Diaphragm Design Forces ASCE7-05 12.10.1.1 2

[crozroz0000]

Equation 12.10-1 in Section 12.10.1.1 Diaphragm Design Forces of ASCE7-05 has the following equation:

Fpx=(nSUMi=x Fi)/(nSUMi=x wi)*wpx

It has been a while since college. I looked at Daniel T. Li's Seismic Analysis Based on CBC 2007 Chapter A found at

http://www.engineering-international.com/

(See attached screenshot of this spreadsheet)

But I am having trouble computing the diaphragm design forces (roof and floor) myself.

 

[msquared48]

Your method in the spreadsheet is not complete. You also have to consider the relative heights of the roof/floor masses above the ground surface to get the proper lateral force distribution to the individual diaphragms.

Mike McCann
McCann Engineering

 

[JAE]

From your main seismic analysis you will have a series of Fx values at each level in accordance with 12.8.3.

These Fx values in 12.8.3 are what 12.10.1.1 refers to when it says in the first sentence: "designed to resist design seismic forces from the structural analysis".

So your diaphragm must resist the forces from 12.8.3...but wait...you also have to AT LEAST design the diaphragm for the forces in equation 12.10-1 (which your spreadsheet apparently calculates).

...but wait again! Your values from Equation 12.10-1 don't need to be more than the given minimum of .2S_DSIw_px and don't have to be more than twice that.
Your spreadsheet appears to include these max and min limits.

 

[crozroz0000]

Thank you Mike and JAE.

JAE, that is a clear explination of the code. That helps a lot.

My next question:
I did not create this spreadsheet and it is locked. I am having trouble finding out how the Fpx column is calculated. If I could only remember how to use sums (F_px=(nSUMi=x F_i)/(nSUMi=x w_i)*w_px) I'd be in good shape.

I have tried to get the F_px column but I just can't seem to duplicate his answer.