shear strength at construction joints

[ampersand]

So here's a (hopefully) simple question...

Contractors always want to pour concrete retaining walls in two pours, with a construction joint at the bottom of the wall/top of the footing. If we call for a "construction joint" on our details, are we jutified in assuming that the full shear strength (2*sqrt(f'c)*b*d) can be developed at the joint.

I've heard that this is standard practice, but I can't help but doubt the integrity of these joints.

Thanks in advance...

 

[miecz]

I don't believe you can use the above formula at the joint.

The shear at this joint is carried by shear friction of reinforcing steel passing through the joint and/or a shear key. Personally, I use shear friction and omit the key. I believe there was some discussion of this topic here recently.

Keep in mind that the wall needs to carry the shear at some small distance (say, the thickness of the wall) above the joint. At this location, I use the formula you cited.

 

[DRC1]

Trying to pour a footing monolithically is very difficult at best. Almost all walls are poured with a joint at the transition between the wall and the footing. The shear capacity of the steel can be used. The footing steel generaly "l"'s up into the wall with the vertical bars slicing in. The "l"'s can be made large enough to take the shear. Alterantively, shear keys are popular. Female keys, that is keyways depressed into the footing are easier to form and are more popular. However they are a potential place for water to collect and can cause problems in cold weather. Male keys, those that rise out of the foundation, can be more complicated to form and place, but not a lot more, but do not collect water.
At any rate you should make provisions for a construction Joint at the walls and the footings. Since the concrtet is not continous, you can not use (2*sqrt(f'c)*b*d), Although the othe methods will allow you to transfer shear.