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- Subject: [SEAOC] Re: [SEAOC] Basement wall with adjacent slab on grade - Help
- From: HARRISENGR(--nospam--at)aol.com
- Date: Fri, 5 Jul 1996 16:16:01 -0400
Dennis : Here in Thousand Oaks ( also called the Conejo valley ) ,. the situation i believe you are describing occurs often on sloping hillside lots where a lower floor or garage has the backside , usually the longer side of rectangle, up against the slope with a full height ( 8' or 9' ) retaining wall. The two side walls are sloping retaining walls from full heaight to 0 .The front side has no retaining , as where cars may be driving into the garage from the street. Above this is a second floor ( or the lowest floor is a basement and the next floor up is called a first floor ) . The second floor is usually larger than the lower floor so there is a floor transition over the rear retaining wall from the slab on grade to the wood floor over the garage or lower floor. I am assuming this is your condition. I don't know if what i do is specifically applicable to your job but i can describe what i do. I have had a problem with a similar condition with a retaining wall where i used a cantilevered wall. The owner called me out 6 months after it was done to see a crack at the top where the cantilever deflected .5 to 1 inch. Because the toe is under more pressure than the heel, it may settle more and be exagerated by the footing width to wall height ratio , as well as normal deflection of the cantilever. On the other hand , since the wall will be backfilled before attachment to the slab , it can not be a simple span. Usually , the wood diaphragm is long and relatively narrow so retaining wall forces may overload it, as there is no earth on the open side pushing back. Also there is usually a stairwell opening in the diaphragm. Another problem is the floor transition over this area since it seems everyone wants tile now days. (What ever happened to wall to wall carpet with thick padding?) . As the wood shrinks with drying, the masonry does not. The truss joists (TJI) minimize shrinkage but there is still the ledger. I try to discuss this transition with the architect to allow for this natural crack area. So, the design i use is conservative to minimize these problems. I use a fully cantilevered wall with toe pressures well under the soil report max. Then i add rebar as required for a simple span condition ( so there is rebar in both faces of the block ). I use the 12" block a couple courses higher than minimum so the block transition from 12" to 8" is not occuring at maximum simple span moment. At the top, i hold the wall short the ledger height and pour a thickened slab over the top with the simple span rebar bending into the slab. The ledger bolts are embedded in the thickened concrete slab above the wall . You could probably save a thousand or two by sharpening your pencil ( and using your computer ) but i see the houses around here go for nearly a million dollars or more and if there is a problem in the foundation it is expensive or not practical to fix. Non deputy inspected masonry is not very accurate anyway. If someone has a better detail at the top connection that may reduce the transition crack, i would appreciate hearing about it. Hope that helps. You didn't leave a fax number to send a detail and since my staff ( computer people ) are off today, i don't know how they e-mail details. Maybe Mon. Tom ...
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