Septic Tank top half as two-way slab? 1

[ctbailey]

Hello to the group.

I've got a precast rectangular tank that I am trying to analyze. The tanks are cast in two sections, a top and a bottom. The slabs are integrelly cast with the walls, forming essentially a two-way slab with a continuous knee-wall around the perimeter. The tanks range in size, but normally around 7 feet wide, 12 feet long, and the "wall" sections are 45 inches tall. Two tanks are cast: a 4" slab top and a 6" slab top.

ACI 318-11 chapter 13 lists applicability to include slabs that are continuously supported by walls to consider those walls as a beam with infinite stiffness.

My question: the negative moment that I envision at the slabtop/wall interface due to being monolithically cast... if ignored, the resulting shear & moment would be conservative?

I understand that ACI 350 handles "environmental underground tanks." Does the ACI 350 approach differ greatly from the ACI 318 approach?

Or... since these tank tops are so small, would a modified Tee-Beam analysis be done for the edges, and a two-way slab analysis be done for that portion of slab that is not included in the "beam flanges"?

Thanks

___
Craig T. Bailey, PE, LLS

http://www.bailey-associates.com

 

[TehMightyEngineer]

I've designed all of our companies traffic rated septic tanks. I usually make an FEA model of the tank using 2D plates in RISA-3D or STAAD to calculate the two-way actions of the slabs.

You'll likely find, as I did, that the 2-piece tanks having a shiplap joint at the middle will act a lot like a roller at those joints near the middle of the wall span, and becoming more like a pin connection at the corners of the walls. Thus, the need for a 3D FEA model to come up with a reasonable interaction of the walls, corners, joints, slab, and manhole openings.

If I didn't have FEA available I'd design the slabs as 2-way slabs supported on infinitely stiff beams. You're correct that if you ignore the flexural restraint at the slab to wall joints you'll conservatively design the positive flexural reinforcement in the slab. I'd also consider some flexural restraint at the edges near corners and envelope the design so that I had both conservative positive and negative flexural moment in the slab to wall joint. This is going to be increasingly difficult to do without FEA if you have vehicle load considerations, but based on those slab thicknesses I'm going to assume these are not H20 rated septic tanks.

For the walls where they're integral with the slab you'll probably have a lot of issues getting accuracy without FEA. I'd idealize the walls as spanning horizontally and design them as rectangular tanks per the PCA rectangular concrete tanks book (note, this uses an older version of ACI 350).

Make sure you actually need to apply ACI 350. Often you do, but for typical residential septic tanks you may only need to design to ASTM C1227 and C890, which reference ACI 318. ACI 350 doesn't differ by much from 318; often your design programs and spreadsheets may only need minor modification to to correctly calculate the reinforcement required. The biggest difference will be the S_d (environmental durability factor). That can sometimes give you a very big penalty to your reinforcement required.

I'd design for a range of fills and provide the precaster with a maximum fill depth allowed for each tank. That will be exceedingly useful.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[ctbailey]

Hi Ian,

Thank you for your time. It is much much appreciated.

A 2D (and certainly not 3D) FEA commercial license is not something I've got access to, but I'm in 100% agreement that this exercise certainly warrants it. This is without a doubt the most indeterminate structure I've been asked about! I think I could "check" my hand calc designs at my other job on a college campus, though...

And what kills me - these Residential tanks have been built and speced for 60 years on gut instinct.

Running through 318 I am all ready finding the positive reinforcement spacing is not meeting 318 requirements, and likely the precaster will scoff when I point this out.

Your point of "do you really need 350?" is well taken - these are residential, and I can find no jurisdictional code authority that calls for ANY code to be applied. (Yes, this is New Hampshire.)

Thanks again. I've got work to do updating some spreadsheets to go 2-way. The precaster may be surprised when they see the rating for even their "Heavy Duty" 6 inch slab tops.



___
Craig T. Bailey, PE, LLS

http://www.bailey-associates.com

 

[TehMightyEngineer]

Uh oh, your precaster is actually a competitor of us then as we definitely sell precast tanks in New Hampshire. So, please understand that I will try to limit any bias to help you out but I wanted you to be aware that in fairness I have an interest in actually not helping you.

You'll do me plenty of favors if you jack up the thickness of their tanks, but wont do them any favors. Your precaster is competing against me and my FEA models where I've cut out every inch of a concrete and any bar I can spare. You're entirely right that he wont like what you tell him his tanks can do because with 60 years of experience they have a good understanding of what will and wont crack a typical tank.

I'd definitely pick up a copy of ASTM C1227 and C890, if there's no design standard specified by local codes then that's what the precaster should meet for industry standards. That's what we design to for residential tanks. Also, be aware that ASTM C1227 allows for load testing rather than rigorous engineered designs, that's often how many standardized tank sizes were created.

Another thing to consider is that many standard septic tanks are limited by standardized form sizes. Obviously reinforcement and such can be changed but your precaster will likely be limited in what they can change thickness wise in their form.

Regarding the reinforcement spacing; design it as plain concrete considering axial compression if the reinforcement is spaced greater than 18 inches. That's how the tanks actually function and how you should look at it.

All said though, it's your stamp. Only stamp what you're comfortable stamping. We had a custom arch form kicking around that we wanted to use but when I ran the numbers on it the form was poorly optimized in it's shape. I wasn't shy telling my boss that we'd be better off junking the form and getting a new one made.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/