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RE: Point load on garage diaphragm

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Read the ASCE commentary on concentrated live loads, C4.3.  The answer to your question is no, you are not required to design the plywood for 3000# over 4.5"x4.5"; that provision is intended to apply only to primary framing members.  You don't necessarily even have to design the floor joist for it, as it is a repetitive member, and the failure of one joist would not cause a collapse.  

HOWEVER, this load is designed to mimic a floor jack lifting a car, which seems likely on your floor, so aside from what you are required to do, I would design all components for this load.  And I would use normal-wt concrete w/ bars to distribute this load among members (and take care of plywood concerns).  Your wearing surface will be much more durable, too.

Gordon Goodell
Alta, WY

-----Original Message-----
From: erik_g(--nospam--at) [mailto:erik_g(--nospam--at)] 
Sent: Tuesday, August 04, 2009 11:03 AM
To: seaint(--nospam--at)
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?