Construction joint between cantilevered slab and support

[SEengineer]

I am working on a baseball stadium project that has a dugout with a roof that cantileveres from the top of a retaining wall. The contractor wants to place a horizontal construction joint at the top of the wall and pour the roof seperately. It would be similar to having a vertical joint in the mid span of a beam, which is obviously a bad idea. I don't like the idea but I can see how it would be difficult to build. In my mind even though I don't like it seems to work as the joint is not through a critical shear plane and the reinforcing will properly developed on each side of the joint. I have not found any additional ACI requirements in additional to typical flexural reinforcing that need to be consided to allow this? Any thoughts are appreciated. I have attached the detail for reference.

 

[SEengineer]

sorry here is the attachment. This is my first post. I'm still figuing this out.

 

[hawkaz]

I gather that this is concrete? I can't open the attachments

This sounds like standard construction practice to me. It is rare (but not un-heard of) to pour a horizontal surface monolithically with a vertical one.

Why is putting a joint in the middle of a beam a bad idea? This is the point where shear would be theoretically zero. As long as the rebar runs through, that would be the ideal place to put a joint.

 

[hokie66]

hawkaz is absolutely correct, in both the joint at the top of the wall and at the midspan of a beam. SEengineer, where did you get this kind of mininformation about construction joint positioning?

 

[BAretired]

I agree that midspan is the best place for a joint. It is not a bad idea...it is a good idea.

BA

 

[slickdeals]

It would be preferred to have it in the middle-third of span length for continuous beams and slabs.

 

[slickdeals]

.......just to give some flexibility in terms of locating the joint. But agree with above, midspan is not a bad idea.

 

[slickdeals]

I found a good response from RAPT on another forum which I am posting here for reference. SEAINT used to be very active, but I think members slowly migrated from over there.

......Just to mess up the logic, the best place to put the joint is NOT at a
point of lowest shear, which is a high moment zone and a critical plastic
hinge area requiring ductility where you want your best concrete, but at
the point of highest shear capacity with relatively low applied shear and
very little effect from induced curvatures from redistribution from plastic
hinges, which is at the point of contra-flexure. At this point, according
to ACI eqn 11.5, the shear capacity is greatest (because a flexure-shear
crack needs flexural tension at the surface to begin the crack). Also, as
the applied moment is lowest, flexural crack control will be best and
ductility is not a problem.

It is also the best point to place it if the first poured slab is to be
stripped before the second is poured (eg prestressed slabs) or stripped and
back-propped (eg RC slabs) as the cantilever will be self-supporting (until
you get a builder who pours the long part first and then strips it
completely as the drawings did not specify back-propping and he missed the
pour sequence note).

So the joint should be at about .15 - .2 x span length from the support.

 

[asixth]

I've done it before and it worked.

 

[marcploks]

this is OK, failure plane is at the interface between wall and slab... watch out for development lengths