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RE: plywood diaphragm restrained fdn wall

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Joe –


Whenever I’ve designed basements (probably not as frequently as you), I’ve always designed the walls as cantilevered walls and required the backfilling (as well as the subsequent compaction) be performed prior to installing the floor framing and floor diaphragm. Regardless of the opposing forces reducing or eliminating the diaphragm loads (I’ve worked on projects where the back wall of a garage was retaining so there would have been high diaphragm loads), there still is the issue of putting the floor joists in compression (kl/r issue) and then the whole problem in the perpendicular direction with the blocking probably creating a sub-diaphragm regardless of opposing forces. Also, transferring that top of wall force into the floor diaphragm is not insignificant.


A cantilevered footing is larger, there is more steel in the wall but the above problems go away. Besides, crews can get to the structure easier if the backfill is in place rather than specifying that the floor framing and diaphragm shall be installed prior to backfilling. That construction sequence makes the area hard to get to.


Just my two cents.


Currently 63 degrees and sunny here in SoCal.


T. William (Bill) Allen, S.E.


Consulting Structural Engineers

-----Original Message-----
From: Joe Grill [mailto:jgrill(--nospam--at)]
Sent: Wednesday, January 11, 2006 8:45 AM
To: seaint
Subject: plywood diaphragm restrained fdn wall


I am working on a foundation for a residential structure for a co-worker in our office.  I took the opportunity to try to set it up on Risa3D as we are trying to minimize the CMU (here most fdn walls are CMU) thicknesses.  By the way, this morning the local axes display miraculously worked (don’t know what I was doing wrong yesterday).  Due to the depth of the foundation he will probably still have to use 12” CMU and use the floor diaphragm as restraint at the top of the wall. 


If a person assumed a rectangular box basement with the same fill on opposing sides then the diaphragm/floor assembly will be in compression, and in my case about 500 plf to the diaphragm.  The floor joists will be perpendicular to this direction so I will suggest he install some blocking at some spacing to get nailing into the diaphragm.


I guess this is another one of our questions regarding the IRC code.  The IRC would allow this for a 9’ high basement wall with no further consideration in the floor diaphragm.  He has 10’ walls.  Will the plywood diaphragm take this magnitude of compression?  It seems that historically it will.


Any comments?




Joseph R. Grill, P.E. (Structural)

Shephard - Wesnitzer, Inc.

Civil Engineering and Surveying

P.O. Box 3924

Sedona, AZ  86340

PHONE (928) 282-1061

FAX (928) 282-2058