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associated with making connections between the precast elements that not only can
withstand the force and deformation demands during an earthquake but that can also be
constructed easily. This report describes research that developed and evaluated practical
methodologies for the seismic design of precast bridge piers. Such methods are needed
for bridge engineers to design economical and safe precast bridges.
Two precast concrete pier systems, a cast-in-place (CIP) emulation system and a
hybrid system, were developed for use in seismically active regions to facilitate the rapid
construction of bridges. The CIP emulation system contains only mild steel reinforcement
and is an emulation of conventional cast-in-place concrete construction. The hybrid
system is reinforced with a combination of mild steel and vertical, unbonded post-
tensioning.
In order to use the CIP emulation and hybrid systems, procedures are needed to
develop economical designs that are not overly conservative, nor prone to excessive
amounts of damage in an earthquake. Two design procedures were examined in this
research: an equivalent lateral force design (ELFD) procedure and a direct
displacement-based design (DDBD) procedure.
The ELFD procedure determines the inertial force on the bridge pier by using
elastic structural dynamics and then reduces the elastic inertial force by an empirical
response modification factor to establish the design force for the pier. A range of
response modification factors ( R ) commonly specified in bridge design (AASHTO 2002;
AASHTO 2004) were considered in this study. The ELFD procedure is easy to
implement, requires no iteration, and is widely used in current practice. One main
drawback of the ELFD procedure is that it is unclear how much damage piers designed
for a particular response reduction factor will experience in a design-level earthquake. In the DDBD procedure, the designer selects a target displacement and then
determines the required strength and stiffness of the pier so that the maximum
displacement in a design-level earthquake is approximately equal to the target
displacement. The target displacement can be selected on the basis of the desired
performance of the pier in a design-level earthquake, so the designer has a clear idea of
the expected damage. The DDBD procedure is more complex than the ELFD procedure
and requires iteration, but simple computer programs can be developed to design piers
with either the ELFD or DDBD procedure, making the effort required to use either
procedure similar.
To evaluate the ELFD and DDBD procedures, the expected damage was
determined for a population of piers designed with both procedures for a design-level
earthquake. Three types of damage were considered: concrete cover spalling, longitudinal
reinforcing bar buckling, and longitudinal reinforcing bar fracture.
The piers designed with the ELFD procedure had an average probability of
spalling ranging between 5 percent ( R =1.5) and 35 percent ( R =5.0) for CIP emulation
piers and 2 percent ( R =1.5) and 37 percent ( R =5.0) for hybrid piers, depending on the
response modification factor used. The average probability of bar buckling ranged
between 0.1 percent ( R =1.5) and 3 percent ( R =5.0) for CIP emulation piers and 0.1
percent ( R =1.5) and 4 percent ( R =5.0) for hybrid piers. Significant variation in the
amount of damage experienced by each pier was predicted because of the variation in the
response of the pier to different ground motions.
The DDBD procedure was used to design the piers for three target probabilities of
cover concrete spalling: 5 percent, 15 percent, and 35 percent. The target displacement
was determined on the basis of the target probability of spalling. Although this research
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