Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Vertical Force Distribution - UBC97

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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.
 
Mike Cochran
 
 
In a message dated 5/17/2004 4:28:46 PM Pacific Standard Time, YI(--nospam--at)summit-sr.com writes:
UBC97 distribute seismic force vertically, basically using the ratio of floor weight times the height.   Consider a three story building, if the roof is a plywood diaphragm, and the floors below are all concrete deck, e.g. rigid diaphragm.  Would the vertical force distribution still occur as described in UBC97, 1630.5?  That is, does the lateral force in the roof diaphragm need to be calculated according to section 1630.5 of UBC97, which mostly like means a higher force than the base shear calculated at the roof level?
 
SEAONC bluebook says that when the structure deformation differs significantly from the assumed liner mode behavior, in this case the roof diaphragm will deform a lot more than the concrete diaphragm below, consideration should be given to alternate methods of distributing the forces sucha s dynamic analysis. 
 
Is there any other method you use in this situation, to avoid doing a dynamic analysis?  Do you just ignor the vertically distributed force in UBC97, and use just the base shear at the roof level as the design force?
 
Appreciate any comments.
 
 
Y i   Y a n g,   P. E.             
STRUCTURAL DIVISION
SUMMIT ENGINEERING INC.
707.527.0775.x15
Santa Rosa, California