Garage Slab & Basement Foundation Wall

[kxa]

I have a situation. In a residential house, two sides of the garage slab are located right next to the 8’ high basement foundation wall. I was wondering if I should do anything to make sure there is not a lot of lateral load being transferred to the basement wall. Is there any standard detail for a situation like this? Should I step the wall footing?
Thanks in advance.

 

[COEngineeer]

Is it slab on grade? I am not sure what you are asking. Just design the wall for that wall depth, make sure they brace the wall when they backfill the foundation. I wouldnt worry about surcharge from the garage. If the span of that 8 ' wall is pretty long (20' maybe), its a good idea to put buttress underneath the slab on grade.

http://www.krmconsultants.com

 

[kxa]

The garage slab is at grade and the length of the basement wall in the back is 21'. To the other side, the length is about 6'. I am concerened about the 21' full basement wall in the back. I plan on stepping the footing from 3' below for the garage to 8' (2' steps) and increasing the thickness of the basement wall in the back fromm 8" to 12".

 

[COEngineeer]

21' then you either have to increase the footing (3' wide maybe) and maybe make the thickness of the wall to 10", OR you can build a buttress undeneath the slab on grade to counteract the overturning moment and keep your footing small and wall thickness 8".

http://www.krmconsultants.com

 

[kxa]

Thanks for your help. I'll specify a 10" wall with a 3' buttress in the mid-span. The floor joists are prependicular to this wall which should also help.
Thanks again.

 

[csd72]

The butress sound expensive. Why not sit the edge of the slab on a corbel and design the slab to span 8' past the 45 degree zone of influence.

 

[COEngineeer]

I think I used the wrong term, it is called counterfort. It is almost like add 3' long 8' high wall. I dont think it is that expensive.

http://www.krmconsultants.com

 

[csd72]

Wouldnt it be cheaper to thicken out the garage slab a bit, put some bottom reinforcing and sit the end of this on the edge of the 10" wall.

The detail at the top of the wall is a bit tricky but you are doing all this work at ground level not in a 10' excavation (with associated digging shoring e.t.c.).

I have even used a similar detail on industrial slabs with forklifts.

That is just my opinion.

 

[kxa]

I was wondering about that but we were all on the same page. The slab will be 6" thick and about 9" thick around all four edges with min. 1-#5 rebar. I don't want to just rest the end of the slab on the subject wall because I may do more damage. One counterfort in the middle shouldn't be that bad in terms of construction cost.

 

[oldestguy]

While you are at it don't let the contractor jet the backfill to the wall.

The result could be a basement full of mud and concrete slabs.

I'd even keep compactors 2 feet plus away from the wall.

 

[jfm77]

Lateral pressure against long foundation wall adjacent to garage slab is often overlooked in residential design. Weight of vehicle wheels, and heavy storage loads, should be considered "surcharge" load, unless slab is properly designed to be supported on the foundation wall. Even then however, vertical load from edge of slab can cause bending moment (due to eccentric load on wall) that adds to bending moment due to inward lateral soil pressure.

Most direct way to avoid this problem is to thicken foundation wall (garage slab side) to form a ledge for supporting slab. Ledge to be 4 inches wide. Load from slab is then distributed directly down to footing (which must of course be widened). Slab must be designed to "span" between foundation wall and firm soil well beyond backfill zone (recommendat minimum "bridge" distance to be 6 feet).......Nominal reinforcement (#4 bars or WWF at center of 6 inch slab) can be adequate......though better design solution is simply to thicken slab within the "bridge" zone, to at least 8 inches for residential garage use.

If slab is "floating"......butts up to foundation wall.....then the wall must be buttresed (with adequate number of piers) or designed to resist lateral pressure (soil + surcharge). This will generally require reinforcement of concrete block wall (vertical rebars). However, as with any foundation wall that must resist lateral pressure, lateral bracing at top of wall is essential for proper design and performance.

Therefore, the increased wall thickness solution ends up being most effective......next "best" is likely "brace-piers" spaced about 6 to 8 feet (depending on wall & soil height). Both of these design solutions minimize lateral force that must be resisted along top of wall.

For concrete block wall (even with brace-piers)...12 inch thickness is highly recommended.

As with any lateral pressure conditions, a concrete wall has much better resistance......since concrete can of course resist lateral pressure in two directions. However, reinforcement / bracing may very well yet be appropriate unless increased wall thickness (to form ledge) is used.