Besides its political infamy, it is (was) a 'famous' post-tensioned concrete structure because it was the first to use UNBONDED PT in banded/uniform arrangement due to its irregular column alignment.
I was there back in late 2001 whilst it was (had) gone through PT repairs due to corrosion. It is a flat plate structure with rectangular columns. Not sure if corrosion of tendons has plagued the structure over the last 10-15 years.
Just checked some reports I had dating back to 1999 on the office building. The spans are irregular but don't exceed 22 ft, constructed of 8" thick, lightweight concrete slabs.
B2 and GR level parking repairs cost projections in 1999 $ exceeded $3M for tendon repairs.
There is an adjacent apartment/condo building too. I think it was framed with similar construction.
It appears that the partial multi-floor collapse was in the stand-alone public parking structure (that make up the large Watergate complex along with office, condo and hotel buildings) and from photos from CNN it does not appear to be PT flat plate construction.
Looked like some significant landscaping was being undertaken on the upper 'green' level at the time of collapse.
That's a good photo of a punching shear failure, and looks like a flat plate. I couldn't find photos that show much of the collapsed structure. There are some photos taken under the part which is adjacent, and that is a waffle plate.
Some of the photos show a portion of waffle which came down, but it is hard to tell whether that is the type structure which failed, or was taken out by debris from above.
A few of the media images seem to show several FEET of soil on the top level AND earthmoving equipment. And then there was this image of the waffle slab:
Yes, the underground nature of this parking structure, with soil and trees on top, is definitely a big issue. Whether or not the top slab was designed for that, we don't know. It is hard to say if the waffle slab in the picture was the top slab or was knocked down by falling debris. The articles called it a 3 storey parking garage, but does that include the "green roof"? Maybe it will become clearer in time.
Was this during construction? I wonder how closely the Contractor was watching their grading thicknesses. It could be another "designed for 2 feet" but had "a stockpile of 6 feet" type of situation.
"We shape our buildings, thereafter they shape us." -WSC
Dirt that had accumulated atop a three-story parking garage at the Watergate in Foggy Bottom caused the May 1 partial collapse that injured two people, shut down several businesses and halted construction at the famed complex, D.C. officials said.
Similar to the I35W bridge in Minneapolis - some stored piles of gravel, etc. but the bridge also had some design flaws I believe.
For a parking structure - assuming a 50 psf live load - with a 1.6 safety factor over a 0.75 phi factor for shear - only about 107 psf ultimate capacity.
For general over-design in shear (engineer's fudge factor of say 1.3) we get a "fall down" capacity of 140 psf.
That only equates to about 21 inches of dirt loosely laid at 80 pcf.
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No, I think there were landscaping renovations going on in conjunction with the other work on the site. Some of the pictures showed at least one mature tree which fell into the "sinkhole", but I think the tree was existing, not new. Hopefully, in time we will find out more about the structure and what actually caused it to collapse.
