Development of a Precast Bent Cap System for Seismic Regions (2011)

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1S U M M A R Y Accelerated Bridge Construction techniques have been sought to replace or rehabilitate, with minimal traffic interruption, thousands of bridges throughout the United States that are classified as structurally deficient or functionally obsolete. The use of precast concrete bent caps has been identified as one approach with many advantages, such as accelerating construc- tion by removing work from the critical path, reducing environmental impact, increasing qual- ity, and improving safety and overall economy. Considerable research has been conducted to develop constructible details with reliable performance; however, implementation in seis- mic regions has been limited due primarily to uncertainty in seismic performance of the connections—bent cap to columns and bent cap to superstructure—and a lack of specifica- tions for design and construction. Precast bent cap systems can be classified as either integral or nonintegral depending on superstructure-to-substructure connectivity. Integral bent cap systems develop longitudinal continuity through girder to bent cap connections. In contrast, nonintegral bent cap systems use bent cap to column connections to provide transverse moment continuity. However, integral precast bent cap systems still require the use of precast bent cap to column connec- tions as well as a superstructure to precast bent cap connection. Additionally, precast connections are typically described as being either emulative or hybrid. Emulative connections are designed to produce a system performance that is similar to (or “emulates”) that achieved by traditional monolithic, cast-in-place (CIP) construction. Bridges using emulative precast bent cap connections are expected to form plastic hinges in the columns and redistribute forces to other members like CIP systems. The lateral force dis- placement response of an emulative system is expected to exhibit full hysteresis loops and sta- ble energy dissipation. This response is characteristic of significant energy dissipation and hysteretic damping achieved through considerable damage and potential residual deforma- tions as assumed in the underlying seismic design philosophy for CIP bridges. Hybrid sys- tems are designed to provide sufficient energy dissipation through controlled rocking around specially detailed joints at the column ends. In addition to dissipating seismic energy, hybrid systems are intended to provide a significant reduction in damage and residual offsets as com- pared to CIP and emulative systems. The primary goal of NCHRP Project 12-74 was to develop and validate design methodolo- gies, design and construction specifications, design examples, and example connection details for precast bent cap systems using emulative and hybrid connections for integral and nonin- tegral systems for all seismic regions throughout the United States. This goal was achieved through a diverse experimental and analytical program focused on a select set of connection details. As an initial phase of this research, a comprehensive review of existing practice in the use of precast bent cap systems was completed. Based on a review of past implementation of precast bent caps and consideration of other promising connection approaches, a series of Development of a Precast Bent Cap System for Seismic Regions

2connection details were developed and selected for further review. Initially, each detail was evaluated for past implementation, expected seismic performance, durability, constructabil- ity, and cost. The details that were finally selected included two emulative details (grouted duct and cap pocket), three hybrid details (conventional, concrete-filled pipe, and dual steel shell) and one integral detail (discontinuous post-tensioned girders spliced through a bent cap). This report summarizes the research efforts conducted under NCHRP Project 12-74 and presents key findings and recommendations to facilitate the implementation of precast bent cap systems in seismic regions. A total of seven 42% scale bent cap to column component tests were conducted including a CIP control specimen, a full ductility grouted duct specimen, a cap pocket full ductility specimen, a cap pocket limited ductility specimen, and three hybrid full ductility specimens. Additionally, one 50% scale girder to bent cap component test was conducted on the integral connection detail. A description of the experimental test program is provided in addition to the presentation of results and associated interpretation of findings. Analytical modeling was also conducted on the hybrid systems to investigate the potential impacts this system type may have on the developed level of inelastic seismic displacement demand experienced during strong ground shaking. Based on the results of the analytical and experimental efforts, design and construction specifications, design examples, and example connection details are developed and presented. A significant number of deliverables are pro- vided as attachments to this report—recommended design specifications, design flow charts, design examples, construction specifications, example connection details, specimen drawings, specimen test reports and an implementation plan. These attachments are available online at www.trb.org/Main/Blurbs/164089.aspx. Based on the observations made during testing, the response of the CIP control specimen was determined to adequately represent the intended response of AASHTO’s 2009 Guide Specifications for LRFD Seismic Bridge Design (1). However, the design and construction of the control specimen and emulative test specimens were based on the 2006 Recommended LRFD Guidelines for the Seismic Design of Highway Bridges (2). Even though the previous, less con- servative code provisions were used to dictate the joint detailing requirements, the CIP and emulative specimens satisfied the intended design objectives. The response of all full ductility emulative specimens was dominated by flexural hinging within the column with controlled joint deformations due to adequate detailing. These specimens all achieved lateral drift capac- ities in excess of a 5% drift ratio indicating that the provided detailing has sufficient inelastic drift capacity. The use of the stay-in-place corrugated steel pipe serving as joint shear reinforce- ment provided sufficient joint shear resistance when subjected to column overstrength demands. The cap pocket limited ductility specimen experienced noticeably more joint dam- age when subjected to lateral loading consistent with column flexural hinging. The damage was due to the relaxation of seismic detailing requirements due to the intended limited duc- tility performance of this specimen. However, even with the intention of limited ductility response, the specimen was able to undergo lateral displacement in excess of a 5% drift ratio. The three tested hybrid specimens varied in terms of column design and detailing. The first specimen used conventional spiral reinforcement at the column end in combination with a reduced amount of flexural reinforcement and the presence of unbonded post-tensioning. The second specimen used a full height steel shell with column longitudinal reinforcement only at the end and terminating shortly within the column. The third specimen used a simi- lar external full height steel shell in combination with an inner shell forming a voided col- umn. For the second and third specimens, full length unbonded post-tensioning was used. The first hybrid specimen was able to undergo lateral displacements in excess of a 6% drift ratio with appreciably less damage and residual offset as compared to the CIP and emulative systems. Ultimate failure for all hybrid specimens was caused by the eventual fracture of col- umn longitudinal reinforcement. The second and third hybrid specimens were again able to

