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Re: Dynamic drift calculations (1998 CBC/1997 UBC)
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- Subject: Re: Dynamic drift calculations (1998 CBC/1997 UBC)
- From: Jake Watson <jwatson(--nospam--at)inconnect.com>
- Date: Fri, 15 Dec 2000 08:52:37 -0700
1. All drift calculations are at "Strength" level. You must leave the 1.4 factor in. 2. My response is an opinion here - no code section to back it up. I would reduce the load to the appropriate level (80%,90%,100% etc.) before deflection. My interpretation is that we design for "design level" forces. So calculate Delta S with design level forces, then magnify by 0.7R to get delta M. Then divide by 1.4 to get to WSD. If you are doing dynamic design note two other things: first, you must design for forces in both the X and Y axis simultaneously. You can use the SRSS method or 100% in one direction and (I think?) 40% in the other. Secondly, if you are using concrete shearwalls you are only allowed to assume 50% of Ig for stiffness (see concrete shearwall section) when calculating deflection. This can have a huge impact on secondary moments. 3. Opinion again here - I would say include and move on. It's not likely to make a difference on most typical buildings for deflection and no one will question you. If it makes a difference it might be worth a call to one of the code Gods at ICBO to find out. Hope this helps. Jake Watson, E.I.T. Salt Lake City, UT "Harper, Mark" wrote: > > Questions: (Reference 1998 CBC VOL 2, based on the 1997 UBC) > > 1. Section 1630.9.1 refers to section 1612.2 for load > combinations to be used with Allowable Stress Design. Does this mean that I > take the dynamic base shear and scale it as required to the base shear > obtained in section 1630.2 without dividing by 1.4 for the drift > computation? > 2. When calculating the drift using a dynamic analysis, and > working stress design do I scale down the load to (80, 90, or 100% depending > on building type) of the static base shear (section 1630.2) to obtain delta > S then multiply that value by .7R to obtain delta M. > That is Delta M = Delta S*.7*R (reference 1630.9.2) > > Then when calculating the forces in the members scale down > the dynamic forces by an additional 1.4 to obtain working stress forces. > > 3. When calculating "E" (section 1630) ROW is equal to 1.0 for > drift calculations and Ev =0 if designing for working stress. However, does > Ev have to be taken into account for drift design? > Example; Dynamic base shear gets scaled down to E = Eh or > gets scaled to E = Eh+Ev, where Eh = the base shear obtained in section > 1630.2? Because the drift calculation "Delta S" appears to use ultimate > design loads I would think that Ev must be included in the design base > shear. However section 1630.9.1 refers only to section 1630.2.1 which would > make me think that the vertical component Ev is not to be included for drift > calculations! > > Thanks for your help! > > Mark Harper PE, SE > HDR INC. > mharper(--nospam--at)hdrinc.com > Phone: (626) 564-2820 > Fax: (626) 564-2873 > -- Jake Watson, E.I.T. Salt Lake City, UT
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- Dynamic drift calculations (1998 CBC/1997 UBC)
- From: Harper, Mark
- Dynamic drift calculations (1998 CBC/1997 UBC)
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