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RE: basic lateral force question

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This is what I have done as well.  You can then use that "relative rigidity"
with a stiff diaphram analysis that distributes loads proportional to the
relative rigidities in combination with the relative torsional
rigidities/effects.

And while wind might govern overall, you will still likely want to run the
seismic numbers as minimum seiscmic loading may still lead to higher loads
to use for the moment frames due to minimum load distribution to the moment
frames required for "dual systems" in the seismic provisions.  In other
words, the requirements that for a dual system of moment frames with shear
walls, the moment frames must be designed to take at least 25% of the
seismic load no matter what happens relative rigidity wise.  Thus, while it
might be likely that the overall seismic loading might be less than wind, it
is possible that 25% of the seismic loading might still be more than what
get proportioned by relative rigidity from wind.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Adam Vakiener [mailto:avakiener(--nospam--at)southernae.com] 
Sent: Friday, February 08, 2008 7:19 AM
To: SEAINT
Subject: RE: basic lateral force question


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.

-- 

Adam Vakiener, P.E.
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
> 
> This is a multi-part message in MIME format.
> 
> ------=_NextPart_000_01DB_01C869BF.3F1D6460
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> charset="iso-8859-1"
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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.
> 
> Thanks in advance,
> Andrew
> 
> 
> Andrew Kester, P.E.
> Principal/Project Manager
> ADK Structural Engineering, PLLC
> 1510 E. Colonial Ave., Suite 301
> Orlando, FL 32803



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