3undergo lateral displacements on the order of a 6% drift ratio; however, the degradation of the grout bedding layer material resulted in a reduction in the lateral capacity under large deformation cycles. The use of a fiber reinforced grout matrix is expected to greatly enhance the integrity of the joint and thereby minimize the loss in lateral capacity. All hybrid speci- mens exhibited reductions in damage and residual displacements when compared with the CIP control specimen. Upon review of the experimental results, the integral system investigated was determined to provide safe and reliable response when subjected to cycled elastic loading and inelastic demands. Capacity design and rational detailing provisions are expected to result in a super- structure system that resists seismically induced demands in an essentially elastic manner. Additionally, service and ultimate level loading should be resisted without major inelastic response in the superstructure. Physical testing indicates that the system studied is capable of resisting cyclic demands in the elastic range without a reduction in stiffness or slip. Large deformation inelastic testing was also performed to consider the potential response to over- loading caused by vertical seismic demands and potential seismically induced relative settle- ments. Testing indicated that the system is capable of resisting inelastic demands in excess of 0.01 radians without a reduction in flexural stiffness. Experimental results indicate that detail- ing of shear reinforcement at the joint should include provisions to develop the full vertical shear demand in well-anchored reinforcement within a short distance to prevent potential shear slip. Based on a review of the experimental and analytical efforts conducted under NCHRP Project 12-74, the following conclusions can be drawn: • The current joint shear design methodology contained in the 2009 Guide Specifications for LRFD Seismic Bridge Design (1) is appropriate for the design of emulative and hybrid, inte- gral and nonintegral precast bent cap systems with minor modifications. • For seismic design categories (SDCs) B, C, and D, the principal tensile stress should be cal- culated and the level of joint shear reinforcement should be based on this stress. For SDCs B, C, and D, if the stress exceeds 0.11 (ksi), joint shear reinforcement should be specif- ically designed. • Minimum joint shear reinforcement should be provided for all SDCs. • Design methodologies and detailing for hybrid systems should be employed to facilitate the implementation of these systems for improving the post-earthquake functionality of the transportation network. • Properly designed and detailed hybrid systems can produce substantially fewer residual deformations and less damage compared to CIP and emulative systems. • In hybrid systems, the contribution of flexural reinforcement should be limited to produce the intended response. In addition, the neutral axis depth should be limited to minimize the magnitude of compressive strains within the section. • Design and detailing of the unbonded post-tensioning and longitudinal reinforcement in a hybrid system should be performed to ensure premature fracture does not occur. • Provisions of the 2009 Guide Specifications for LRFD Seismic Bridge Design (1) for the design of multicolumn integral connections should be updated for consistency with the design of multicolumn nonintegral connections. • For the cap pocket connections, the use of a supplementary hoop at the top and bottom of the corrugated pipe should be employed. • Proposed equations for anchorage of reinforcement within grouted ducts and the cap pocket connection should be implemented. • Future provisions of seismic design and detailing requirements should be developed for knee joints for both CIP and precast bent caps. ′fc

4• Alternate connection details are provided for structures located in SDC A with SD1 less than 0.10. However, a minimum of vertical stirrups in the joint are recommended, as well as the extension of column longitudinal reinforcement as close as practical to the top of the bent cap. • Grouted joints for use in seismic applications should be limited to 3 in. in thickness and should be reinforced with hoops to maintain the spacing of lateral reinforcement within the plastic hinge region. • For hybrid columns and integral closure joints, grouted connections should employ a 3 lb per cubic yard fraction of polypropylene fibers to enhance the integrity of the joint. • The studied integral bridge connection provides sufficient strength, stiffness, and safety for implementation throughout the nation’s seismic regions and is capable of resisting substantial inelastic demands made by vertical motion or seismically induced foundation movements. • To provide reliable inelastic behavior, the integral system should be specially detailed at the girder end, including bottom flange confinement and well-anchored shear stirrups. • Open soffit, integral precast bent caps should be designed considering an appropriate tor- sional resistance mechanism to distribute column overstrength actions into the superstruc- ture girders. Based on these recommendations, a series of recommended updates to the AASHTO LRFD Bridge Design Specifications (3), 2009 Guide Specifications for LRFD Seismic Bridge Design (1) and LRFD Bridge Construction Specifications (4) were developed. These updates are available online as attachments at www.trb.org/Main/Blurbs/164089.aspx.