Page 445 - Forensic Structural Engineering Handbook

12.36 MATERIAL-SPECIFIC FORENSIC ANALYSES

FIGURE 12.30 Close-up of shear failure with temporary
shoring in place. (Courtesy of CTLGroup.)

cantilevered outward around the perimeter of the structure. The second-story column was
supported by a solid section of the waffle slab spanning from the bottom of the second-story
column to the top of the first-story column.
To provide deflection control for the long spans used in the interior of the building, the
waffle slabs were post-tensioned.
Figure 12.32 shows reinforcing details in the vicinity of the cantilever portion of the slab.
Because of negative moments over the interior column, post-tensioning was located near the
top of the slab. The bottom of the slab had extremely light reinforcement consisting of one
no. 4 bar in each stem of the waffle slab. The resulting reinforcement provided less than min-
imum flexural reinforcement at the bottom of the slab. Consequently, the full depth of the
section could not be used to carry shear forces. Rather, shear forces were more closely
related to an effective depth between the top of the slab and the depth of the post-tensioning
tendon. This depth resulted in an inadequate strength to carry the shear that was applied.
Properties of concrete and steel used in construction were found to be consistent with
those specified. Review of the original construction documents disclosed no indication that
shear strength had ever been checked anywhere in the building. Consequently, it was deter-
mined that the cause of failure was primarily due to a failure in the design to provide ade-
quate shear strength in the cantilever sections carrying the load from the second-story
column to the first-story column.