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Another Sewage Treatment Wall Collapsing
7

Another Sewage Treatment Wall Collapsing

Another Sewage Treatment Wall Collapsing

(OP)
Even though this seems to be a poorly constructed project, I've got to believe that there is a design aspect to this.
Note that the Gatlinburg Wall Collapse looked very similar to this one.

RE: Another Sewage Treatment Wall Collapsing

It will be interesting to see the final reports...

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Another Sewage Treatment Wall Collapsing

There's mention of checking the design documents for hydrostatic pressure.

... but there's not a hint that they were checked for the increased effective hydrostatic pressure exerted by a load of Imported Norwegian Magic Rock.
 

Mike Halloran
Pembroke Pines, FL, USA

RE: Another Sewage Treatment Wall Collapsing

Thanks for keeping an eye out for these, Jed.  The end walls just zipped off like the one you posted earlier.

RE: Another Sewage Treatment Wall Collapsing

Nice pick up Mike, that stood out to me too. First, what the hell are these special sewage treatment rocks that we need from Norway? Last time I checked the east coast has a pretty wide variety of rocks... And like you said, did they design the walls for that additional load?

From skim reading the structural section of the report, seems like enough red flags were raised that the owner should have perhaps taken more drastic action. I know it is very expensive and they may not have many alternatives to shutting down sections of WWT plant, but look at their options NOW.

And this was just built @ 5 years ago! Not to overstate the obvious, but this should not be happening, especially in the US on municipal projects where public health and safety are paramount.

 

RE: Another Sewage Treatment Wall Collapsing

(OP)
From the report I linked to earlier, I'm more suspicious of the inquiry about the ACI 350 design.  I suspect that the invesigating engineers did some rough calculations, couldn't make them come out and wanted to see the design company's calculations to confirm their suspicions.
Of course there's almost never one single cause for a disaster like this. Several things had to go wrong.

RE: Another Sewage Treatment Wall Collapsing

Using 350 vs. 318 wouldn't make a difference in the basic ability of the tanks to resist the required loads.  All 350 does is provide more reinforcement to keep cracks and leaks less likely.

The extra reinforcement in 350 would of course add to the capacity of the walls, etc., but if designed correctly, under 318, there shouldn't have been a collapse.

 

RE: Another Sewage Treatment Wall Collapsing

(OP)
Except that ACI 318 allows you to use a load factor of 1.4 for Fluid Loads, where ACI 350 requires 1.7 or more.
That's what I'm wondering about.  It's unlikely to be something marginal because I doubt most engineers would raise a concern on a 5% or 10% difference.

RE: Another Sewage Treatment Wall Collapsing

ACI 350-06 has 1.4F.  ??

 

RE: Another Sewage Treatment Wall Collapsing

(OP)
ACI 318 still has 1.4 as far as I know.

RE: Another Sewage Treatment Wall Collapsing

I didn't see a 1.7 in 350...that's all I was asking.  What version of 350 has the 1.7?
 

RE: Another Sewage Treatment Wall Collapsing

(OP)
I'll have to check.  I think it bottoms out at 1.7 if all the spacing of reinforcing meets requirements. I'm in MikeHalloran's neck of the woods today, so I don't have my references.

RE: Another Sewage Treatment Wall Collapsing

1.4...1.7...whatever.  That's not what made those end walls fall off.  I don't know about the rock.  That could have contributed.  But like the other failure which Jed posted a few weeks ago, the detail at the wall intersections, as designed and/or as constructed, was obviously the weak link.  

RE: Another Sewage Treatment Wall Collapsing

This video was in a link from the original article.

Agree Hokie, I remember the other W VA failure was similar, where the front walls met the side walls. There was a "pull-away" failure of the front wall apparently from inadequate development of the horizontal reinforcement. But that W VA wall was fairly tall, this one seems like it is more in the 10-12' range judging from the video.

The pic from the main article- look at the walls in the background, there are horiz rebar stubs visible extending out from the side walls. So from this limited info I would speculate, if I must,that there were no hooks and the wall simply pulled away from the side wall (I can't imagine them detailing it like this!), or they did provide hooked bars at that connection and they failed in tension.

WWTP tanks are not my area of expertise, but square box tank structures with uniform pressures don't seem THAT complicated... Any other theories from some tank designers? If there were that many leaks as the peer review report indicated, could that be from rapid corrosion?

