Page 440 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.31

earthquake design requirements call for special moment frames or intermediate moment
frames, effective depth greater than called for may lead to structural members that are critical
in shear rather than flexure. Consequently, effective depths that are too great can sometimes
require repair.
An effective depth that is significantly smaller than called for must always be evaluated.
In doing the evaluation, measurements of cover cannot be relied upon to determine effec-
tive depth. Rather, cover measurements must be used in combination with total thickness
to determine safety of a structure.
Another common problem that occurs during construction is the misplacement and/or
shifting of reinforcing bars that results in inadequate concrete cover. This is especially crit-
ical and leads to corrosion, spalling, and cracking when the concrete surface is exposed to
weather or earth backfill.

TEMPORARY AND PERMANENT
REMEDIAL REPAIRS

There are many ways to repair damaged or failed concrete structures, depending on the type and
extent of the problem, accessibility, aesthetics, etc. Only some are mentioned here for illustration.

Shoring
Safety of the public, as well as of workers, is paramount in any investigation. The first
course of action after a failure should be to stabilize any structural and nonstructural element
with cables and shoring.
Although shoring is a temporary method of repair, supports must be designed to carry
all loads that may be put on them. Off-the-shelf shoring of known capacity should be used
when possible. Also, foundations for the shoring must be adequate to carry loads that will
be applied.
More permanent repairs can be provided after the problem has been identified.
Reinforced concrete provides many options for permanent repair. Often, it is economical
to simply shore up portions of the structure, remove low-strength or damaged areas, put in
additional reinforcement, and recast the concrete.

External Prestressing

Often, structures are already in place, and it is not practical to remove concrete. External
prestressing is one method that can be used to strengthen the structure. This is particularly
useful where questions arise because of beam-column joint capacity. Figure 12.26 shows
external prestressing used to strengthen a structure.
When external post-tensioning is used, it is important to attach the tendons to the beams
at intervals along the span. If the tendons are not attached, the capacity of the beam will be
reduced.

Supplemental Reinforcement
Where space is available, another method of strengthening concrete structures is to provide
supplemental bonded overlays. Figure 12.27 shows a reinforcing cage in place, ready to
receive concrete. Externally applied reinforcement in concrete, in this case, strengthens
both the columns and the beams.