Seismic Calculations per ASCE 7-05

[Irontrussman]

We build mostly R1 and R2 for the New England area and therefore aren't usually concerned about seismic calculations, but now I have a Daycare center going into Massachusetts. I'm trying to correlate the Mass. 780CMR code values with those obtained from ASCE 7-05. My question: in ASCE 7-05, equation 12.8-7, what is the definition of the upper limit of h_n, the highest level? Is it to the top of the roof ridge or is it to the uppermost diaphragm at which a seismic lateral force is to be applied? And is this the same height required for 780CMR, section 1612.5.1.2.1?

 

[ssweetland]

My take on this is that hn is taken to the center of mass of the highest diaphragm. In your case, a pitched roof, would be about mid height between eave and ridge.

 

[Irontrussman]

ssweetland,
Thanks for your reply.
Are you familiar with 780CMR? My thinking is that h_n is to the highest connection of the vertical elements of the seismic resisting system to the diaphragms that comprise and transmit the inertial loads to those vertical elements. A lot of 780CMR looks like it was lifted directly from ASCE7, with some differences on percentage of snow load to be used, and they give values for effective peak velocity-related ground acceleration (A_v) and effective peak acceleration (A_a) both equal to 0.12g. Are these the same quantities as S_s and S₁ used in ASCE7?

 

[ssweetland]

Sorry, not familiar with 780CMR.