Moment transfer from a grade beam to a caisson

[ARPASEVAN]

Hello,

ACI 318-19 has changed section 25.4.3 substantially (Development of standard hooks in tension) and made moment transfers from grade beams to caissons a bit more challenging. Is the following approach reasonable or too conservative?

Let's say I have a 200 ft-k of moment to be transferred to the caisson. This moment can be converted to a couple with and arm of 9.75".

3.375" (centroid of the 5 right hand side rebars) + 6.375" (centroid of the left half of the concrete)

200x12/9.75= 246k or 246/(.9x60)= 4.56 sq.in. of rebar. I have 5x1.27=6.35 sq.in. so rebar is OK.

In the best case scenario I will need to develop #10 rebars 22", doable, or in the worst case 22x1.6x1.2=42", not doable.

Has anyone faces a similar situation? Any comments or suggestions will very much appreciated.

Thank You,

Vic.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1679277839/tips/Caisson-Q_qq4jrv.pdf[/url]

 

[1503-44]

I don't think I'd particularly want to transfer grade beam moments into cassions.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[HTURKAK]

I could not get the reason or say motivation for the cutting the 10 # 10 rebars at the top of the pile then planting 8 # 10 dowels..


What about the shear transferred to the pile ?

I would extend the 10 # 10 rebars to the grade beam and satisfy lapping requirements ..



My opinion ..





Not to know is bad;
not to wish to know is worse.

NIGERIAN PROVERB

 

[ARPASEVAN]

Yes, you are correct HTURKAK and that is what needs to be done. The development length in the grade beam will still be an issue and may require a deeper grade beam. Shear transfer will be with shear friction and will easily work. Thank You.

 

[MotorCity]

Is your 200 ft-k moment internal to the beam or is it an external reaction being transferred to the caisson?

I ask because usually where you have a cold joint, I would consider the grade beam to be pin or roller supported at the caisson (not fixed).
If the grade beam was cast integral with the caisson, you would have more of a moment connection.

The other concern I'd have is whether or not the top of caisson has the flexural capacity to resist that moment.

 

[ARPASEVAN]

MotorCity, The moment comes from retaining walls at either side of the grade beam. The connection is pinned and they are poured at different times. Thank You.