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

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51.1 Background Thousands of bridges throughout the United States have been classified as structurally deficient or functionally obsolete, and many are in need of immediate repair or replacement. A great number of bridges rated as structurally deficient or functionally obsolete combined with the seismic design cate- gory (SDC) for Site Class D soil are located in regions sub- jected to seismic actions (5). To replace or rehabilitate these structures with minimal traffic interruption, Accelerated Bridge Construction (ABC) techniques have been sought. The use of precast concrete bent caps is one approach to accelerating construction, as it removes much of the work from the critical path. Other advantages of precast bent caps include reduction in environmental impact due to decreased on-site construction time and removal of environmentally hazardous operations to less intrusive locations; increased quality as bent caps are fabricated in a more controlled environment; improved safety for construction workers and the traveling public due to reduced exposure to hazardous conditions; and improved overall economy (6). Precast bent caps such as those shown in Figure 1.1 have been used to meet a variety of project objectives. Considerable research has been conducted to develop constructible details with reliable performance (7, 8, 9). However, implementation in seismic regions has been limited because of (1) uncertainty about the performance of connections—bent cap to columns (or piles) and bent cap to superstructure—especially in assuring adequate ductility, strength, and stiffness; (2) a lack of specifications for design and construction; and (3) potential congestion of connections for higher seismic regions (10). Precast bent cap systems can be classified as either integral or nonintegral depending on superstructure-to-substructure connectivity. When the superstructure is connected to the supporting bent cap by a cast-in-place (CIP) pour, closure pour, post-tensioning, and/or other means, longitudinal moment continuity can be developed. This integral connection creates longitudinal framing action and thereby provides redundancy in the load path, and, in some cases, can reduce the displace- ment demand and demand on the foundation. Nonintegral connections are typically produced by supporting the super- structure on bearings at the top of the bent cap. The bent cap to column connection provides a moment connection in the transverse direction, but moment transfer does not develop between the superstructure and substructure in the longitu- dinal direction. Thus, the longitudinal strength and stiffness of the bridge are based on the cantilever response of the sup- porting columns and soil-structure interaction at the abut- ments. Figure 1.2 shows plastic hinging for these conditions (11). Because of their simplicity, nonintegral bent cap systems are expected to be more widely implemented than integral systems, especially in regions of low to moderate seismicity, where such a system can provide suitable performance. For higher seismicities, nonintegral systems may still provide an economical solution for shorter span bridges, although provisions such as CIP diaphragms and additional seat width should be incorporated into the design. It should be noted that integral precast bent cap systems still require the use of precast bent cap to column connections, such as those used in nonintegral systems, in addition to a superstructure to precast bent cap connection. Precast connections are typically described as being either emulative or hybrid, depending on the use of “wet” or “dry” connections, respectively. Wet connections use CIP concrete or grout to connect precast elements, whereas dry connections often employ mechanical devices for connection. Many seismic regions around the world such as the United States, Japan, and New Zealand have used emulative connections, which are designed to produce a system performance that is similar to (or “emulates”) that developed by monolithic, CIP con- struction. As shown in Figure 1.3, the San Mateo-Hayward Bridge widening used partially precast construction and limited on-site concrete pours to produce emulative response, increase the speed of erection, and lower cost (12). C H A P T E R 1 Introduction

6Bridges using emulative precast bent cap connections facil- itate nonlinear response through the distribution of inelastic actions some distance into the column, termed the spread of plasticity (13). The lateral force displacement response of an emulative system is expected to exhibit full hysteresis loops and stable energy dissipation as shown in Figure 1.4. This response is characteristic of the significant energy dissipation assumed in the underlying seismic design philosophy for CIP bridges, and it helps ensure life safety. Emulative performance is commonly used in seismic regions despite the potential for large inelastic deformations that can lead to significant residual displacements and regions of severe, and sometimes irrepara- ble, post-earthquake damage (10). Significant improvements in the seismic performance can be realized through modifica- tion of conventional design and performance approaches. The use of controlled rocking in bridge piers can serve to reduce seismically induced residual displacements while also reducing the damage experienced (14). Combining the use of unbonded post-tensioning and reinforcement, systems can exhibit appre- ciable energy dissipation, reduced residual displacements, and Figure 1.1. Precast concrete bent cap system used in crossing of State Highway 36 over Lake Belton, TX (7). Figure 1.2. Potential plastic hinge locations (11). Figure 1.3. San Mateo-Hayward Bridge widening project (12).

