Construction Joint Location in a Two-Way Slab
Construction Joint Location in a Two-Way Slab
(OP)
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.
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.






RE: Construction Joint Location in a Two-Way Slab
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.
RE: Construction Joint Location in a Two-Way Slab
RE: Construction Joint Location in a Two-Way Slab
RE: Construction Joint Location in a Two-Way Slab
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.
RE: Construction Joint Location in a Two-Way Slab
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).
RE: Construction Joint Location in a Two-Way Slab
If you use saw cut joints, I would still place waterstop at the future joint locations.
RE: Construction Joint Location in a Two-Way Slab
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.
RE: Construction Joint Location in a Two-Way Slab
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.