existing pile cap deficiency

[structSU10]

I have an existing building with relatively thin pile caps (20" thick) for all pile arrangements. Per ACI equations, they are very deficient for shear and moment(stress ratios up to 3.5). I have depth to retrofit due to the floor slab being raised - I was thinking of either making a composite cap, or provide a pier with large enough footprint to increase the shear perimeter. Any thoughts for or against either option? Other concepts? I don't know the condition of the current cap, and with the large overstress that could be an issue.

 

[r13]

I don't know the condition of the current cap, and with the large overstress that could be an issue.

It seems there is no physical indication of the pile cap deficiency, but through the check against current code. When was the building constructed, and why the floor slab being lift? Need more details, a sketch may help.

 

[KootK]

1) Could we bet a sketch or description of the geometry of the typical pile layout?

2) How much extra depth above do we have to play with here?

3) Is the load delivered to the cap pure axial or is there moment too?

4) One strategy that I'd attempt before getting deep into retro would be an aggressive analysis using the CRSI manual methods. They are a bit different, and a bit more aggressive than the straight ACI stuff I think. That said, 20" does sound pretty thin and I doubt this would be enough to save the original design.

5) Depending on the size of the thing, I might lean towards the big pier idea. Doing the composite thing in situations where the forces tend to be quite localized always makes me a bit nervous because it's usually difficult to localize the shear friction etc that you employ to make the thing composite. It becomes a bit of a load path inconsistency. Much depends on the proportions though.

 

[structSU10]

here are some shots from existing drawings:

geometry match CRSI - this building is from 1985. there are 2,3,4, and 5 pile caps, all the same 20" thickness:

The floor is being raised 3' for floodproofing.

 

[KootK]

Unless you'll be modifying the bearing condition of the columns to move the location of load application up higher, it would seem that you're kinda stuck with the composite retrofit. And, even at that, it may not fix your punching shear problem if you have one. See detail below for a spitball idea. Is the problem equally present at the interior and exterior column locations?

 

[KootK]

Thanks for the drawing. Old structural... sexy.

Just another way to approach things.

 

[structSU10]

Yea my thought was to encase the column in concrete with headed studs to transfer to a concrete column wrap - wasn't sure what to think of load transfer with existing loads in place.

I wonder if I could do something like this:

1. build concrete cap over pile cap, boxing out existing steel column
2. weld shoring beams to existing column
3. jack shoring beams to take up building loads
4. cast / dry pack below shoring beams to engage on existing cap. or could cast collar around existing concrete to lock into the existing new surround collar.

basically a method to directly preload the new cap construction and pull forces out of the existing 'low' bearing. doesn't help the perimeter condition though.

 

[r13]

If the entire building is to be raised 3' (grade beams and columns included) for flood concern, and it is the final elevation, then I'll just simply raise the top of the pile cap 3' to fill the gap in between. The existing surface should be mechanically roughened to 1/4" magnitude as recommended by ACI, and apply bonding agent before pouring the new concrete. To ensure the positive connectivity, you can drill and grout vertical reinforcement on perimeter of the existing cap to tie the two together. After all, you won't have shear problem, but to assure the existing bars are adequate in tension, which can be easily calculated by truss analogy. I sense there will be bigger problem, if the existing tension reinforcement is in adequate, that rebuild the pile cap maybe necessary.

 

[KootK]

At the exterior columns, maybe you can install deep beams adjacent to the column pilasters to get the caps at least working like one way shear elements.

 

[Boiler106]

This is a pretty awesome guide for all types of retrofits, regardless of seismic zone:

Link

 

[JAE]

You may have already done so - but did you take advantage of ACI's provision to ignore shear in a cap when the piling are within a certain proximity to the applied load footprint?
I can't recall the section in ACI 318 as I'm away from my desk.

Whenever I see a unity of a magnitude of 3.5 on an existing design check I have to stop myself and ask "what am I not seeing" that the original engineer saw or did.
Granted, the original engineer could have made an enormous mistake like this but it doesn't always happen to that extent.

Just wondering. Like I said, you may be perfectly right with your 3.5 value.

 

[hokie66]

How do the columns capacities compare to the pile group capacities? If they are all only W8 columns, it looks to me like there may be excess capacity in some of the piles.

Using strut and tie analogy won't do you much good without adequate anchorage of the reinforcement, which looks to be lacking. And the 2 and 3 pile caps have hardly any reinforcement.

It looks you need a scheme to increase the column bearing footprint, whether your idea, KootK's, or something else.