7reduced structural damage when designed using a rationally founded approach (5). The combination of unbonded post- tensioning and mild reinforcement is termed “hybrid systems” throughout this report and is depicted in Figure 1.4. Tobolski (5) provides detailed information regarding the design and performance of hybrid systems for bridge applications. 1.2 Implications for Bridge Design and Construction The results published in this report summarize a com- prehensive research program that was aimed at the ultimate implementation of precast bent cap systems throughout U.S. seismic regions. The analytical and experimental program conducted served to validate the expected performance of a variety of precast bent cap details when subjected to seismic loadings. The results presented herein facilitate an under- standing of the design and construction efforts required for safe and economical implementation of precast bent cap sys- tems throughout the United States. Symbols and variables used throughout this document follow standard AASHTO convention. 1.3 Key Results from Initial Report The first step in the research program was to develop promising precast bent cap connection and system details for use throughout the United State’s seismic regions (10). To perform this task, a significant review and survey of state departments of transportation (DOTs) and engineers was conducted to identify prior uses of precast bent caps throughout the world. During this survey, more than 60 proj- ects that used precast bent caps for all levels of seismicity were identified in 23 U.S. states, Puerto Rico, New Zealand, Europe, and Saudi Arabia. However, the majority of these previous applications were in regions subjected to minimal seismic actions, and almost all details were for nonintegral applications. In addition to gathering information regarding prior implementation, the survey was aimed at obtaining insight into why these details have not been widely used. Engineers and agencies expressed reservations about using precast bent caps in seismic regions because of the lack of prior research and performance data for these systems. Many state DOT officials indicated that they would not allow the use of precast bent caps for higher seismic regions without validated design methodologies. Various other concerns were voiced regarding potential fabrication complexities as seismic demand increased due to heavy congestion at connection regions. Many fabri- cators did indicate that the use of post-tensioning would be a potentially advantageous way to reduce this congestion and provide a more constructible system. From a construction standpoint, many contractors indicated that they have reser- vations due to the potentially small tolerances that would be required to erect these precast systems. Many persons indi- cated that the required tolerances are achievable, but require increased care during construction that would in turn result in construction cost increases. Based on the review of previous details and seismic research along with the input from industry, a variety of connection concepts were developed and evaluated for expected seismic performance, constructability, durability, advantages, and disadvantages. During this effort, the following nonintegral connection types were developed, as described in Tobolski et al. (10): • Grouted duct connection • Bolted connection • Grouted sleeve coupler connection • Cap pocket connection • Welded connection • Partially precast shell connection • Conventional hybrid connection • Concrete filled pipe hybrid connection • Hollow dual shell hybrid connection Additionally, a variety of precast, integral bent cap systems were presented. However, based on significant interaction with the NCHRP Project 12-74 panel, a future integral connection Lateral Response Conventional Rocking HybridJointed Figure 1.4. An overview of idealized lateral response for various column systems (5).

detail was developed and tested. Review of some connection details resulted in recommended details that were ready for implementation for limited ductility connection applications. These limited ductility details were the grouted duct connection, grouted sleeve coupler connection, and partially precast shell connection. The proposed connections for testing and validation were presented as well as a rigorous experimental and analytical program for performing the required validation studies. 1.4 Summary of Experimental Specimens During the course of this research program, a number of promising precast bent cap details were investigated through experimental testing. These specimens were developed to meet a variety of performance objectives for sites located throughout U.S. seismic regions. A summary of these specimens is provided in Table 1.1. 8 Table 1.1. Summary of experimental specimens. Code Specimen Name Specimen Type Specimen Purpose CIP Cast-in-place control specimen Beam-to- column emulative Control specimen detailed in accordance with Recommended LRFD Guidelines for the Seismic Design of Highway Bridges (2) for high seismic applications. GD Grouted duct specimen Beam-to- column emulative Grouted duct specimen designed to provide high ductility response with similar response to CIP specimen. CPFD Cap pocket full ductility specimen Beam-to- column emulative Cap pocket specimen designed to provide high ductility response with similar response to CIP specimen. Detail uses a corrugated metal pipe to provide stay-in- place form and joint shear reinforcement. CPLD Cap pocket limited ductility specimen Beam-to- column emulative Cap pocket specimen design with alleviated seismic detailing intended to provide limited ductility for regions of low to moderate seismicity. Detail uses similar corrugated metal pipe detail to CPFD. HYB1 Conventional hybrid specimen Beam-to- column hybrid Hybrid specimen detailed with conventional spiral confinement reinforcement and full-length mild reinforcement. Detail is intended to be a hybrid detail most similar to traditional CIP construction. HYB2 Concrete filled pipe hybrid specimen Beam-to- column hybrid Hybrid specimen using full length steel shell acting as confinement and shear reinforcement. Mild reinforcement used only at joint to provide energy dissipation and terminated into the column. HYB3 Dual steel shell hybrid specimen Beam-to- column hybrid Hybrid specimen using two full length shells (outer steel and inner corrugated metal pipe) acting as confinement and shear reinforcement. Mild reinforcement utilized only at joint. Dual shell detail intended to reduce weight of column section for precasting. INT Post-tensioned integral specimen Girder-to- beam emulative Integral specimen using precast post- tensioned girders spliced through the precast bent cap. Girders are discontinuous at bent cap leaving a cold joint between the members. Intended to provide a constructible precast option for high seismic regions requiring integral response.