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construction joint location in a two-way slab
question: where should construction joints be located in a two-way slab?
i am involved in the construction of an underground tank with a two-way concrete roof slab. the column spacing is about 22.5 and the tank is 270' x 90'. i have been asked by the contractor where he should locate the construction joints. there will be two construction joints for the slab. this will divide the slab into three seperate pours. should the construction joints be located at the mid-span where the shear is almost zero - or - should the joints be located at the third points where the moments are almost zero.

the mid span seems more logical to me.
the flexural bars should be extended continuously thru the joint and fully developed so that there will be full moment capacity. if there is only bottom steel at the construction joint, it seems advisable to add top steel bars (fully developed) also across the joint to keep it closed tight.
if the span is fully loaded the shear might be nearly zero theoretically but if the span is half loaded the mid-span shear could be approximately 1/4 of the end shear. design a shear key or shear friction mechanism to transfer that amount of shear.
i agree with jike - midspan.
i'm looking at a very similar project we did (buried reservoir; two-way slab design). the columns are spaced at 20'-0". there are construction joints spaced at midspan in each direction. the joints are waterstopped with a 6 inch pvc waterstop and the top reinforcing is stopped at these joints. the top reinforcing does continue at wall strips, however.
thank-you jedclampett and jae
jed, out of curiosity what did you use for a base slab thickness and reinforcing for your tank and how did you deal with the control joints there.
on another thank that i am doing, i would like to use a 10" slab on grade with 1 layer of reinforcing centered so that they can cut the control joints to 1/4 of the slab thickness without cutting the top mat of reinforcing. this seems reasonable enough to me, but i've seen a lot of 12" slabs with a top and bottom layer of reinforcing, and i am not sure what the reasoning for that is.
concerning the slab on grade. i design a lot of foundations for mechanical equipment. the design should be based on an analysis of the mat that takes into account the positive and negative moments that are developed due to the applied and reactive forces.
that said, i often end up with 12" slabs reinforced top and bottom (see aci 318 for minimum reinforcing requirements for slabs). most of my joints are shrinkage joints that run transverse to the slab. i have used three different arrangements with respect to the rebar and the joint. where i don't need the moment capacity i simply stop the top longitudinal bars 1 1/2" from each side of the joint, and place a transverse bar on each side of the joint. where i do need the moment capacity i have lowered the top steel to 3/4" below the bottom of the joint, and i have also specified bars that are bent to bend around the joint.
there is not much giudance in the literature for this type of situation as most of the information is for slabs on grade that function as floors (see for example aci 302 and aci 360).
we designed based on snwa (southern nevada water authority) design standards. they encourage the use of 7 1/2 inch thick base slabs with #5 @ 12" ew centered. the base slab is basically flat but there are sloped walls (~1:5) at the perimeter. the sloped walls are 7 1/2 inches thick also. the control joints (more accurately, construction joints) basically line up with the joints of the roof slabs at 20'-0" centers.
if you use saw cut joints, i would still place waterstop at the future joint locations.
jed i went to the snwa web site, but wasn't able to find this design standard. do you have a link to this standard?
when you refer to sloped side walls, is this a 1 vertical in 5 horizontal slope to a thickened slab edge. does this also occur at the interior column pads too? i'm not quite sure i understand.
i don't think the design standards will be available on the internet. they have produced four volumes of "facility engineering guides" known as fegs. most of the volumes are just standard specifications, but one has a lot of structural design standards. maybe if you call them, they can cut you a copy.
as far as the sloped sides, we usually use a "hopper bottom" design. the outside walls are shallower than the deepest part of the reservoir. this reduces the shear and moments on these walls. the transition is made by sloping the slabs from the exterior walls to the deepest portion. we've used slopes of 1:1 to 1:5. right now 1:2 is preferred. they're tough to build, but a good contractor can do it. they need to work up the hill, starting the pour at the base.
as far as the columns, the ones over the deep area just go a thickened slab. same at the sloped slab.
i've also done reservoirs with vertical walls. as you might expect, the wall thicknesses get pretty excessive.

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