Another Sewage Treatment Wall Collapsing 7

[MikeHalloran]

>>> The pressure is 62.4 pcf times the depth of the liquid.<<<

True for clear water.

Where do you factor in the additional weight of slurries and such?



Mike Halloran
Pembroke Pines, FL, USA

 

[JedClampett]

We do water and wastewater treatment plants predominately. 63 pcf times the height works fine 99% of the time.

 

[hokie66]

Jed,
But those fancy rocks would add to the pressure, wouldn't they?

 

[JedClampett]

Yeah, I guess they would, but we've never treated sewage with rocks.
I'll have to check around.

 

[a2mfk]

Jed- Good point, I was thinking about how quickly a fluid retaining structure gets load-tested, while most buildings go their lifetime without seeing anything close to the worse case loading scenarios.

So Jed, ATSE, others, what about my original question about the failure mechanism in these tanks, looking at the pictures of horizontal rebar stubs extending out from the inside walls. Any guesses?

 

[ATSE]

a2,
Stub rebar at wall ends as shown in the photo make no sense to me either - it appears like bar ends. Very improbable that the bars all fractured uniformly in tension with the same stickout. Guessing that horiz bar lap was lacking.
Just guessing again, but possibly due to creep and lack of tension capacity at the T-intersection, the perimeter wall started to separate from the perpendicular web walls, with the wall behavior changed from 2-way action to 1-way action, and base moment demand increased 4x, immediately hinging at the base.

 

[a2mfk]

So maybe the front wall rebar was not properly lapped with the side walls (or the corner bars were improperly lapped), and they failed in tension by pulling out of the side walls, taking some concrete with it of course. The picture then shows those horizontal rebar that is remaining.

A surprisingly clean failure though. Thanks for your 2 cents.

 

[hokie66]

The other failure which Jed referenced was even cleaner.
thread507-296127
That one looked like there were ferrules in the ends of the cross walls, and the end walls were cast later, and the screwed in bars failed. Would seem impossible to install hooked bars into ferrules...maybe that's what they proved. Could have been the same in this one, but they didn't even try to hook the bars.

 

[Elite7]

Looks like inadequate bond. But how did it stand for so long then?

 

[JKW05]

The load factor for F in ACI 350-06 is 1.4. However, there is also an "environmental durability factor" (Sd) that is to be included which can vary from ~1 to over 2 depending on the size and spacing of the reinforcing. This is to minimize the size of cracks as JAE noted previously.

Another consideration that needs to be included in the size of the horizontal reinforcing in rectangular tanks is the direct tension forces that the walls experience from counteracting the fluid pressure on the end walls.

 

[JedClampett]

I kind of like hokie66's theory. Maybe the divider walls were tilt up. At least the hardware looks like ferrules used to lift tiltup panels. An inexperienced designer might have used tiltup hardware (it has a load rating, after all) with threaded rods developed into the long walls. Maybe the rods weren't engaged enough into the ferrule or they were cross threaded or any number of installation issues. Or the threaded material was more brittle than reinforcing.

 

[JLNJ]

I don't know much about them, but those media rocks must increase the lateral wall pressures significantly - maybe 30% or more over water-only pressures.

 

[JedClampett]

Now the plant staff is worried, for good reason, about the remaining walls collapsing. And the original design firm is not exactly going out on a limb with a warm fuzzy.

 

[KevinMonroe]

For the Binghamton Plant (in NY) one of the papers had a pdf of the City's contract with the forensic firm that's investigating the failure. The contract gave a description of the tank - dimensions, wall thickness, reinforcing. Doing a quick check showed that the horizontal reinforcing was about 1/3rd of what ACI 350 would require. And ACI 350 doesn't require 3 times what ACI 318 does. That same article also showed pictures of water pouring through a gap between the wall that failed and an intersecting wall - it was in the process of pulling away.

My bet is that when the reports come out, they will indicate that there was either an error in the design calculations, or the wrong reinforcing was called out on the drawings, or the shop drawing had the wrong reinforcing and it wasn't caught during the review.

 

[hokie66]

The shotgun approach to litigation has begun.

http://www.theithacajournal.com/art...ile-20-million-sewage-treatment-plant-lawsuit

 

[JedClampett]

Putting aside the owner's culpability in hiring incompetent engineers and/or contractors, I'm glad they're suing. This tends to focus not only the litigants, but shows the entire industry that this is a serious business.
Any time a client complains about our invoices, I think I'll attach those photos.

 

[hokie66]

I agree completely, Jed. Twelve defendants does seem a lot for a wall collapse, but perhaps the lawsuit casts a wider net than just the structural failure.

 

[bridgebuster]

Only 12 defendants? Binghampton's not a big town. I was on jury duty 25+ years ago, a lawyer told the jury panel they have to sue everyone because that's the only way they can find out who is guilty.

Being a cynic, I can't help but think that lawyers do this to help their colleagues by generating billable work for other lawyers. Think of it as a professional courtesy. Meanwhile engineer's tend to be like prostitutes fighting over a customer.

It's a shame that we as engineers can't be like lawyers and find ways to create work. Look at how many TV commercials there are by lawyers suing for everything and anything.

 

[JedClampett]

Here's a forensic report on the failure. Unfortunately it exhibits only a little more understanding of the problems of water retaining structure design than the original design firm.
It's a classic case of the blind leading the blind.

 

[MikeHalloran]

When I was in college, working summers as a draftsman, my engineer employer did a slurry system installation, where a basement room was converted to a slurry tank. I asked the engineer who was working on it why we had specified a lot of reinforcement added to what were clearly very sturdy poured concrete walls. He explained that the 62.4 psf x water depth rule had to be multiplied by the specific gravity of the slurry in order to predict the pressure at depth.

The forensic report repeats the 62.4 psf number, as if the tanks were filled with just water. One look at photo 7 of the forensic report suggests otherwise. Anybody know the specific gravity of the magic special imported particulates in that tank?

Mike Halloran
Pembroke Pines, FL, USA