Precast 3-Sided Culvert Footing

[TehMightyEngineer]

So, I have an interesting task at work where I need to make the attached supporting system work for a series of 3-sided precast culverts for a job. The presumptive soil bearing pressures are good and the 3-sided structure has been designed for the pin-roller support condition it will face. The obvious failure mode is the outward thrust from the culvert will put overturning forces on the tapered "legs" of the footing. In my mind this would be resisted by the passive soil pressure of the compacted backfill. Due to the geometry of this structure there are no inward thrust reactions.

My question is how crazy is this footing setup and what are your thoughts on potential issues arising from it?

If it can't work, then it can't work. But on paper I can't see any reason it shouldn't work and it could definitely save a client a ton of money using this if it works.

Edit: Helps if I upload the pdf...

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

Just the usual issues with mobilizing passive pressure:

- how much movement is required for it to do the job?

- how accurately can you predict the movement required to do the job?

- from a design and durability perspective, can the parts and pieces accommodate the movement?

I don't know culvert design real well. Any chance the top might get washed away under flood conditions?

Other than that, it looks like a clever solution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

This job is a utility road for remote access to utilities, settlement will be less of a concern (though obviously we would like to minimize it). I do a lot of 3-sided culverts and have observed they tend to react with some lateral thrust but usually not as much as predicted in our idealized models. Lots of extra stiffness in actual service.

Good points on the passive soil pressure; I can ensure they bring in good structural fill and compact it well. Should be able to get ideal conditions to develop the passive pressure. These will not be excavated in the future without either being taken out of service or with no load placed on them so I'm not too worried about the soil being removed in the future.

I'm going to detail the shear keys in each piece to accommodate some rotation without self-destructing. In practice I would expect the actual deflections to be very small. In my mind this reduces the available passive pressure but also eliminates the requirement for it.

There's definitely a chance the interior soil could be partially scoured away; around 4-5 feet of compacted interior soil with rip rap. Also, note that the service reactions are all outward so inward fill shouldn't destabilize the structure unless it undermines the footing. Plus these are private utility road structures to allow access by construction equipment so the expected service life of these structures is reduced. I will likely clarify with the client to make sure they understand they're potentially not getting a 75 year structure.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

My gut feel is that it should go like this:

1) Design the cap as though it were pin supported at the legs.
2) Deal with the cap reactions and the passive pressure resistance solely in the base.
3) Estimate base lateral movement.
4) Make sure #3 doesn't result in a curvature reversal in the cap legs (unlikely).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

1) I enveloped the design of the "cap" 3-sided culvert for both pin-pin and pin-roller. Should cover that.
2) Sounds good.
3) Sounds good.
4) Not sure how you could get this. Unless you're considering the passive/active soil pressure confining enough to generate fixity? If so I can reinforce the leg to death. Boss has authorized me to do crazy things if it results in helping get these out of the plant faster.

Do you think I could get any strange eccentricity/bearing failure modes in the "footing" slab even without any moment transfer between the leg and the slab?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

4) I agree. Concern recinded.

I feel pretty good about the footing. The taper on the low wall rubs me the right way. Good call with that I think.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

The whole story behind this is one of our forms is out of action and the customer can't wait. So the boss had an interesting idea of using a highway barrier form to cast the "leg" of the footing and pour a "footing" slab using aluminum panel forms or letting the customer use cast-in-place. From there, it's been a lot of "well, I don't like it but can't see why it wouldn't work".

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

Yeah, precasters do tend to be "can do" folks.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.