A PRE-TENSIONED, ROCKING BRIDGE BENT FOR ABC IN SEISMIC REGIONS.

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1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska A PRE-TENSIONED, ROCKING BRIDGE BENT FOR ABC IN SEISMIC REGIONS. John Stanton, Marc Eberhard, David Sanders, Travis Thonstad, Jeffrey Schaefer, Bryan Kennedy, Olafur Haraldsson and Islam Mantawy. ABSTRACT A new, rocking, pre-tensioned concrete bridge bent system has been developed that reduces onsite construction time by precasting the beams and columns, minimizes post-earthquake residual displacements by the use of locally unbonded, pre-tensioned strands in the columns, and reduces earthquake damage by means of rocking connections at the ends of the columns. Cyclic tests of the critical connections have demonstrated that the system can deform to drift ratios of around 6% with minimal damage and negligible residual displacements. Shaking table tests of a 25% scale, two-span bridge at the University of Nevada, Reno will be used to evaluate the dynamic performance of the system.

2 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska A Pre-tensioned, Rocking Bridge Bent for ABC in Seismic Regions. John Stanton 1, Marc Eberhard 1, David Sanders 2, Travis Thonstad 3, Jeffrey Schaefer 3, Bryan Kennedy 3, Olafur Haraldsson 3 and Islam Mantawy 4 ABSTRACT A new, rocking, pre-tensioned concrete bridge bent system has been developed that reduces on-site construction time by precasting the beams and columns, minimizes post-earthquake residual displacements by the use of locally unbonded, pre-tensioned strands in the columns, and reduces earthquake damage by means of rocking connections at the ends of the columns. Cyclic tests of the critical connections have demonstrated that the system can deform to drift ratios of around 6% with minimal damage and negligible residual displacements. Shaking table tests of a 25% scale, twospan bridge at the University of Nevada, Reno will be used to evaluate the dynamic performance of the system. Introduction Within the United States, the design of reinforced concrete bridges in seismic regions has changed little since the mid-1970s, when ductile details were first introduced. In seismic regions, nearly all bridge bents today are constructed of cast-in-place reinforced concrete. These cast-inplace bridges have typically met life-safety requirements, but are slow to construct and suffer damage during earthquakes. New structural systems and construction methods are needed to improve speed of construction on site, to minimize residual displacements after an earthquake and to reduce post-earthquake damage. A new concept has been developed that addresses each of these three concerns. This paper describes the concept, its constructability characteristics, and the results of quasi-static testing of subassemblies representing components of the new system. Shaking table tests on a 25% scale model bridge using the system will be conducted in 2014 at the University of Nevada, Reno Network for Earthquake Engineering Simulation (NEES) facility. 1 Professor, Dept. of Civil Eng., University of Washington, Seattle, WA Professor, Dept. of Civil Eng., University of Nevada, Reno, NV Graduate Research Assistant, Dept. of Civil Eng., University of WA, Seattle, WA Graduate Research Assistant, Dept. of Civil Eng., University of Nevada, Reno, NV Author: Stanton,J.F. A Pre-tensioned, Rocking Bridge Bent for ABC in Seismic Regions. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 Structural Concept The new system is a development of the one tested by Davis et al (2012). It is shown schematically in Figure 1 and has the following key features. The columns and cross-beams are cast off-site and then assembled rapidly once they arrive on-site. Precasting significantly reduces the on-site construction time and offers solutions when the site is environmentally sensitive, such as over water. Further, the critical components of the system are cast under factory conditions, which reduces the probability of errors. Construction is further accelerated by using a wet socket connection between the column and the spread footing (Haraldsson et al. 2013). In this connection, the precast column and the footing reinforcement are placed in the excavation, then the concrete is cast in place around it. To facilitate the transfer of forces into the surrounding concrete, the base of the column has a roughened exterior with a saw-tooth detail. No bars cross the interface between the precast column and cast-in-place footing. The column longitudinal bars are not bent out as they are in conventional construction, but instead, they are developed using mechanical anchors within the part of the column that is embedded in the footing. This design facilitates transportation, increases safety (no protruding bars), and improves performance (compared with bent-out bars). Post-earthquake residual displacements are reduced by pre-tensioning the precast bridge columns with tendons that are unbonded over the central part of the column and bonded at the ends. They are designed to return the system to its original plumb position when the ground motion stops. They are situated in the center of the cross-section. The column is cast horizontally to facilitate pretensioning, and the section is octagonal to provide a flat to surface for good finishing. Figure 1. Precast, Pretensioned Rocking Column Bent Concept

