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Re: Vertical Force Distribution - UBC97[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Vertical Force Distribution - UBC97
- From: Mlcse(--nospam--at)aol.com
- Date: Tue, 18 May 2004 01:05:15 EDT
I believe you still need to use 1630.5 for the vertical distribution of lateral forces, unless you can prove that the lower floor level (lateral resisting system) is 10 times stiffer than the upper floor level. This is likely hard to do if the footprint size of the roof is the same footprint size as the floor levels and your lateral resisting system is the same all the way up (moment frames, brace frames, shear walls,etc). You might be able to figure a lateral resisting system combination (upper floor level vs lower floor level) out that does meet this 10x stiffer requirement, but I don't think its very likely unless you are using wood shear walls. You would likely have to do a Dynamic analysis.
Thats one reason in the multi-story buildings using the static procedure you end up with a very large force acting at the roof (typically Dynamic First Mode effect for static vertical force distribution on the building). Our office encourages the building owner in these cases to use a concrete fill over the steel deck for a rigid diaphragm at the roof otherwise you can have a very large torsional redistribution of forces once you go from the flexible diaphragm (trib area) to the rigid diaphragm at the lower floor level below. The use of the rigid diaphragm also gets rid of the flexible diaphragm wall anchorage issue.
Its hard to make a flexible roof diaphragm work for a two story building with a rigid second floor, and I imagine even more difficult for a three story building (second and third floor rigid, roof flexible). To get the 10x stiffer part you might have to use plywood shear walls for the upper part and concrete shear walls for the lower part. Granted this is usually done for three and four stories of wood over one or two levels of concrete flate plate (rigid diaphragm) above grade. Of course plywood shear walls might not be allowed depending on the fire rating of the building.
In a message dated 5/17/2004 4:28:46 PM Pacific Standard Time, YI(--nospam--at)summit-sr.com writes:
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