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Re: Point load on garage diaphram

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The 3000 lb force is a very conservative value for a residential garage (refer to ASCE 7 C4.8.3), and is likely to never happen; however, you still need to make sure that the floor assembly can take it. 
Your single concentrated 3000-lb force is another conservative assumption on top of the first one.  Realistically, you can assume the load as 2 forces of 1,500# each, placed for maximum moment (this is what it ultimately will be).  Then, for an 8-inch span, the maximum stress in the 1-1/8" plywood will be about 1,800 PSI.  This is less than the 1650 PSI*1.25=2063 PSI, OK.
If you go to a 6" joist spacing - which is not unheard of in such situations - the plywood will be good without the duration factor,  A Ford Excursion may be parked in your garage with one wheel on a jack forever. 
V. Steve Gordin, SE
Irvine CA
----- Original Message -----
Sent: Tuesday, August 04, 2009 10:02
Subject: Point load on garage diaphram

Designing a residential garage floor system. Wood framing, Lt-weight concrete over plywood. To design this to code a 3k pt load must be placed on an area 4.5 x 4.5 inches. After designing the joists & beams for this load I now must design the plywood spaning btw'n the joists for the 3k pt load placed at mid-span.

Even at 8" o.c. joist spacing & 1-1/8" Struct I plywood:

Max moment is 6.0 k-in & the allowable bending stress is 1650psi. Based on this I will need 1-5/8" thick plywood.  Am I missing something?

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