Design of underground precast Structure

[nader666]

Hi everyone,

I am the review engineer of a new sewer pump station project which has a 23' deep & 10' diameter grease trap. The contractor has submitted a shop drawing for the grease trap (copy attached) with a monolithic base and solid rings on top, the submitted shop drawing shows 4" thick wall with 6X6-w4xw4 WWM reinforcement (copy attached). I request structural calculation because I did my calculation and it did not even come close to that wall thickness and reinforcement. I have attached the calculation submitted by the project design engineer. He argued that ASTM C478 is not the proper standard because there is ASTM C1613 which is specifically for Concrete precast grease trap ( copy attached). The structural design requirement on ASTM C1613 has stated to use ACI 318 Strength Design Method or Alternate Design Method. I think because the grease trap is an environmental concrete structure, ACI 350 is the right code to design the grease trap. The project engineer still insist that his calculation is correct and because the grease trap is already available at the precast yard, he is fighting to get the shop drawing approved and have the contractor use it. I think he has used the minimum reinforcement for non-bearing wall and I think As should be calculated to resist the maximum tension( As = Tmax/0.9 Fy).
Please advice if the attached calculation acceptable.

Thanks

 

[IDS]

I have marked the other post (which has the wrong file attached) for deletion.

I don't have time to go through the calculations in detail, and I'm not familiar with the codes, but 4 inches does seem very thin for a structure of this size and depth. I'd recommend finding an engineer familiar with design of this sort of structure, and standard practice in your area.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[bridgebuster]

It sounds like you have an environmental structure; excerpt from ACI 350: "Environmental engineering concrete structures are defined as concrete structures intended
for conveying, storing, or treating water, wastewater, or other liquids and non-hazardous materials such as solid waste, and for secondary containment of hazardous liquids or solid waste".

However, unless your town requires the use of ACI 350, the contractor may be correct about the design code; another excerpt from ACI 350: "Precast concrete environmental structures designed and constructed in accordance with ASTM or AWWA standards, with the exception of circular tanks, are not covered in this code."

Without going through the design, it just feels flimsy to me. 20' deep; 10' diameter, and 4" walls. As a sanity check go to the NYCDEP site and look at their sewer standards. They have an 11" wall for that depth and diameter. They use 4" walls for deep manholes but the diameter is 4 to 5 feet.

 

[TehMightyEngineer]

The diameter does seem large for a 4" wall. I would also agree that ACI 350 would only apply if your town required it as the ASTM standard doesn't seem unreasonable for the application. However, unless you can find something specifically wrong it's the project engineers stamp on there, not yours. As a reviewer it seems to me that your scope is making sure the design was done conforming to the project design requirements, not necessarily to double check the design methodology except for making sure that it was designed in accordance to the standards used. It's a grey area but it seems to me that you've stated your concerns with the project engineer so either you accept his assurances, find an example to support your calculations, or you find some way to arbitrate it.

But that's just me.

Maine EIT, Civil/Structural.