RE: Another Sewage Treatment Wall Collapsing

Just to get the geography right, this one is in New York, and the earlier failure was in Tennessee.

RE: Another Sewage Treatment Wall Collapsing

a2mfk  -  I don't often see standard hooks provided for horizontal bars in tank walls unless geometry requires it. The need for horizontal reinforcing is obviously higher near the corner, so running corner bars the full ld or 1.3ld into the wall can be beneficial in giving you extra area of steel where its needed.

What is the gradation of those special rocks? I bet they're pretty granular - lots of surface area for microbes. If its like a coarse poorly graded filter sand or pea gravel I can imagine some really significant lateral loads developing there at saturation.  

RE: Another Sewage Treatment Wall Collapsing

You are correct Hokie, I forgot where Gatlinburg is. Up near your old stomping grounds, at least that neck of the woods...

RE: Another Sewage Treatment Wall Collapsing

(OP)
As far as the post from a2mfk, I design a lot of these and the main lesson I would pass on is that while these are not complicated, the forces are more "real" and unrelenting than normal building type structures.  When you design a building for a 100 psf LL and a 90 mph wind, it's unlikely that they're ever going to see either one. You might get 30 square feet of bookshelves where there's 100 psf or the wind might approach 90 mph at a 60 degree angle to the wall, but neither is close to the design loads of 100 psf over every square foot of the structure plus a 90 mph wind over the whole wall of the building, plus increases for corners and edges.
Compare that to a water or wastewater tank.  The pressure is 62.4 pcf times the depth of the liquid.  It can't be less than that. This pressure is likely to be there for a long period, maybe years at a time.
There are some companies, big and small, that design for these loadings all the time.  We have procedures in place to assure a good design product. Even we make mistakes.  We learn something from every project. But on occasion, companies dabble in this work. They're the local favorites, they know somebody, they're owed the job, whatever. If they don't respect what they're doing, mistakes are made.  I'm not going to say that they can't do a good job.  It's just a different mindset.  
When hokie66 says "1.4, 1.7 whatever?", he might be right, as long as everything else is done perfect. It's a dirty little secret, but a lot of the load factors, phi factors, cracking calculations, etc., are there to protect from normal mess-ups in construction, design and operation.
Enough of my soap box ranting.  Just remember, next time you see a tank full of poop, thank people like me.  And if it's running down the creek, thank someone else.

RE: Another Sewage Treatment Wall Collapsing

For engineers not familiar with rectangular fluid retaining structure design, a common error (among many) is to ignore the combined tension and shear at wall intersections (Tees and corners).  High tension and shear interaction, combined with peak flexure at intersections, can cause these unzipping failures.

Instead of single hooked bars, I like U-shaped bars at corners and Tees, along with vertical bars at all inside corners.
In addition to that, we always use corner bars (45 degrees in plan) to address the reduced concrete shear capacity due to the aggravating additional net tension.

As much as I dislike tort lawyers and "expert witness" engineers that usually seem to just feed on their own, they do serve a purpose in these unfortunate events.  Bottom feeder engineers will steal jobs away from the experienced engineers in the short term, but also get purged from the system from time to time.
 

RE: Another Sewage Treatment Wall Collapsing

>>> 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

RE: Another Sewage Treatment Wall Collapsing

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

RE: Another Sewage Treatment Wall Collapsing

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

RE: Another Sewage Treatment Wall Collapsing

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

RE: Another Sewage Treatment Wall Collapsing

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?

RE: Another Sewage Treatment Wall Collapsing

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.

RE: Another Sewage Treatment Wall Collapsing

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.

RE: Another Sewage Treatment Wall Collapsing

The other failure which Jed referenced was even cleaner.
thread507-296127: Tank Collapses and Kills Two
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.

RE: Another Sewage Treatment Wall Collapsing

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

RE: Another Sewage Treatment Wall Collapsing

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.

RE: Another Sewage Treatment Wall Collapsing

(OP)
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.

RE: Another Sewage Treatment Wall Collapsing

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.  

RE: Another Sewage Treatment Wall Collapsing

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.

RE: Another Sewage Treatment Wall Collapsing

(OP)
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.

RE: Another Sewage Treatment Wall Collapsing

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.

RE: Another Sewage Treatment Wall Collapsing

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.