4 Damage to the system is minimized by incorporating a new rocking detail at the ends of the columns, as shown (for the base of the column) in Figure 2. The column is detailed so that it cracks on a plane at the top of the footing, and the column then rocks as a rigid body at the crack. Each end of the column is fitted with a steel jacket (or shoe ), which consists of circular steel pipe welded to an end plate. The shoe forms a discontinuity at Figure 2. Base Detail for Precast, Pretensioned Rocking Column Bent Figure 3. Construction of Precast Column Bent (without pretensioning) which the crack naturally occurs and it confines the end of the column to minimize damage to the concrete during rocking. The connection detail at the top of the column is similar in principle but the column section is reduced at its interface with the cap beam (Figure 1). This detail allows the use of a precast cap beam, which significantly shortens on-site construction time, without the

5 need for a large and potentially damaging opening in the cap beam. It also provides a convenient seat for the cap beam during construction, thereby avoiding the need for other, temporary support mechanisms. The primary longitudinal bars are debonded near the interface to distribute their elongation over a region long enough to reduce the peak strain and prevent bar failure at the design deformation. Additional bars, which are discontinuous and welded to the base plate of the shoe, are included in the system to help dissipate the local compressive forces at the rocking interface, and to ensure that any cracking is concentrated at this interface rather than in the body of the column above the shoe. Rapid On-Site Construction A non-prestressed version of the precast bent system was deployed in Washington State as part of the construction of a bridge over Interstate-5 (Khaleghi et al. 2012). The bridge had two spans, tall abutments on the ends and a center pier with four columns (Fig. 3). The precast columns were connected to the cast-in-place spread footings using a wet socket connection (Haraldsson et al. 2013). The tops of the precast columns were connected to the precast cap-beam using a large-bar connection (Pang et al. 2010), consisting of large bras grouted into ducts. Large bars are used to reduce the number of bar fit-ups, which facilitates on-site construction. Experiments (Steuck et al 2009) have shown that the bars can be fully developed within the depth of the cap beam. The cap-beam was made in two segments because of weight constraints. No major problems were encountered during construction, and alignment was straightforward (Khaleghi et al. 2012). The placement of each cap-beam segment took less than 30 minutes. The precast, pre-tensioned rocking system has construction details similar to those of the nonprestressed system, so it could be assembled similarly, as illustrated in Figure 4. Seismic Performance Figure 4. Construction Sequence The resistance and damage progression of the top and bottom connections were evaluated through quasi-static tests of a column-to-spread-footing connection subassembly (PreT-SF-Rock) and a column-to-cap-beam connection subassembly (PreT-CB-Rock).

6 The columns were designed to have a strength similar (at 42% scale) to that of a typical reinforced concrete column. For both subassemblies, the octagonal columns had a diameter (flatto-flat) of 20 in. (508 mm) and a cantilever length of 60 in. (1524 mm), resulting in a cantilever span-to-depth ratio of 3.0. The columns were subjected to a constant axial load while cyclic, lateral displacements were applied to the column. The loading setup is shown in Figure 5. Figure 5. Loading Setup for Subassembly Tests The cyclic performance of the subassemblies (Figure 6) greatly exceeded that of a comparable conventional reinforced concrete column connection (e.g., Pang et al. 2010). For peak drift ratios up to approximately 6%, the columns returned to their undeformed geometry upon unloading. The columns continued to resist nearly 100% of the peak lateral load after having been subjected to two cycles of deformation at a drift ratio of 10.4%. Even after those extreme load cycles, the residual drift ratio was less than 1%. No spalling or bar buckling was observed during the tests, and the grout in the top connection suffered only cosmetic damage. The longitudinal bars for the column-to-spread-footing specimen fractured after being subjected to a drift ratio of 5.9%. The column-to-cap-beam bars, which had a longer debonded length, fractured after being subjected to a drift ratio of 7.0%.

