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- To: seaint(--nospam--at)seaint.org
- Subject: Propped cantilever foundations was RE: plywood diaphragm restrained fdn wall
- From: Jim Wilson <wilsonengineers(--nospam--at)yahoo.com>
- Date: Thu, 12 Jan 2006 19:28:36 -0800 (PST)
Bill Allen <T.W.Allen(--nospam--at)cox.net> wrote:
Neil ?In the (relatively few) cases I?ve been involved with, I?ve had close coordination with architect and contractor where I?ve been able to feel comfortable that the construction sequence was going to be correct. The way I?ve been able to ?sell? this is by showing the architect (and contractor) the additional hardware that would be necessary to reduce the footing size (the first thing they see). After I walk them all the way through the alternative, I have always gotten my way. And I do verify this condition on the jobsite. I can?t tell you how many times I say (and write): ?Backfill before floor framing?.Regarding the reinforcing on the inside face, in my cases I?ve managed to design the walls with the steel at the center (thicker block and more steel) and have checked the wall section for a propped condition (just not designed or detailed for it). That being said, I have designed most of my basement walls for live load surcharge, etc. for a propped condition and have designed the connection of the floor joists to the masonry wall (as well as checking the wall reinforcing) for this condition. It?s a superposition thing which isn?t very difficult.Regards,T. William (Bill) Allen, S.E.Consulting Structural Engineers-----Original Message-----
From: Neil Moore [mailto:nmoore(--nospam--at)innercite.com]
Sent: Wednesday, January 11, 2006 11:04 AM
Subject: RE: plywood diaphragm restrained fdn wallBill:
This has been an issue in El Dorado County for some time. What some of the engineers are doing are exactly what you are doing, but they are also designing the wall as propped, which would require some tension steel on the inside of the wall. Then the floor diaphragm does have to be designed for the lateral loads. You might check with your geotech also.
The problem is usually no matter what you design it for, it will get built different or the sequence won't be paid attention to anyway.
Neil Moore, SE, SECB
neil moore and associates
consulting structural engineers
shingle springs, ca
distressed structures investigations
At 10:39 AM 1/11/2006, you wrote: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
From: Joe Grill [ mailto:jgrill(--nospam--at)swiaz.com]
Sent: Wednesday, January 11, 2006 8:45 AM
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.
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
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