Subject: basic lateral force question
Date: Thu, 7 Feb 2008 19:25:49 -0500
3 story office building with some CMU reinforced shear walls along the core of the building. On the second floor the floor extends out beyond the CMU walls on the north and south sides to have covered parking. Project in Florida, ZERO SEISMIC but 120mph wind.
A rigid diaphgragm (floor) of either a hollow core slab system or deck and concrete on steel joists.
I can use steel moment frames at the very N and S sides of the building, and then at the middle I will use some of the CMU shear walls and elevator/stair cores.
I have the SDI Manual and have been looking through it. Are there any limits on diaphgram cantilever I should be aware of (other than allowable shear force limits and deflection)? Both with steel deck, conc and steel deck, or hollow core?
***Not any other than what you have listed. Deflection is a bit difficult to calculate, but there are procedures.
I am thinking with hollow core I may need to increase the 2" topping and provide a reinforcing mesh in excess of what we normally would do for gravity and normal shear...
***I would agree.
Also, if I go the moment/braced frame route, how do I distribute the story shear between CMU walls and the frames? With the rigid diaphgrams at the floors I would normally assume this is a function of the rigidity of each shear wall and frame. Or can I just use trib area method? I know how to do this with all CMU walls, but how do I calc relative rigidities of CMU walls and steel frames?
***You should use relative stiffness. Jim Amrhein, "Reinforced Masonry Engineering Handbook" has some good guidance in this area. He has some coefficients of rigidity in the back that are helpful. You can find a used copy for about $25. New is about $150.
I am thinking the CMU walls are going to be MUCH stiffer than the steel frames, so that the steel frames would not see much of the load... I am talking about doing a torsional shear analysis to distribute the shear forces, but with a mixed system I am not sure how that would work. CMU- been there, done that, may go that route if possible.
Kind of need a starting point here because I think I can do it a few different ways.
Thanks in advance,
Andrew Kester, P.E.
ADK Structural Engineering, PLLC
1510 E. Colonial Ave., Suite 301
Orlando, FL 32803