Page 18 - Advanced Design Examples of Seismic Retrofit of Structures

Introduction Chapter 1 9

created from a composite of new and old components. Examples of such com-
posites include filling in openings in infill frames and using existing columns
for chord members for “new” shear walls or braced frames. If existing lateral
force resisting elements are to be used in conjunction with new ones to provide
the required stiffness, the potential for degradation due to poor detailing in the
existing structure must be considered. If loss of lateral stiffness of the existing
elements will reduce the overall strength to levels that could cause P-delta insta-
bility, the existing elements should be discounted and additional new elements
employed.

1.5.3 Improvement of Local Behavior

Rather than providing retrofit actions that affect the entire structure, deficien-
cies may be eliminated at the local, component level. This can be done by
enhancing the existing shear or moment strength of an element, or simply by
altering the element in a way that allows addition deformation without
compromising vertical load carrying capacity. As previously discussed, certain
yielding sequences are almost always benign: beams yielding before columns,
bracing members yielding before connections, and bending yielding before
shear failure in columns and walls. These relationships can be obtained by local
retrofit in a variety of ways. Columns in frames and connections in braces can be
strengthened, and the shear capacity of columns and walls can be enhanced to be
stronger than the shear that can be delivered. Concrete columns can be wrapped
with steel, concrete, or other materials to provide confinement and shear
strength. Concrete and masonry walls can be layered with reinforced concrete,
plate steel, and other materials. Composites of glass or carbon fibers and epoxy
are becoming popular to enhance shear strength and confinement in columns,
and to provide shear-only strengthening to walls. However, such materials must
be used with caution, because moment capacity can be inadvertently added, the
material can be incorrectly designed to be incompatible with the existing ele-
ment, or the system can be applied incorrectly in the field.

1.6 FUTURE DIRECTIONS; TOWARD SEISMIC RESILIENCE

Risk and resilience strategies are not equivalent. Risk-based strategies are most
effective when hazard probabilities are known or can be estimated. However,
ignorance of emergent hazards does not justify a lack of preparedness. Three
recent disasters, the Fukushima nuclear reactors, Deepwater Horizon, and Hur-
ricane Katrina, reinforce the view that some degree of ignorance in complex
systems is irreducible. Therefore, an exclusively risk-based management
approach is never fully justified, and lack of attention to resilience will exacer-
bate the consequences of inevitable failures. Risk management begins with haz-
ard identification. This step has some difficulties because hazards are usually
unknown, inestimable, or very low-probability. Also, high-consequence events,