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Missing Seismic/Wind Inputs in Build 6258

Missing Seismic/Wind Inputs in Build 6258

Missing Seismic/Wind Inputs in Build 6258

(OP)
In regards to Seismic inputs for horizontal vessels using building code IBC 2006 ground supported (or ASCE 7-05 ground supported) in Compress Build 6258 implementing ASME Sect VIII Div 1, can anyone tell me where to set the inputs for "spectral response acceleration over 1 second period" and also for "response modification factor?"  According to the manual, these should be displayed in the "Seismic Code" menu, but are not present in the current menu (perhaps removed from the latest builds?), or in the html generated saddle report.

I realize that these factors can be automatically interpolated from an ASCE table and are, in all likelihood, being left out as inputs in Compress for this general reason; however, I have a client in California with specific CA building code requests and need to, at a minimum, verify that the inputs meet or exceed their design requests.  Thanks in advance to any who respond.

- ShaZmoto

RE: Missing Seismic/Wind Inputs in Build 6258

(OP)
Thanks for the prompt answer, Tom! I'm posting Tom Barsh's reply here for others who may come across this same issue.

=============================
COMPRESS provides inputs for the S1 spectral response acceleration, response modification coefficient R, and long-period transition period TL, as shown in the screenprint below.
 
However, these inputs are not provided for horizontal vessels supported on two saddles. Factor S1 applies only to "flexible" structures, it does not apply to rigid structures. This is true for ASCE 7 and IBC seismic codes (the IBC adopts the seismic analysis of ASCE 7).
 
COMPRESS assumes that horizontal vessels supported on two saddles may be treated as a "rigid structure". Consequently, the input for S1 is not provided on the seismic code dialog for this type of vessel. Note that if you create a vertical vessel then an input for S1, etc, is provided.
 
The natural frequency of the horizontal vessel on two supports is sufficiently high that it can be treated as a rigid structure. In some cases the saddles may be unusually tall and flexible. In this case a better treatment might be to use the "building mounted" seismic code option in order to account for the flexibility of the supporting structure; the vessel itself is still treated as a rigid structure.

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