Existing Mechanical Tower Foundation not following ACI Seismic Detailing Requirement 4

[FRACH23]

Hi guys. I reviewed an existing steel frame tower (for a food processing plant) on piled foundation. My initial check goes fine by strength and serviceability, but i was worried that the RC pedestals did not follow appropriate seismic detailing (located at high seismic risk zone). It was like around 500x500mm cantilever pedestal with 8-d20mm main bars just hooped by d10mm ties. My concerns are:

1. Roughly the equally-spaced bars were at 190mm on center spacing, wasn't it supposed to have cross-ties by ACI318 seismic detailing?
2. If such cross-ties were really required for seismic detailing, the trouble was, the pedestal is already existing. Does this conclude to demolition though by strength the pedestal is still good? Or retrofit by any means possible without totally knocking down the pedestal?

 

[dik]

note it if non-conforming and if concerned about the strength, notwithstanding ACI requirement, then repair as needed.

Dik

 

[FRACH23]

Hi Dik, its OK for strength, but its construction detail does not conform with ACI requirement to have cross ties. Is there any acceptable repair on this other than demolishing? Say when you have the pedestal carbon fiber wrapped, could that escape from the rule somehow?

 

[dik]

If adequate for strength, then note it as non-conforming and that the strength is adequate, and live with it...

if not comfortable with that approach, then do a little research, and if still not comfortable, then recommend that it be reconstructed to conform.

Dik

 

[MrHershey]

Wrapping with FRP would probably be a good bet here. When we've had cross tie issues in the past, we've justified it by actually post-installing FRP through the column where column ties are missing in conjunction with a perimeter wrap.

 

[KootK]

I'd easy up to the demolition option slowly and cautiously. That's an extreme solution that won't be well received, especially if it's not justified.

I don't know much about your system but I would be surprised if your pedestal is actually an element expected to undergo serious inelastic deformation. When that is the case, there's usually a way around adherence to the high ductility requirements.

Another approach, if you truly have a lot of excess capacity, might be to simply disregard the presence of the vertical bars that do not have ties.

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.

 

[dik]

"I'd easy up to the demolition option slowly and cautiously. That's an extreme solution that won't be well received, especially if it's not justified."

Great advice...

Dik

 

[MotorCity]

If you are trying to get around a code requirement, its best to get the AHJ to buy in to your rationale and perhaps show that its ok by analysis. Code requirements are there for a reason and any deviation should have AHJ input/approval.