RE: Another Sewage Treatment Wall Collapsing

(OP)
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.

RE: Another Sewage Treatment Wall Collapsing

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

RE: Another Sewage Treatment Wall Collapsing

(OP)
I wondered about that myself. But that isn't even the worst mistake.  Since they didn't have the calculations, they should of studied every possible mechanism that might of been assumed. I think they could easily show that the design was inadequate no matter what load carrying pattern (cantilever or two way) is used. Plus they assumed the water was up to the top of the wall in their design calculations. I agree it should of been designed that way, but in the pictures, the liquid is a good foot or so below the top of wall. If the wall failed at a lower water level, the stresses at that level need to be checked.
They dwell on reinforcing lap lengths. But to prove that they had something to do with a failure needs a study of the stresses in the reinforcing at the lap location. After all, development lengths are allowed to be reduced if the analysis shows the bars aren't fully loaded.
This sloppy report, with its multiple offenders, will result in a muddying of liability, and no one being held accountable.

RE: Another Sewage Treatment Wall Collapsing

I just skimmed the report, and agree with Jed.  One thing which caught my eye was the statement that lapped bars are required to be spaced rather than touching, which is obviously a misuse of the wrong section of ACI318.

All those leaking cracks prior to the collapse, and in walls that haven't collapsed, shows the lack of adequate horizontal reinforcement.  And the little stubs of bars sticking out of the interior walls answers the question we had from the original photos.

RE: Another Sewage Treatment Wall Collapsing

Whole lotta unbonded rebar there!  Elite7 noted as well.

RE: Another Sewage Treatment Wall Collapsing

I don't think that bond was the issue.  The unbonded bars in the wall got to be that way when the wall collapsed.

The wall was clearly deficient in both vertical and horizontal reinforcement (only about .0038 Ag horizontal in a water retaining wall).

The wall wasn't cantilevered off the base slab, but according to the drawings was supposed to be pinned to the base slab as a propped cantilever.  The photos show very short lengths of bar protruding from the base slab, rather than the hooked bars which should have been there, similar to the internal wall.  The whole cantilever was dependent on passive resistance of the earth, which would have been fill material.  When the ground finally moved enough, down she came.

The failure of the report to investigate the actual mechanism of collapse is inexplicable.

RE: Another Sewage Treatment Wall Collapsing

Another report has been issued, by the well respected firm Simpson Gumpertz & Heger.  I don't think it is online, but if anyone comes across it, please post.

RE: Another Sewage Treatment Wall Collapsing

The first report had a ton of problems.  Why no mention of the lack of 90deg hooks at the ends of walls and the base slab (they're shown in the details)?  That, to me, is the primary failure mechanism. It looks like the tension at the bottom of wall and the intersecting walls caused the failure of the 6 inch dowels embeded therein.

  It is a very poor report.  It seems that the investigators had never designed a water holding structure.  

It will be interesting to see the SGH report and its findings.

RE: Another Sewage Treatment Wall Collapsing

(OP)
Gumpmaster, the lack of hooks probably was the last straw. But if the original design assumed a cantilever or a propped cantilever mechanism and it was designed correctly, the wall should have worked, hooked bars or not. The horizontal reinforcing was so light (#5's @ 9") that it doesn't seem likely that any horizontal bending was assumed.
I think it's also very possible that one-way (vertical) bending was the planned design, it was done poorly, the wall went into two way action, and the inadequate horizontal steel and lack of support from the side walls (planned or unplanned) was the end of the story.
The calculations would be a big help (hopefully they haven't "disappeared"), but more complete drawing information would go a long way.

RE: Another Sewage Treatment Wall Collapsing

Jed,
Gumpmaster has identified the same major deficiency I saw, but which not mentioned in the report by the insurance company's engineer.  Granted, the wall vertical reinforcement was inadequate, but that is not why it fell.  Neither was the woefully inadequate horizontal reinforcement.  It collapsed because of the connection between the wall and the tank base slab.

RE: Another Sewage Treatment Wall Collapsing

Disappointing in that SGH did not discuss the actual failure.  They obviously were not asked to do that.

RE: Another Sewage Treatment Wall Collapsing

This failure has me thinking.....

A contractor is currently asking our firm on a water treatment plant to use mechanical splices for the intersecting walls in a water tank.   Currently, we have designed and detailed corner bars with lap splices in both directions.   

