Pile Caps and Grade Beams - Pour Sequence

[GalileoG]

Hi all,

I have concrete grade beams that span into the side of pile caps (top of pile caps is at the same elevation as top of grade beams).

I have designed the grade beam / pile cap system as a monolithic system. The contractor is proposing to pour the pile caps first, with bars sticking out of the pile caps that will lap with the grade beam top and bottom bars, the grade beams will be poured at a later time.

With this pour sequence, I have concerns about the shear interface between the pile caps and grade beams. I feel that the shear transfer between the grade beam and the pile cap would be compromised. Can this pour sequence be accommodated in any way? Is this an acceptable pour sequence? How would you address this?

Thanks!

 

[jayrod12]

you could put a shear key in the side of the pile cap at the connections to increase shear interaction.

 

[SlideRuleEra]

GalileoG - The pour sequence the Contractor has proposed is common. It simplifies setting reinforcement steel, forming, and concrete placement. IMHO, and with all due respect, designs that rely on complex monolithic concrete shapes to succeed are a little too "theoretical". Using jayrod12's suggestion, take another look at your calculations, the constructions joints will probably be satisfactory. If not, perhaps that area should be redesigned anyway to no longer rely on monolithic construction.

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[hokie66]

GalileoG's concern about shear at the joint is valid. Depending on the magnitude of shear, key or dowels could be used. If the shear is always downward from the grade beams, the sides of the pile caps could be sloped. But when modifications are proposed, the contractor may decide that it not too difficult after all to cast the whole thing monolithically.

 

[KootK]

You'll need enough reinforcing crossing your pour joint to make shear friction work there. You can count any rebar that already crosses the joint for your flexural design so, in the end, you likely won't have to do much more than appropriately roughen the interface (shear keys, bush hammering, etc.).

I find it helpful to remember that shear friction needs to be satisfied at all locations, even when monolithic construction is employed. The only difference is that, with monolithic construction, your coefficient of friction is high enough to preclude an interface shear failure the vast majority of the time. As Slide Rule has suggested, if a design only works with monolithic construction, it might be worth revisiting the design.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

KootK

Where do you get the idea that "shear friction needs to be satisfied at all locations..."?

 

[KootK]

Hokie,

For better or worse, I got the idea from my own reasoning. And it has proven to be a controversial opinion, as you can imagine. I've been meaning to create a post of my own on the issue. Hopefully you'll weigh in on that post when the time comes.

I sympathise with the OP. Whenever I find out about a new cold joint in something that I imagined would be monolithic, terror grips me, my heart rate increases, and my breathing gets shallow. I imagine that a new failure plane has been created. Then, I remind myself that there was always a non-critical shear friction plane at the location of interest and that all that has changed is that the coefficient of friction on that plane has been reduced a bit from its monolithic value. I was hoping to give the OP some peace of mind by sharing my take on it.

KootK

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.