huangyhg

sog shear reinforcing under wall
i am hoping someone can point me in the right direcion. i am looking for recommended shear reinforcing details for a slab-on-grade with a heavily loaded wall. going thicker with the slab is not a practical option at this point and was wondering if there are any references out there that specifically address this situation. i can find lot with suspended slabs but nothing that seems specific to a slab on grade. is this not a recommended practice? particularly if the slab needs to remain water tight? i am currntly entertaining the idea of a "u" or "z" sirrup but the spacing is going to need to be pretty tight.
i appreciate any thoughts you may have.
thanks,
waytsh
put in a thickened footing or separate footing under the wall.
refer to "designing floor slabs on grade" by ringo and anderson.
consider strips of shear studs like those used on elevated slabs supported at columns. if you turn your problem upside down, the loading is very similar: a distributed load on the slab (soil reaction) and a point support (wall load).
a more desirable solution would be to install a thicker footing designed for the wall, but it sounds like the slab base, reinforcing, and wall forms are already in place (?).
don't forget that aci 318, section 15.7 states that the minimum footing thickness is based on a minimum depth to rebar, d = 6".
so with 3" cover you have about a 10" slab minimum.
my slab is currently a uniform 15" thick. it does not make sense to me but the owner and contractor insist that it is going to be much cheaper for them to install the shear stirrups instead of just thickening the slab. i have considered the shear studs and may go with them if there is not a good reason for avoiding shear reinforcing. maybe the owner and contractor will then realize just how expensive this shear reinforcing is going to be.
the wall is bearing on the slab on grade and is supported by granular fill? can you place a concrete pour under the sog in the vicinity of the wall? should be inexpensive just a matter of excavating, compacting and placing concrete... what is the consequence of the slab cracking? is it worth the risk? even with large amounts of steel, the slab can still crack.
can the wall width be increased? a small pedestal? is the problem beam shear? or punching shear? can you use shear friction? with large bars? a recent project has 35m@6"
a 15" thick slab ought to be able to support alot of load. how much load are we talking about? joint locations?
i assume it already has top and bottom reinforcing. how much reinforcing? are you sure you need shear reinforcing?
what is the soil modulus?
dik, thanks for the suggestions. the wall is precast so unfortunately i can not go any thicker than what it already is. the problem is punching shear. i had thought about shear friction since i will have a substantial amount of flexural reinforcing under the wall. i didn’t think i could “double dip�on the reinforcing if it is already being used to resist flexure…or are you suggesting additional reinforcing specific to that purpose?
maybe i better mention my analysis approach as well in case that is flawed. i have analyzed the slab on staadpro because of the complexity of the loading and the wall system. for my worst case load combination i am getting a shear stress under my wall of 108.2 psi. which translates to about 19.5 k/ft. which i am then checking against aci 318 eq. 11-3 or 11-5. for my model i used a uniform thickness of 15�but in reality the slab is only 12�thick under the wall because of the keyway. so my d is approximately 8.5. my concrete is 4,000 psi so i am getting an allowable shear strength of 9.68 k/ft for a one foot wide section.
is this a rational approach or am i oversimplifying?
thanks,
waytsh
sorry jike, you posted as i was writing and i didn't notice before submitting. the unfactored load from that particular wall is dl = 10.5 k/ft and ll = 13.9 k/ft. the wall is 14" thick. i will have top and bottom mats in the slab under the wall. it is going to be #5 bar @ 8" o.c. but i may need to change the bottom mat to something like #9 bar @ 4" o.c. to meet the flexural requirements. i haven't determined what i am going to do there yet.
soil modulus = 90 pci
sounds like you did a beam on elastic foundation analysis.
is there any way to increase the concrete strength?
or
is there any way to eliminate the keyway?

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