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# RE: basic lateral force question

• To: SEAINT <seaint(--nospam--at)seaint.org>
• Subject: RE: basic lateral force question
• Date: Fri, 08 Feb 2008 07:19:02 -0500

```Andrew,

```
When I have had rigid frames combined with shear walls I have calculated the deflection of each under a hypothetical lateral load (say 1 kip or 10 kips). I then simply use the inverse of this deflection as the rigidity of each element.
```
```
You are right that the shear walls will take most of the load. It is hard to make a rigid frame anywhere near as rigid as a shear wall.
```
--

Structural Engineer
Southern A&E, LLC

```
```From: "Andrew Kester, P.E." <akester(--nospam--at)cfl.rr.com>
To: "seaint" <seaint(--nospam--at)seaint.org>
Subject: basic lateral force question

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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? 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...

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? 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.

Andrew

Andrew Kester, P.E.
Principal/Project Manager
1510 E. Colonial Ave., Suite 301
Orlando, FL 32803
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