Page 444 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.35

• The concrete’s durability was compromised due to extensive cracking and leakage through
cracks. In the absence of repairs, corrosion of embedded steel reinforcement and related
concrete deterioration would likely occur relatively early in the service life of the garage.
The findings indicated that significant repairs were required to ensure structural integrity
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and provide a durable structure. Four repair alternatives were considered: demolish the
superstructure and rebuild the garage, using the remaining foundations; remove and replace
slabs and strengthen deficient beams; install supplemental below-slab framing spanning
between joists and strengthen deficient beams; and place a bonded reinforced-concrete over-
lay and strengthen deficient beams.
Important criteria in evaluating repair alternatives included cost, construction schedule,
construction methods, the effect of repairs on the remainder of the garage (structural and
operational), and durability. The bonded reinforced overlay alternative was chosen for sev-
eral reasons: its estimated cost was lowest of the four options; the repairs could be completed
in approximately 4 months, with minimal impact on the rest of the garage, because the work
could be phased; its construction methods were proved, and many contractors have overlay
experience; and the new overlay slab would provide a durable wearing surface.

Case Study of Shear Failure in Waffle Slab

Approximately 8 years after construction was completed, a sudden shear failure occurred at two
columns around the perimeter of a two-story office building. Figure 12.29 shows the failed slabs
after curtain walls had been removed. Figure 12.30 shows a close-up of one of the shear fail-
ures. Temporary timber shoring was placed under the column to provide emergency support.

FIGURE 12.29 Shear failures of columns on second-story building. (Courtesy of CTLGroup.)

Once the safety of the building had been secured with emergency shoring, an investigation
to determine the cause of the failure was begun. Samples of concrete and reinforcement
were obtained to determine material properties. Simultaneously, plans for the structure
were obtained and verified by field inspection.
Figure 12.31 shows framing at each perimeter column for the building. As can be seen,
columns for the second story did not line up with columns for the first story, but were