During an inspection of the elevated deck for a 16-year-old parking garage, extensive cracking was found above all 14 interior columns. The cracking pattern indicated a deficiency in shear or flexural capacity that could lead to a collapse of a large portion of the deck.


Improper bar placement affects slab flexural strength and resistance to punching shear at the columns. Both are directly proportional to the effective depth--the distance between the bottom of the slab and center of the top layers of reinforcing. While the intended effective depth for the 10-inch-thick slab was about 8.3 inches, the actual value was as little as 5.5 inches. This reduction in effective depth reduced structural capacity by up to one third. To correct the problem the shear capacity of the slab had to be increased at the slab column junction.


The contractor chose to transfer load to the columns through shear friction by roughening the column surface directly below the deck and installing concrete shear collars. To install the shear collars the contractor drilled horizontal holes through the column and epoxied three #7 bars into place to support the shear collar reinforcement cage and to supplement hoop reinforcement. The shear collar reinforcement cage was epoxy-coated and consisted of circular bars and vertical ties.