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Re: UBC 97 Drift
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- Subject: Re: UBC 97 Drift
- From: Eddie Gonzalez <Eagonzal(--nospam--at)ENG.CI.LA.CA.US>
- Date: Fri, 26 Mar 1999 08:30:45 -0800
Mr. Serhal, Based on my continually developing understanding of UBC97, drift calculations are primarily based on load combinations found in 1612.2.1, particularly 12-5 & 12-6, as noted in Section 1909.2.3. Exception 2 of 1612.2.1 is not recommended to be applied per Seismology Committee per Dr. Ghosh's paper in Building Standards. Section 1612.3 do not apply. 1612.4 is for increasing forces on special elements. One may use Section 1928 as alternate but 1) alternate strength reduction factors must be used 2) Correct error of 1.5 load factor for E in LC 5 & 6. Section 1630.9.1 specifically calls for a mathematical model in accordance with Section 1630.1.2 which specifically requires that "1. Stiffness properties of reinforced concrete and masonry elements shall consider the effects of cracked sections." One must also account for tranlational and torsional deflection, as well as panel zone deformation (steel bldgs) when calculating Ds I believe your I=0.35Ig you obtained from recommended values when calculating magnification factors of slender columns, Section 1910.11.1. If this is what you are checking than it seems in accordance with the code. For Shear walls, you must comply with Section 1921.6.6.5 where Dt may equal Dm and you must calculate the curvature requirements. I believe there is a requirement of limiting the stiffness of the boundary elements but I can't find it right now. The draft Blue Book seems to recommend a revised formula of Dt as Dt = RDs, such that Di = Dt - Dy You are right. The requirements are very stringent. Compare the UBC 97 with UBC 94 (incorrectly assuming I=Ie=Ig): D97 = 0.7RDs = 0.7R(1.4D) = 0.7 = 1.87 D94 0.375RwD 0.375(1.4R)D 0.375 Some expect that this large deflection results may pose a challenge when addressing deformation compatibility and banging into adjacent buildings during design. My 0.02 cents, ed gonzalez >>> Sleiman Serhal <mony(--nospam--at)destination.com.lb> 03/26/99 12:51AM >>> When computing Ds for a shear wall according to the UBC 97, do u use in your mathematical model an I of 0.35Ig if the wall cracks under elastic forces - due to E only It seems to me that this would be too penalizing especially that we multiply Ds by the R factor to get Dm which is the value actually used to determine if the allowable drift has been exceeded. Some engineers tell me they use an I = 1Ig ! And is the 1.2D + f1L + E including P-D the only combination to use for drift calculations ? Thanks in advance, Moni Serhal
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