Strap Beams for Pile Caps Near Property Line

[msegerman]

Hi,

Does anyone know of any good resources for this? I feel like this is different than a traditional strap footing.

Thanks,
Matt, EIT

 

[BAretired]

Why?

BA

 

[msegerman]

I'm not sure if the beam would have continuous uniform support. It seems to me like it would behave more like a deep cantilever with concentrated loads... I feel like a strut and tie approach would be applicable. I can't really find any resources for this anywhere. It would be easy to just say Mu = Pu*e with no significant shear (load is within d of support typically), but I'm an engineer and don't want to guess. I want to be certain I am taking the correct approach.

...there would be some shear as a result of the cantilever action (causing some uplift at interior column), but I don't think that will have a huge influence on my design.

 

[hokie66]

What you described, and how you would approach the design, is a "traditional strap footing". The strut and tie approach is appropriate for the region near the property line where you have the strap beam rectifying the eccentric loading.

 

[msegerman]

Hokie,

I tend to agree with you. There are plenty of examples for common strut and ties applications (e.g., deep beams, corbels, etc.) but not really any for pile cap strap beams. For the d-region (which I agree with you is really only the cantilevered portion), do I require the minimum horizontal and vertical shear reinforcement as required by ACI? I think this is uncommon in pile caps. Also, for the node above the pile, how do I determine the width of that... that is would it be governed by the width of the beam, the width of the pile cap, or the width of the piles themselves? Thanks for the input.

-Matt, EIT

 

[WARose]

Bowles book on foundation design has a passage on it (i.e. p.486-489, of ‘Foundation Analysis and Design’, 5th Edition). It is for a strap not in contact with the ground (as they typically aren’t) that connects two spread footings.

But I think your situation is a bit different (IIRC your problem from the other thread you posted on this) as you are trying to restrain the rotation of the tip of a single pile by connecting it to the cap of another pile group. That sort of problem screams some sort for FEA or some other computer solution because when modeling the rotational stiffness of the pile group on the far end, the contribution will be influenced by localized affects and won’t lend itself to a simple model (like a single free head pile).

Also (another reason for the computer solution is the fact that you may want to consider some differential settlement (and its influence on the forces in the strap; I typically have). Granted settlement in piles tends to be low, but I’ve never gone without superimposing those forces on my other results.

And I'd forget about the deep beam stuff.....the distance from cap to cap (relative to the "d" of the strap probably precludes it anyway.

 

[hokie66]

The cantilever part (the D region) is exactly like a corbel, so you can use your strut and tie references for that part. The width of the node would depend on the orientation of the beam. If it is on top of the pile cap, the beam width rules. If it is monolithic and with the depth of the pile cap, then the pile cap may increase the available strut width. The backspan is just another beam.

I see no reason for a fancy FEA of this as WARose suggested.

 

[WARose]

I see no reason for a fancy FEA of this as WARose suggested.

Without seeing a layout of the thing that’s difficult to say. I can think of some scenarios where it wouldn’t…..others where it would be. If (for example) a row of piles were not directly under the strap, the (flexural) stiffness of the cap would play a role in the forces developed and their distribution (this doesn’t lend itself to a simple model). In the scenario of settlement, the afore mentioned layout could potentially relieve some forces in the strap. The possibilities are endless.

I just mention that from my standpoint as I could set that up (in STAAD) very quickly. (In fact, just about as fast as I could do the hand calculations.) But, to each his own.

 

[msegerman]

Thank you for all the input. I think that I'm going to size my beam so that it's 4" shallower than my exterior pile caps (my piles are embedded 4" into cap) and have the top of the beam be flush with the top of the pile cap. I will design top steel reinforcing using a simple strut and tie model. I will size the width of my beams to fit all my rebar in one layer with approximately 3" spacing. I don't think I'm going to include any horizontal or vertical stirrups within the d-region (cantilevered region between the exterior column and exterior pile) as it seems like this is rare in pile caps and my boss doesn't seem to want to call for anything too unconventional on our drawings. For the back span, I will provide minimum shear reinforcement (roughly #4 @ 12" - 2 legs). If anyone agrees or disagrees with this approach, I'm open to all opinions. Thanks again.

-Matt, EIT