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Existing Pile Cap Retro-fit

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ChipB

ChipB

Structural
Apr 21, 2001
347
We are doing an expansion at a power plant. I have a 3x2 pile cap which is 3' thick w/ #8@12 E.W.:

X X X
|-|
X X X

which is apparently not good under the new loading conditions. (FWIW, it is not sufficient under the original loading either.)

I didn't have access to the original 1990 calculations, so I just took the value of the brace (or) connection could support to determine the lateral loading value. Original pile capacity was 120T. I have a new geo report which allows me to go up to 145T. I'm at capacity for these piles, so it doesn't surprise me the cap is insufficient (more so) by calculations.

The baseplate of the column is 2'-4" below top of slab (actually 2'-6" mat, not slab), and the pocket is filled with grout. My thoughts were to chip out the grout, weld (2) W24x103 along each column flange (W24 web to W14x145 column flange) Here goes:

W24
----- CL (Sym About)
| |
|| W14 .
||-------|
|| .
|
-----

and set the flanges of the W24s on W18x143 frame work beneath the slab level. Grillage would likely better describe what I'm attempting to do. The flanges of the W24 will hit about 2" from the CL of the W18. I'm going to stiffen the W18 flanges substantially where this occurs. I'm spanning these W18s out to the edges of the original grout pocket. I will then be about 9" from the CL of the outside piles.

Has anyone out there done anything similar?
Anybody got a better idea? (please)
 
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Sounds like a reasonable approach. A couple of suggestions:

1. Since your can't really calculate the detail forces and material costs are small compared to labor, don't skimp on beam sizes. Flange of W14x145 is 1.09" thick, web of W24x103 is 0.55". Get a better match on these two thickesses... say a W24x176 with 0.75" web, or larger. This will allow more substantial welding.

2. Go with something larger than a "beefed-up" W18x143. Perhaps a W18x192 (or so) that does not have be "beefed-up". No need to do any more labor than necessary - not just the cost, but members may not be able to be positioned exactly as planned (because of field conditions).

I don't think that you will have any trouble.



[idea]
 
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Questions:

1. When you say the cap isn't adequate under the new loading conditions - what specifically is failing? It sounds like pile punching shear as the W18 grillage would be used to spread the load out and avoid the shear...correct?

2. Are you adding anything to the bolting between steel and cap? i.e. via more bolts from W18 flanges to the cap?

3. Is the value capacity of the brace based upon seismic? Does it include the [Ω] overstrength factor?

 
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SRE:
If I changed my beams out it does a couple of things:
I was already worried about the clearance to make the weld on the flange/web interface with the W24x103. I think the 3.5" wider flange of a W24x176 would make it impossible.

Going with a larger grillage beam would produce more eccentricity of the above mentioned framing as once again, the wider flange forces me to move it further from the column, thus still inducing the local flange bending.

However, very good points. You got me thinking deeply on my commute! There is nothing that I know of that tells me I can't design a plate column along the W14 flanges. The flanges could be located (nearly) directly over the web of the grillage beams, thus eliminating (minimizing) the flange bending of the grillage beams. I'm not limited to the book.

Sound reasonable (workable)?
TIA
Chip
 
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JAE,
Basically, I have a 7'x9' pile cap. The base plate on this column is big. 3"x24"x2'-4". I don't know why they did it so big as they appear to have missed something along the way. According to our calculations, the axial stress in the column is 36.4ksi BEFORE the addition if I remember right. That's f[sub]a[/sub] not F[sub]a[/sub]. Column schedule lists it at A36. We are reinforcing the heck out of it already.

I used CRSI and checked M[sub]u[/sub] short direction using an effective pedestal of 26"X30" (2" Grout called out). That was ok.
Long direction M[sub]u[/sub]---> N.G.
Punching Shear: N.G.
One way deep beam as calculated in CRSI: N.G.

Bolting between the beams and cap:
Yes, I'm going to bolt the beams to the cap. My thoughts were to use (4) 100T jacks, (1) at each corner of the grillage, and shift some of the load off the column and onto the grillage, then grout beneath the grillage beams where they are to be bolted. After curing, remove the jacks and fill the pocket back up with grout.

Value of Brace:
I'm just guessing at what they did. They couldn't find their calcs when asked. I took the capacity of the brace, or the bolts, whichever was greater. No overstrength factor. This building supports the boiler, so our thoughts are we have not changed the loading by more than 5%,

Chip
 
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Chip - I see your points. The plate column sounds like a good approach. To emphasize... try to make allowances in your design for field dimensions that could be a little different from expected values.

If I may offer an educated guess why the foundation is mismatched with the existing loads:
Electric generating stations are typically designed and built on the Fast Track, with the boiler itself being on the Critical Path of the schedule. The foundation HAS to be ready to receive the boiler, when the time comes. To accomplish this, the boiler foundations are designed early, sometimes even before a boiler manufacturer has been selected. These "generic" foundations are sized conservatively to take just about any reasonable combination of loads... but it does not always work out that way. Loads may be larger than anticipated and the result is what you have come across.

[idea]
 
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