7 (a) Column Connection to Spread Footing (PreT-SF-Rock) (b) Column Connection to Cap Beam (PreT-CB-Rock) Figure 6. Measured Effective-Force vs. Drift Ratio Responses for Rocking Connections

8 Planned Shaking Table Tests To complement the component tests described above, shaking table tests on the system will be conducted at the NEES facility at the University of Nevada, Reno in The structure will consist of a two-span, three-bent bridge constructed at 25% scale. The columns of the bents have been designed to perform similarly to those already tested statically at the University of Washington. Figure 7 shows the shaking table specimen. Figure 7. Planned Shaking Table Specimen Conclusions A new column bridge bent system has been developed for use in any seismic region. It accelerates bridge construction, it re-centers after even an extreme earthquake, and it minimizes seismic damage. Its seismic behavior is characterized by rigid-body rocking, and re-centering is achieved by pretesnioning that is debonded over the central region of the column. Damage is minimized by suitable confinement detailing. Field experience with a similar, non-prestressed system suggests that the new system can be constructed rapidly. Quasi-static tests of a column-to-spread-footing subassembly and a column-to-cap-beam subassembly indicate that the new system will perform better than a reinforced concrete bridge constructed with conventional seismic detailing. The column re-centers even after excursions to large drift ratios, the column suffers almost no damage, and lateral strength is maintained out to very high drift ratios. The dynamic performance of the system will be evaluated in upcoming shaking table tests of a 25% scale, two-span bridge constructed using the system.

9 Acknowledgments This research was supported by the National Science Foundation George Brown Network for Earthquake Engineering Systems Research Program (Award # ), the Pacific Earthquake Engineering Research (PEER) Center and the Valle Foundation of the University of Washington. The findings and conclusions contained herein are those of the authors alone. The quasi-static tests were conducted at the University of Washington with the help of graduate students Lisa Berg, Spencer Livermore, Kevin Martin, Tony Nguyen, Max Stephens and Hung Viet Tran. Further help was provided by undergraduate students Sam Adiputra, Matt Brosman, Nathan Clemens, David Lam, Scott Laws, Kevin Tsuchida, Hin-Kei Wong and Chase Young. The assistance of Professor Donald Janssen and Laboratory Manager Vince Chaijaroen is also gratefully acknowledged. References 1. Davis, Phillip M., Janes, Todd M., Haraldsson, Olafur S., Stanton, John F., and Eberhard, Marc O. (2012). Unbonded Pre-tensioned Columns for Accelerated Bridge Construction in Seismic Regions. Journal of Bridge Engineering, ASCE (submitted November 2012) 2. Haraldsson, O.S., Janes, T.M., Eberhard, M.O. and Stanton, J.F. (2013). Seismic Resistance of Socket Connection between Footing and Precast Column. Journal of Bridge Engineering, ASCE, Sept-Oct, pp Khaleghi, B., Schultz, E., Seguirant, S.J., Marsh, M.L., Haraldsson, O.S., Eberhard, M.O. and Stanton, J.F. (2012). Accelerated Bridge Construction in Washington State -- From Research to Practice, PCI Journal, Autumn, pp Pang, J.B.K., Eberhard M.O. and Stanton, J.F. (2010). Large-Bar Connection for Precast Bridge Bents in Seismic Regions. Journal of Bridge Engineering, ASCE, May-June, pp Steuck, K.P., Stanton, J.F. and Eberhard, M.O., (2009). Anchorage of Large-Diameter Reinforcing Bars in Ducts, ACI Structural Journal, 106(4), July-August, pp