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RE: IBC 2003 Seismic

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All of the answers are contained in the NEHRP Provisions and Commentary. I would suggest that you look through these documents to help you understand the rationale. This issue was discussed at length during committee meetings and the reasoning was developed and placed in the NEHRP.

Harold Sprague

From: "Larsen, Trevor" <tlarsen(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: IBC 2003 Seismic
Date: Thu, 9 Mar 2006 08:21:20 -0600

Any insight at all on this topic would be greatly appreciated!

1. 1615.2.1 defines S(aM) as the maximum spectral response. 1615.2.4 defines S(aM) as the median spectral response. Which is it? Is there a relationship between the S(a) given in the general section and the S(aM) given in the site specific section?

2. 1615.2.5 defines S(DS) and S(D1) in terms of S(a). It's my understanding that S(DS) and S(D1) would be used in the tables to pick a design category. My question is: what is the purpose of the statement S(MS) and S(M1) shall be taken as 1.5 times S(DS) and S(D1) respectively? In 1615.1.2 S(MS) and S(M1) Are calculated and then multiplied by .667 to get S(DS) and S(D1). The "flow" seems to be, "find S(MS) and S(M1) and then calculate S(DS) and S(D1)", not the other
Way around.

3. 1617.1.1.1 talks about the effect of gravity and seismic being additive or counteracting on another. Please explain how you add or subtract a horizontal load and a vertical load. Is there a simple example of what these equations are trying to convey?



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