huangyhg

anchoring an exterior cmu fdn wall to s.o.g.
i am looking for a rational way to specify dowels attaching the top of an 8" cmu exterior wall that has earth on both sides to the 4" s.o.g. of a single story building. retained height of earth is < 6" for all practical purposes. have seen some engineers just run the slab over the top of the cmu with no dowels, or provide a "shoe block" for the top course of the block and run the slab into that. another detail i have seen is to thicken the slab edge to 8" or more near the wall and dowel into that from the wall.
a fourth option is to have no dowels, and keep the the top of the masonry wall even with the top of the slab, and put premolded filler between the concrete and cmu. this option is attractive in this cold weather because it allows the building to be closed in without having to first place the s.o.g. so i am considering it, although never did this before. doweling the wall to the slab in at least some nominal, minimal way seems desirable. it will stiffen the wall and help keep it from wandering out of plane.
these exterior cmu walls do have out-of-plane forces due to seismic/wind forces on the facade, but they are low. a short portion of the exterior wall is used for shear walls, and in those locations it makes sense to use at least a #4 at 4'-0" o/c dowel from the wall into the slab. elsewhere i'm not so sure.
how does your office typically treat these conditions?


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typically we do not connect the exterior cmu wall to the sog because the differential settlement between the sog (not loaded) and the loaded strip footing can cause cracking at the sog-wall interface. the wall supports in this case are the footing and the roof diaphragm, typically. as you pointed out it also means that we can cast the slab at a later date, protecting it from construction damage that would occur if it were cast earlier. this is especially important for slabs that will be exposed when finished. if the wall needs the extra support for purposes of reducing the wall span we connect with dowels, cast-in-place normally. this also works in the event that the lateral shear force cannot be transferred to the earth by the wall footing alone. the slab then picks up with it's additional weight and so on. on residential structures the load-bearing walls are less loaded and usually the chair block at the top performs quite well.

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