Slab bearing ledge on stem walls


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Old 06-21-06, 08:29 AM
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Slab bearing ledge on stem walls

For attached garages and concrete porches, are they required? Just recomended? I guess all walls would be ideal in theory, but whats common practice? And how the heck is it done on an ICF foundation?
 
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Old 06-21-06, 03:55 PM
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Slab bearing ledge on stem walls

You have two choices for walls if you are considering a garage area.

The first is a floating slab. This is supported by the soil and is independant of the walls. This is usually the most economical in terms of reinforcement and thickness.

The second is a slab supported by a ledge on the walls. Since the soil under the slab will eventually settle somewhat (it doesn't make any difference how much), the slab ends up spanning between the two, three or four walls that support it. This slab should be designed as a structural slab and is usually thicker, has more reinforcement and the placement hieight of the steel in the slab is more critical.

If you are a "quick and dirty" builder, you build a slab on grade, but set it on the wall ledge. This works OK for a basement with light loads, but for garages most builders are gone when the problem really occurs.

For ICF system, each system will have its own suggested details for the slab support. Some systems have good engineering support and details and some systems just graduated from making foam plastic meat trays for Walmart hamburger and have no idea of construction. Because an ICF system is hidden from any visual inspection, pay particular attention to the critical areas that can easily have poor concrete placement and voids (corners, opening and under ledges that carry loads).

In all cases the design of the slab supported on its edges depends on the size, connections, support and loads.

I just bought a townhouse with a badly cracked slab on grade suported by a ledge with my eyes wide open. I tore out and replaced the slab and found several inches of air supporting a good portion of the old floor. Now I have a good garage floor.

Dick
 
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Old 06-22-06, 08:02 AM
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Ok. I'm in Wisconsin, footings have to be 48" deep if attached to the house. So best bet would be let the slab float in the attached garage? Or was your town house a case a bad compaction?
 
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Old 06-22-06, 08:45 AM
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Slab bearing ledge on stem walls

My townhouse situation was a combination of poor soil (clay) backfiled into the garage area with poor compaction. The scrap wood, empty beer cans and sheet rock scraps from the builder were the main reasons and this is very common for townhouses and even custom homes. This is usually one of the last areas to pour concrete, so it collects a lot of junk. Basements don't have this problem because the slab is poured much earlier so backfill can be done to get good access to the house.

The slab would have had no problem if the material under it was OK. A slab sitting on the ledge of the stem wall would have broken up much quicker. The 20 year old concrete was very tough to break into smaller pieces.

Dick
 
 

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