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1997 UBC Seismic
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- Subject: 1997 UBC Seismic
- From: George Barclay <gbarclay(--nospam--at)lgt.lg.com>
- Date: Mon, 27 Apr 1998 16:32:17 -0400
I am working on a new design using the 1997 UBC. (one story with a partial basement, ordinary steel braced frame) This is the first time I have used the '97 UBC and I am somewhat confused by a particular topic in the seismic provisions. In section 1630.1.1, there is a new "seismic force amplification factor" (omega) that is required in the equation for Em (eq. 30-2). Em is defined as the "estimated maximum earthquake force that can be developed in the structure". The equation for E (eq. 30-1), does not require this 'omega' factor and E is defined as the "earthquake load on an element of the structure resulting from the combination of horizontal component, Eh, and the vertical component Ev". These definitions tend to lead me to believe that eq. 30-1 is to develop the seismic forces on equipment, components and cladding or nonbuilding structures and eq. 30-2 is for the entire building system. Backing up to sections 1612, "Combinations of Loads", sections 1612.2, 1612.3.1, and 1612.3.2 give the various load combinations for strength design, allowable stress design and alternative load combinations. These combinations all use the term "E" for earthquake. Section 1612.4, "Special Seismic Load Combinations", gives two additional combinations that use the term "Em" for earthquake. I assume that these are in reference to equation 30-1 for "E" and 30-2 for "Em". Section 1612.4 states that these combinations "shall be used as specifically required by Chapter 16, Division IV, or by Chapters 18 through 23". The problem, in my opinion, is that chapter 16, Division IV, (seismic provisions) is not clear when to use this "Em". Section 1630.3.1 states that the 'omega' factor shall be used for "specific elements of the structure, as specifically identified in this code". Unless I have missed it, the body of Division IV of chapter 16 does not mention this 'omega' factor again until section 1634.5, "Other Nonbuiding Structures". Is this the only type of building element for which the 'omega' factor is used? If so, then why are these 'omega' factors included in table 16-N, which are the R and 'omega' factors for entire structural systems? I used the 1994 UBC to develop the base shear and the number turned out to be 5.1% of the building weight. I then used the 97 UBC, including the 'omega' factor and divided by 1.4 in order to compare with the 94 UBC and the number turned out to be 19.1%. This seems to be an unreasonable increase. I then removed the 'omega' factor from the 97 UBC number and this resulted in a base shear of 6.9% of the building weight. This would seem to be a more reasonable increase over the 94 code. Any guidance on this issue would be greatly appreciated. George Barclay, P.E.
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