Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

[SEAOC] Fwd: [SEAOC] Basement wall with adjacent slab on grade - Help

[Subject Prev][Subject Next][Thread Prev][Thread Next]
---------------------
Forwarded message:
Subj:    Re: [SEAOC] Basement wall with adjacent slab on grade - Help
Date:    96-07-05 16:16:16 EDT
From:    HARRISENGR
To:      seaoc(--nospam--at)power.net

     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 

...