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Flat Slab Collapse in Windsor, Ontario, Canada

Flat Slab Collapse in Windsor, Ontario, Canada

RE: Flat Slab Collapse in Windsor, Ontario, Canada

Even if the boom lift did initiate this collapse, I'd say the cause is serious understrength.
There wasn't much spare capacity there, collapse would have happened anyway, sooner or later.
Rebar corrosion would have to be a suspect.

RE: Flat Slab Collapse in Windsor, Ontario, Canada

(OP)
Agreed, the lift was probably the straw the broke the camel.  

I've parked on that structure several times (my favorite thai restaurant is out front).  I've never thought that the workmanship of the concrete looked very good.  Should be interesting to see how this one turns out.

RE: Flat Slab Collapse in Windsor, Ontario, Canada

My eyesight is not as good as it used to be, but I can't see any reinforcement through the columns.

BA

RE: Flat Slab Collapse in Windsor, Ontario, Canada

I'd be thinking real hard about the other portion across the access road to the lower story that has not failed...yet - the one where the cars can still be driven off!

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

RE: Flat Slab Collapse in Windsor, Ontario, Canada

That's not the same one, Doug.  That failure was some time ago, and was also discussed here.

RE: Flat Slab Collapse in Windsor, Ontario, Canada

Oh, sorry.

I thought it didn't look quite right, but with the yellow painted columns and Canadian location I thought it must be the same one.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
 

RE: Flat Slab Collapse in Windsor, Ontario, Canada

There does seem to be a bit of a pattern.  Canada, flat plate punching shear, parking structure, asphalt topping...

RE: Flat Slab Collapse in Windsor, Ontario, Canada

I wonder if the AC topping was an after thought, the weight not being incorporated into the original design.

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

RE: Flat Slab Collapse in Windsor, Ontario, Canada

Isn't there a professor from a Canadian University that leads the way in punching shear reinforcement. I have just specified propriety stud-rails on a flat project that I am working on.

RE: Flat Slab Collapse in Windsor, Ontario, Canada

(OP)
More info, the structure was built in 1948.  From what I can gather, I think the adjacent level has been roped off, and the three buildings have also been evacuated.

asixth, you are probably speaking of Dr's. Dilger and Ghali out of the University of Alberta, they developed stud rails back in the 80s, and since they have been marketed throughout the world. They also developed the code equations used in the canadian concrete code for punching shear reinforcement.

the canadian code is actually pretty far along compared to ACI when it comes to punching shear, due to Dilger and Ghali's research being accepted there first.  Although being built in 1948, this one obviously predates that..

RE: Flat Slab Collapse in Windsor, Ontario, Canada

If constructed in 1948, then it was designed using the older Working Stress Design and there is added safety for flexure and for shear in the method.  In addition, I would have thought that there would be corrosion issues.  The area uses a fair amount of salt de-icer and there could have been some degree of exfoliation of the concrete surface as well as reduction in steel area. There could have been added toppings to increase the dead load.

Dik

RE: Flat Slab Collapse in Windsor, Ontario, Canada

Looking at the top of the columns, I don't see the failure cone that we assume in a two way shear check, i.e. d/2 from the face of the column. The failure plane here is exactly the perimeter of the column. Doesn't that mean shear reinforcing would not be useful?
 

RE: Flat Slab Collapse in Windsor, Ontario, Canada

Good question, Ailmar, and I think you are right.  The first thing I thought of in looking at those photos was that the columns may have been poured too high, reducing the slab depth.  Punching shear has been studied to death, but I still don't believe in reinforcing against it.  Using depth, better concrete, and larger effective perimeter are the better paths to take.

In the 1948 structure which failed, a lot of things could have contributed, including overloading, poor concrete, poor construction practice, deterioration due to salting, the list goes on.  This appears to be a flat plate, and in 1948 I think this type floor was uncommon and perhaps ill understood by some.  Flat plates in the early days were primarily used in lift slab construction.   

RE: Flat Slab Collapse in Windsor, Ontario, Canada

The 50 psf live load we design for in these garages always seemed intuitively too light to me.

And now we've skinnied it down to 40 psf. A few inches of asphalt overlay and most of that's used up.
 

RE: Flat Slab Collapse in Windsor, Ontario, Canada

JLNJ... the 50 psf is adequate for cars and most trucks (not big ones) and some jurisdictions have reduced this to 40 psf. Realistic normal loading with vehicles parked bumper to bumper, and in all the drive isles as well as parking spots will yield a load of between 15 and 20 psf.  A typical car space is about 300 sq.ft.  Design loading is OK...

Dik

RE: Flat Slab Collapse in Windsor, Ontario, Canada

From what I can see, the whole portion of slab (with vehicles still in same relative positions)went down in one shot. With the boom/lift at one end, it is interesting that the complete slab appears to have collapsed. With the lift creating local concentrated loads, you would not expect that. So I agree it was probably the 'straw that broke the camels back', and led to a progressive, quick collapse. The date of the structure, lack of reinforcing, design (at that time), corrosion etc. created a scenario where punching shear was unfortunately illustrated as a textbook case. I am actually going to Windsor this afternoon for a family function and if I can sneak out.....might have a look.

RE: Flat Slab Collapse in Windsor, Ontario, Canada

It happens too often, an engineering failure gets investigated and no report released. When structural failures occur, the forensic findings should be released for the entire engineering industry. How are we meant to review the unified set of principles that we design for to ensure they are adequate.

My belief is that 22kip (100kN) crane (whether it was loaded to 100kN or not is another question) is the 'straw which broke the camel's back' and that an engineer should have been consulted prior to putting that crane on the floor. No competent engineer would have certified that floor to be structurally adequate to support that load. The deck looks 5-6" with not much reinforcement. Assuming any structural plans of the carpark would be long gone by now.

RE: Flat Slab Collapse in Windsor, Ontario, Canada

In this case, the report should have a good chance of being released, as there would be no one left to blame for the design or construction shortcomings.

RE: Flat Slab Collapse in Windsor, Ontario, Canada

(OP)
Well, here's some more info.  Appears that an additional layer of asphalt was put on the deck recently, by the previous owner.

http://www.windsorstar.com/Asphalt+repair+overloaded+parking+structure+engineer+says/3261984/story.html

A lot of factors seems to be adding up here, for the perfect storm so to speak. Extra asphalt, boom truck, rebar corrosion, shotty construction.  It's pretty amazing when you think how much it takes to overcome the safety factors.   

RE: Flat Slab Collapse in Windsor, Ontario, Canada

There you go...  

The "Lets install a protective layer layer of asphalt to protect the corroding rebar" scenario.

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

RE: Flat Slab Collapse in Windsor, Ontario, Canada

Looking at the photos, it looks like a fairly thick layer of asphalt was added... a couple of inches or so.

Dik

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