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Re: Garage load on composite deck + concrete

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Kevin,
 
I was also very skeptical about what I inherited with this morning.  The problem became mine this morning without warning and needed to make a decision as quick as I could.  Actually, it was the contractor that questioned the slab thickness and called the office.  A good call on his part.  Masons were working quickly vertically and the contractor (a pretty good guy) was wanting to make sure they had beam pocket/plate elevations fixed.  He was hoping he didn't have to stop work.  I think I did the right thing.  As I replied to Paul's post I was very concerned about shear, and not so much flexure.  I told him I thought we needed more slab thickness and we could accommodate that.  Not a huge area, so not a huge increase in cost.  Monday I will have to talk to him about reinforcing and cracks over the beams.  I think he will be acceptable to that.
 
Joe Grill
 
 
----- Original Message ----- From: Kevin Elbury
Sent: Friday, April 30, 2004 5:29 PM
Subject: RE: Garage load on composite deck + concrete

The Brits allow you to use an effective width being a parabolic curve from support to support, with the maximum effective width being '0.6L+W' at the mid-span and 'W' at the support where 'W' is the width of the load (i.e. say 10" for a wheel)
 
For points at a distance of 'X' from the support, the effective width is W + 2.4*X*(1 - X/L). This makes sense, the farther you go out into the span, the less stiffness you have and hence more lateral distribution.  I can't find anything about what minimum transverse moments you would have to design for to achieve this though.
 
I am skeptical that a 4" deck has sufficient transverse bending or shear capacity (i.e. only 2" of concrete in this direction) and in fact, shear capacity will likely govern as the wheel leaves the transverse support beams.  I think a thicker slab is in order.  I've said this before in previous posts on residential garage floors - what about the guy who backs his one ton truck full of gravel onto *your* garage slab.
 
Kevin Elbury


From: John P. Riley [mailto:jpriley485(--nospam--at)peoplepc.com]
Sent: Friday, April 30, 2004 1:24 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Garage load on composite deck + concrete

JG:
 
Just shooting from the hip here, but I think you can consider that the load is carried by a width of slab 40% of the span.  In other words, 0.4(6')= 2.4' in lieu of the 1' wide section you mentioned.  I don't remember where the 40% came from, but I remember that I did something like that with cored slabs in the past.
 
John Riley
-----------
 
 

I was just presented a design (done by another) for a 2? composite deck with a concrete topping of 2? over the deck (4? total).  The deck and concrete system is supported by steel beams spaced at 6? o.c..  This design was for a residential garage.  I did preliminary calculations equating the moment from the allowable superimposed uniform loads (from the deck manufacturer) with the moment from a concentrated load as required by both the 2000 IBC (2000#) and the 2003 IBC (3000#) assuming that the load will act on a 1? wide section.  I found the deck system will not work.  What do others of you do?  It looks to me like the concrete thickness will have to be increased to where a reinforced slab thickness above the deck will be adequate by itself for flexure and shear.  On the other hand, one of the manufacturers I spoke with this morning suggested getting a copy of the ?Composite Steel Deck Design Handbook? by SDI.  Will that be of help in this instance?   Of course this is an ASAP thing as construction is continuing and I don?t have a copy.

 

Thanks,

Joe Grill