How to Check if Concrete Spalling Reinforcement is needed

[hitch22]

Hello fellow engineers,

I'm trying determine what caused the corners of a concrete support to crack. This is a 40 year old concrete structure supporting a vessel. It looks to me like bearing failure. However, my calculations say only 30% of the concrete bearing capacity is being used. I've checked my calculations and I still get to the same conclusion. After a little bit of thought I think there's concrete spalling because there's not enough reinforcement at the bearing area to resist spalling.

The plate bearing on the concrete extends past the last stirrup. I think the stress is dispersing out of the plate and creating tension on the concrete. The problem is that I don't know how to check this. I don't know if the numbers back up my suspicion.

Would someone please give me some help? I have attached a file to illustrate the scenario. Thank you very much in advance!

 

[hitch22]

Maybe I should have specified that the load on the steel plate is approximately 99,000 lbs unfactored. The plate is 16" by 18" and it's 1.5" thick. The concrete is assumed to be 3000 psi as this is the value shown on the drawings from 1970.

 

[StructDave]

There are other possible resons for the spalling...
Rebar corrosion? You didn't mention any rust staining so let's assume that's not a factor.
Concrete damaged by chemical attack, freeze-thaw exposure, or simply deficient in the first place?
Did the load change at any time? Yes your bearing stress seems low on average, but is there any moment present? Or, was the load ever increased in the past?
Consider whether the damage is all around; if only one side, might be an indicator of moment present.

If you've ruled these things out, and if the crack/spall looks clean, then yes you probably have a compression or shear failure from the load being (partly) outside the stirrups.

In deciding what to do with it, if you conclude the rebar is sound and the concrete not deficient, consider a carbon fibre wrap around the pier. This is fairly new on the market; check with your SIKA rep. I am not connected with Sika, but my people have used this is a design to stabilize some old concrete. (Can't give you results yet!)

 

[hitch22]

Hi StructDave,

No I haven't seen any rebar corrosion. Has the concrete been damaged by freeze-thaw exposure? I think freeze-thaw will make the problem worse since water will fall in the cracks and they should grow. But I don't think freeze-thaw exposure caused this damage. Yes it's possible that chemical attack caused this damage because it's in a refinery, but I don't have a way to verify this.

You mentioned moment, but I see this damage on both sides of the structure. Wind load could generate a moment, but I already checked this situation. It doesn't seem to be a concern. The load from the vessel is about 98,000 lbs and the wind load on the structure is about 1200 lbs. so in the grand scheme wind causing bearing on the structure is not much of a problem.

 

[racookpe1978]

What are the distances and dimensions of your problem?

CL of plate to edge of concrete?
size of plate?
exactly what the load bearing steel going down to the plate is shaped (the load won't be centered in the exact center of the plate if it is an angle iron or WF member.)
Thickness of plate?
WxLxH of concrete?
Size of that corner chamfer of the concrete?
Dist from edge of concrete to cl of the vertical rebar?

 

[hitch22]

Hi rackookpe1978:

CL of plate to edge of concrete? [highlight #FCE94F]12"[/highlight]
size of plate? [highlight #FCE94F]1 1/2"x16"x18"[/highlight]exactly what the load bearing steel going down to the plate is shaped (the load won't be centered in the exact center of the plate if it is an angle iron or WF member.) [highlight #FCE94F]I'm not sure of the name because it's not listed in the vendor drawing of the vessel. I only only see that it's an I-shape.[/highlight]
Thickness of plate? [highlight #FCE94F]1 1/2"[/highlight]
WxLxH of concrete? [highlight #FCE94F]20"x45"x30" This is the cantilever portion of the support. It's a single beam on top column making a T-shape. The entire width of the beam is 10'0" and the diameter of the column is 30".[/highlight]
Size of that corner chamfer of the concrete? [highlight #FCE94F]The chamfer built into the corner is 2" by 2".[/highlight]
Dist from edge of concrete to cl of the vertical rebar? [highlight #FCE94F]About 6".[/highlight]