the seismic design force for nonstructural components.  

I think I understand the definitions of z and h and the purpose of the z/h term in the equation.  However, what I find unclear is why the increase in the seismic design force for a nonstructural component on the 2nd floor of a 4 story building is the same as if the same component is located on the 4th floor of an 8 story building.  It seems the absolute height above the base of the structure would have some affect on the seismic design force.

What am I missing here?  Any input is greatly appreciated.
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RE: ASCE 7-05 13.3.1 Seismic Design Force - z/h Question

Taller buildings have longer periods, and thus less actual seismic demand.  That is one aspect.

Another is that the z/h factor is trying to consider the whipping action of a building...sort of like if you've ever snapped a wet towel at someone -the tip of the towel (z/h = 1) moves at incredible speeds while the part halfway down the length (z/h = 0.5) is considerably slower.  So a 3 ft. towel vs. a 6 ft. towel may not produce all that much different velocity.

Now...no bathroom humor is allowed after this post  smile


RE: ASCE 7-05 13.3.1 Seismic Design Force - z/h Question

Thanks for the reply JAE.  I was not taking into consideration the effect of a taller building's longer period on seismic loads.  Also, I guess I need to think more in relative terms and less in absolute ones.

RE: ASCE 7-05 13.3.1 Seismic Design Force - z/h Question


I was once in a room when a mild one struck(Richter magnitude around 4.5). Yes, everything in the room including my skull was whipped and I can clearly understand your point of view now.


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