Thursday, July 5, 2007

There is no 13th floor...until now.

Part of the deck (floor or roof surface) of a 14-story building under construction collapsed today in Greenwood Village, CO, which is about 30 minutes south of Denver proper (in an area we call the Tech Center). From the photos, it looks like it was a post-tensioned cast-in-place slab that collapsed. Despite the fact that it was top news here in town today, I had to hear it from Miss Kitty via an IM chat this afternoon. According to one local news channel's website, a portion of the 14th floor collapsed onto the 12th floor. "There is no 13th floor, said officials" the website reported.

Um, yeah. You do have a 13th floor. As comic Mitch Hedberg observed, "If your building goes from 12 to 14, everyone on the 14th floor knows what's up."

Here's a photo showing the section of the building that fell, just one floor.
The website reported that the fall may have had something to do with some formwork being removed and some supports underneath giving way. Here's a closeup of the collapsed section.

The long stringy things that are hanging down are the post-tension rods that are now not in tension because the concrete around them is gone. You put the rods in sleeves, then pour concrete around them, then when the concrete is cured you use a machine (I think it's a machine, some type of tool; my mom used to work on post-tensioned CIP slabs) to stretch the cables. Stops on the ends of the cables hold them in tension, and the slab then gets its strength from the tension of the cables pulling against the concrete. If that makes no sense, don't fret. I'll do an upcoming Detail of the Week on it.

See the vertical wood sticks to your left of the hole? That's shoring--supports holding up the concrete as it completes curing. Funny they said that about the formwork. When I read the news report online this afternoon, Norman came over to my desk to look at the damage. "Wonder if there's an RFI in there about 'can we take the formwork off early?'" he said.

"What, you think that's what it was?" I asked him.

"Maybe," he replied. "Every project I've ever done and ever seen done with cast-in-place slabs has an RFI where the contractor wants to take the formwork off earlier than the structural engineer says to, just 'cause a lot of engineers make them leave it on longer than necessary."

"Hmm." I didn't have much of a reply. Wheatlands was precast concrete, so there was no arguing from the contractor. I'm even more troubled by the website's indication that construction was behind schedule, and rushing to finish is a good way to make mistakes. I don't know that rushing and being behind is why maybe just maybe the formwork was popped off early and the floor fell in and 14 people went to the hospital, but.... It sure doesn't make them look good.

But it doesn't end with the contractor. According to the news report on the website, the owner added another floor and made changes that possibly pushed the schedule behind. Adding a floor to a building that's already been designed and engineered sounds like a nightmare to a Pixie. I thought the staff at Wheatlands finally choosing radiology equipment a month after the CDs went out was a pain in the ass. Reckon I don't know pain like this project knows. 14 people in the hospital kind of pain.


Miss Kitty said...

Triskadekaphobia is SO 13th-century.

Wilderness Gina said...

Tendons are stretched by hydrolics. Buildings use smaller ones; 1 or 2 guys can carry a "puller". Highway bridges use ones that are moved around by crane. they all use metal 'gromits' to hold the tendons. That Photo looked to me like there wasn't even any concrete on the part that fell. There wasn't enough rubbish under the slack tendons.

Mile High Pixie said...

Mom, I knew you'd know how that thing was put together. That photo may have been taken after they cleared off the rubble to get a guy out. It certainly does seem overly clean, though, doesn't it?

faded said...

This is an unusual failure. It broke at the edges of the columns. Did they clean the site up? If the tendons had been loaded I would have expected the failure to spread across the entire floor. There is much here that is not visible.

Mile High Pixie said...

I think they did clean the site up by the time this photo was taken. It's a strange failure for sure.

Formwork said...

Still no answers on what caused the failure?

Mile High Pixie said...

In what few news articles I could find online about it after July 5, it looks like the project was behind schedule, and a few folks who spoke to the media said "there was a problem with the concrete." My guess is that they took the formwork off too soon before it was done curing, and there may indeed have been a mix issue with the concrete; the two issues combined caused it to collapse. My two cents, anyway--I'm not an engineer or a contractor.

Anonymous said...

You can see the first section of the slab (on right) is concreted. It looks to me like they were in the process of casting the deck when the formwork gave way. Probably due to not following the formwork design i.e. not install bracing. All the columns around are still intact. When the deck collpased they stopped the pour.