I have used these mechanical splices before, but never in a liquid retaining structure.  In light of this, has anyone used these in the past on tank structure, and would you continue to use them in light of the failure?   I realize that mechanical bar splices may not have been the cause of the failure, but it has me questioning their use.

Thoughts??

RE: Another Sewage Treatment Wall Collapsing

(OP)
If they're threaded couplers, like Lenton Formsavers, http://www.erico.com/products/FormSaver.asp and they're properly installed, I wouldn't have a problem with them. They market their couplers as developing 125% of the bar strength. They look beefy, too. There's other systems I'm not as familiar with, but I suspect they're OK, too.
When you think about how little of the ultimate capacity is utilized if these bars are designed correctly, you appreciate how only a cascading series of errors can cause a catastrophe like this.

RE: Another Sewage Treatment Wall Collapsing

Speaking of rebar couplers, let me share this:

We had a job were we specified the couplers shown in the link below. During the inspection, we noticed that not one of the coupler screws were tightened properly (the tops torque off when properly tightened).

The Contractor commented, "Huh, I never saw those twist off before!" Which is a little unsettling.

So keep an eye out for this. It's clear in the manufacturer installation instructions that the screwheads torque when properly installed, but sometimes that information doesn't always make it to the site.

http://www.daytonsuperior.com/Lists/Product%20Catalog1/Product.aspx?List=3c62f7a2%2D1783%2D4ee6%2Da0c4%2D2642507b1472&amp;ID=113&amp;NodeId=420&FilterType=&amp;Filter=

"We shape our buildings, thereafter they shape us." -WSC

RE: Another Sewage Treatment Wall Collapsing

Failures like these are good topics for CEU seminars.

RE: Another Sewage Treatment Wall Collapsing

How do you use threaded couplers or ferrules at an intersection?  Looks to me like you would not be able to turn the corner bars into the ferrules.

RE: Another Sewage Treatment Wall Collapsing

Anyone notice that the engineering report lists the lap splice length as the primary cause of the failure? The report says the actual lap splice length was 5'-4" and should have been 8'-8". From the equation listed at Section 12.2.3 of ACI 318-08, I calculate 46" as the minimum length for a Class B Splice vertical bar and 60" for a Class B Splice horizontal bar. This also agrees with a CRSI design table that I have.

I would usually avoid a lap splice at this location but by my interpretation of ACI 318, the lap splice length meets the code.
 

RE: Another Sewage Treatment Wall Collapsing

(OP)
I agree with CivilPipe that blaming lap splice lengths is bogus. I'm sure that there have been failures due to lap splice inadequacy, but to just state that the lap splice lengths were inadequate, and throw that out there as a cause of failure is negligent.  As I stated above, development lengths can be reduced based on bar stresses, and they're the basis of lap splices. And that is assuming that they were correct in the first place.
This kind of report has lawyers licking their chops.
  

RE: Another Sewage Treatment Wall Collapsing

Yeah, I think we came to that conclusion above.  The wall fell because it came loose from its supports, not because it failed in bending.  The report was amateurish.

RE: Another Sewage Treatment Wall Collapsing

Any one hear anything new on this project?  

"We shape our buildings, thereafter they shape us." -WSC

RE: Another Sewage Treatment Wall Collapsing

(OP)
Here's a fresh article on this disaster.
Unfortunately, the pattern for these problems is that the people who know the technical details aren't talking. Just like the Gatlinburg, Tennessee wall failure, I'd sure like to look a the design drawings. They blamed that on a corrosion affect at the tie in walls, which I still don't completely believe.

RE: Another Sewage Treatment Wall Collapsing

Damn, now they want me to subscribe.

Mike Halloran
Pembroke Pines, FL, USA

RE: Another Sewage Treatment Wall Collapsing

Found this online after reading some of the initial news reports of the lawsuits. It appears that a post construction audit was done on the facility due to the magnitude of deficiencies noted during construction. I haven't read the audit so I am not sure if it contains any valuable information but thought that someone my like to look at it.

RE: Another Sewage Treatment Wall Collapsing

Has this issue ever come to a conclusion? Has the tank been reconstructed?

"We shape our buildings, thereafter they shape us." -WSC

RE: Another Sewage Treatment Wall Collapsing

I'm sorry JC. Is there supposed to be a link to a story in your OP? I can't see the story you guys are talking about.

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