Precast bridge columns with energy dissipating joints
- PI: M. Saiid Saiidi, University of Nevada, Reno
- Co-PI: David Sanders, Nevada
- Research assistant: Sarira Motaref, Nevada
- Sponsor: California Department of Transportation
- Project manager: Li-Hong Sheng
- Project dates: October - December 2009
Project summary
Conventional bridge construction involves a time-consuming process associated with traffic delays and risk to public safety. In contrast, prefabricated bridge systems can expedite construction, thus minimizing traffic delays and construction site safety risk.
Under moderate and strong earthquakes, it is essential for bridge columns to dissipate energy through nonlinear deformations in plastic hinges. Existing details for precast segmental columns offer minimal energy dissipation as a result of the discontinuity of longitudinal reinforcement; therefore, precast members are not used in high seismic zones.
A series of innovative precast concrete segmental columns are being developed and studied at the University of Nevada, Reno through a research project funded by the California Department of Transportation. The purpose of the study is to develop precast columns that are able to dissipate energy under cyclic loading.
The first phase of this project involved analytical and experimental study of a segmental concrete column incorporating an elastomeric bearing pad in the plastic hinge.
The second phase of the project includes designing and testing three different segmental concrete columns with different low-damage plastic hinges.
In one column a conventional reinforced concrete detail is used and it is called SC-2 (Segmental with Concrete). The other two columns incorporate ECC (Engineered Cementitious Composite) referred to as SE-2 (Segmental with ECC) and FRP wrap referred to as SF-2 (Segmental with FRP) at the lower two segments, respectively.
SC-2
SC-2 is a one-third scale precast concrete segmental column with longitudinal steel dowels connecting the base segment to the footing. Unbonded post tensioning is used to connect the segments and to minimize the residual displacements.
Energy dissipation will take place mostly through the yielding of the longitudinal bars at base segment.
The column will be subjected to the Sylmar earthquake (Northridge 1994) record with increasing amplitudes until failure.
SC-2 column will be a benchmark based on which the advantages of other specimens consisting of innovative materials at plastic hinges will be evaluated and quantified.
SF-2
SF-2 is a one-third scale precast segmental concrete column in which the first two segments are wrapped with two layers of FRP. The longitudinal steel dowels connect the base segment to the footing. A central high strength unbonded post tensioning rod is used to connect the segments and minimize the residual displacements.
Energy dissipation is expected to take place mostly through the yielding of the longitudinal bars at base segment.
The column will be subjected to the Sylmar earthquake (Northridge 1994) ground motion. The amplitude will be increased until failure.
FRP wrapping provides confinement for the concrete and increases its ductility. It is expected to observe less concrete crushing, due to the gap opening, at the interface of base and second segments.
SE-2
SE-2 is a one-third scale precast segmental column with longitudinal steel dowels connecting the base segment to the footing. The base segment and second segment are made from ECC. ECC is a fiber-reinforced cement-based composite engineered for high tensile ductility, compressive, and tensile strength. It is expected to observe more ductile behavior and fine cracks instead of crushing at interface of base and second segments. Unbonded post tensioning is used to connect the segments and to minimize the residual displacements.
Energy dissipation will take place mostly through the yielding of the longitudinal bars at base segment.
The column will be subjected to the Sylmar earthquake (Northridge 1994) record with increasing amplitudes until failure.
PFEB
The final test specimen is a two column bent referred to as PFEB (Precast FRP-ECC Bent) incorporating two monolithic precast columns, footing and cap beam that were built separately.
To evaluate the seismic performance of precast bridge pier construction two hexagonal molds were shaped in the footing during the construction to allow embedding of the columns. The embedded length was selected 1.5 times of column diameter. The embedded length was designed in such a way as to transfer the full plastic moment of the column and provide complete rigidity. The embedment gap was filled with high strength grout after assembling process. Pipe pin hinges have been used on top of the columns and connect them to the cap beam.
The northern column is a conventional concrete column which is incorporating ECC material in plastic hinge area. The southern column is an innovative system that makes use of external FRP tube. The fibers in the FRP tube are aliened in 55 degrees to perform as both longitudinal and transversal reinforcement. The expected ultimate capacities of both of the columns are the same at a drift of approximately 5%.
The bent will be subjected to the Sylmar earthquake (Northridge 1994) record with increasing amplitudes until failure.