Concrete Ring Wall Foundation for Tank
Concrete Ring Wall Foundation for Tank
(OP)
Hi all,
The project is a retrofit of an existing 12.8m high x 48.9m dia tank supported on a 0.45m wide 2.3m deep ring foundation. Due to the retrofitting there will be an increase in loads. I am doing a check to confirm the existing reinforcement provided is sufficient. This involves revising the existing structural calcs.
Due to the additional line load, the vertical reinforcing req'd is 0.02% which is well below that ACI318 min of 900mm2 (0.2%) that has been provided. No issues there.
However, due to the additional hoop stress, resulting from the increased surcharge acting above the ringwall, we are above the allowable tensile stress of the 35MPa concrete 3.43N/mm2> 3.29N/mm2.
I am still satisfying the maximum crack width of 0.3mm from ACI-244
Maybe I am not understanding the structural theory between load transfer between concrete and steel but the calculation approach seems flawed as the entire hoop stress is applied to the concrete alone without considering the reinforcement. Wouldn't there be some load taken by the reinforcement?
In short I am looking for a way to make this work, within the codes and rules of course.
Any help appreciated.
The project is a retrofit of an existing 12.8m high x 48.9m dia tank supported on a 0.45m wide 2.3m deep ring foundation. Due to the retrofitting there will be an increase in loads. I am doing a check to confirm the existing reinforcement provided is sufficient. This involves revising the existing structural calcs.
Due to the additional line load, the vertical reinforcing req'd is 0.02% which is well below that ACI318 min of 900mm2 (0.2%) that has been provided. No issues there.
However, due to the additional hoop stress, resulting from the increased surcharge acting above the ringwall, we are above the allowable tensile stress of the 35MPa concrete 3.43N/mm2> 3.29N/mm2.
I am still satisfying the maximum crack width of 0.3mm from ACI-244
Maybe I am not understanding the structural theory between load transfer between concrete and steel but the calculation approach seems flawed as the entire hoop stress is applied to the concrete alone without considering the reinforcement. Wouldn't there be some load taken by the reinforcement?
In short I am looking for a way to make this work, within the codes and rules of course.
Any help appreciated.
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RE: Concrete Ring Wall Foundation for Tank
1) Must be an error, reinforcing steel is assumed to carry 100% of the tensile hoop stress.
2) Test to see if the actual concrete strength > 35 MPa. Since the existing tank/foundation are certainly >> 28 days old, concrete strength may exceed nominal.
Also, passive soil pressure outside the ring wall is typically ignored. Do the calcs to see how the amount of passive pressure needed to "pass" compares with total passive pressure available. If the percentage is low enough, you may want to take a closer look at existing conditions to see if reasonable restrictions on operation are practical.
...and there is always the possibility that the ring wall analysis fails. In this case all may not be lost... perhaps a new, concentric ring wall can be constructed outside (and in contact with) the existing ring wall to contain the existing ring wall.
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RE: Concrete Ring Wall Foundation for Tank
Also, to get from the vertical load under the tank to the horizontal pressure involves a fudge factor that can be varied somewhat. API-650 recommends 0.5 for normal fill or 0.3 for granular fill, as I recall. But if you have any basis to more accurately determine that pressure, it may be simpler than modifying anything. You may also have some leeway on load factors used.
RE: Concrete Ring Wall Foundation for Tank
Consider wrapping the existing ring beam with post-tensioning. This will help control or improve the existing cracking, while adding any needed new tension strength. Then, cast a new outer ring beam, as much as anything, as protection for the post-tensioning. But, some of the final tensioning may take place after this new beam section is cast.
RE: Concrete Ring Wall Foundation for Tank
Regarding your question about the share of load between steel and concrete, the steel contribution before cracking was probably ignored for convenience which is about 10% conservative for this reinforcement quantity. However there doesn't seem to be any account taken of tensile stresses due to restraint so the cracking stress is overestimated.
It's a little hard to comment without having ACI224 but it looks like a flexural cracking calculation has been applied to axial tension so some loss of accuracy. Is your job just to update the old numbers or should a new crack width calculation be substituted? EC2 perhaps.
RE: Concrete Ring Wall Foundation for Tank
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RE: Concrete Ring Wall Foundation for Tank
The only issue you have with the calculation is the stress in the concrete is 3.43 N/mm2, a mere 4.2% above the allowable stress 3.29 N/mm2. Am I right? As SRE pointed out before, after cured in ages, the concrete would have a much higher strength than the originally specified strength - 35 MPa in this case. Knowing the age of the tank, you can estimate present concrete strength, using readily available charts in any of the reinforced concrete design textbooks.
Also you have to keep in mind is the conservatism in the design codes regarding strengths, allowable stresses, amplification of loadings, after adding all up, is the 4.2% that difficult to accept? It's your judgement call.