(AP Photo/Lynne Sladky Miami-Dade College, Wednesday, Oct. 10, 2012 in Doral, Fla)  Red line diagraming by Jeff Buster

The fairly hi-res image taken by Lynne Sladky provides an opportunity for the arm-chair (a serious study -three workers dead) forensic study of the precast concrete garage structure failure.

All pre-cast parking garages are potential  pancake collapses -  Houses of Cards   The name of the commercial parking structure game is to get the per car cost as low as possible.   This garage was designed to hold 1,800 cars and cost 22 million. - about $12,000 per car space.  

FAILURE FORENSICS - (even assuming that there was a crane accident - say a precast T beam was dropped or whatever)

What is a shear wall?

A shear wall is a structural component which resists parallelograming - or leaning.  The taller and heavier a structure is, the more critical shear walls become.  A shear resistant component can be a compressive element (concrete), a tensile element (e.g. "X" crossed cables) , or a combination of the two.  

In the image above, red lines have been drawn around the shear walls in the area of the collapse.  It appears that the exterior wall of the building (look at the corner on the RH side near the brick building) had no shear walls.  WHAT WAS HOLDING THE CORNER COLUMN VERTICAL IN BOTH EXTERIOR WALL DIRECTIONS?  (answer: wish and prayer)

The shear walls are set back in the second column line.   The shear walls appear to be precase - not cast in place concrete.   The stair well - stair wells being the classic shear wall structure - also appears to be pre-cast concrete.

The shear wall on the RH side, in the second column line, has failed right down to the first floor - allowing the entire wall to move out, dropping every plank to the ground. 

(I was also very poor construction practice to have many workers in the structure while the crane was setting concrete planks - if that was the case)   

As OSHA investigates, the failure sequence will become better known.   My hunch is that the shearwall design and installation (and ongoing attachment of shearwall precast elements during construction) was inadequate.   I wouldn't be surprised if the entire garage is taken down.