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5 CHAPTER 1 Introduction 1.1 Background longitudinal framing action and thereby provides redundancy in the load path, and, in some cases, can reduce the displace- Thousands of bridges throughout the United States have ment demand and demand on the foundation. Nonintegral been classified as structurally deficient or functionally obsolete, connections are typically produced by supporting the super- and many are in need of immediate repair or replacement. structure on bearings at the top of the bent cap. The bent cap A great number of bridges rated as structurally deficient or to column connection provides a moment connection in the functionally obsolete combined with the seismic design cate- transverse direction, but moment transfer does not develop gory (SDC) for Site Class D soil are located in regions sub- between the superstructure and substructure in the longitu- jected to seismic actions (5). To replace or rehabilitate these dinal direction. Thus, the longitudinal strength and stiffness structures with minimal traffic interruption, Accelerated Bridge of the bridge are based on the cantilever response of the sup- Construction (ABC) techniques have been sought. The use of porting columns and soil-structure interaction at the abut- precast concrete bent caps is one approach to accelerating ments. Figure 1.2 shows plastic hinging for these conditions construction, as it removes much of the work from the critical (11). Because of their simplicity, nonintegral bent cap systems path. Other advantages of precast bent caps include reduction are expected to be more widely implemented than integral in environmental impact due to decreased on-site construction systems, especially in regions of low to moderate seismicity, time and removal of environmentally hazardous operations where such a system can provide suitable performance. For to less intrusive locations; increased quality as bent caps are higher seismicities, nonintegral systems may still provide fabricated in a more controlled environment; improved safety an economical solution for shorter span bridges, although for construction workers and the traveling public due to provisions such as CIP diaphragms and additional seat width reduced exposure to hazardous conditions; and improved should be incorporated into the design. It should be noted overall economy (6). that integral precast bent cap systems still require the use of Precast bent caps such as those shown in Figure 1.1 have precast bent cap to column connections, such as those used been used to meet a variety of project objectives. Considerable in nonintegral systems, in addition to a superstructure to research has been conducted to develop constructible details precast bent cap connection. with reliable performance (7, 8, 9). However, implementation Precast connections are typically described as being either in seismic regions has been limited because of (1) uncertainty emulative or hybrid, depending on the use of "wet" or "dry" about the performance of connections--bent cap to columns connections, respectively. Wet connections use CIP concrete (or piles) and bent cap to superstructure--especially in or grout to connect precast elements, whereas dry connections assuring adequate ductility, strength, and stiffness; (2) a lack of often employ mechanical devices for connection. Many seismic specifications for design and construction; and (3) potential regions around the world such as the United States, Japan, congestion of connections for higher seismic regions (10). and New Zealand have used emulative connections, which Precast bent cap systems can be classified as either integral are designed to produce a system performance that is similar or nonintegral depending on superstructure-to-substructure to (or "emulates") that developed by monolithic, CIP con- connectivity. When the superstructure is connected to the struction. As shown in Figure 1.3, the San Mateo-Hayward supporting bent cap by a cast-in-place (CIP) pour, closure pour, Bridge widening used partially precast construction and limited post-tensioning, and/or other means, longitudinal moment on-site concrete pours to produce emulative response, increase continuity can be developed. This integral connection creates the speed of erection, and lower cost (12).
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6 Figure 1.1. Precast concrete bent cap system used in crossing of State Highway 36 over Lake Belton, TX (7). Bridges using emulative precast bent cap connections facil- large inelastic deformations that can lead to significant residual itate nonlinear response through the distribution of inelastic displacements and regions of severe, and sometimes irrepara- actions some distance into the column, termed the spread of ble, post-earthquake damage (10). Significant improvements plasticity (13). The lateral force displacement response of an in the seismic performance can be realized through modifica- emulative system is expected to exhibit full hysteresis loops tion of conventional design and performance approaches. The and stable energy dissipation as shown in Figure 1.4. This use of controlled rocking in bridge piers can serve to reduce response is characteristic of the significant energy dissipation seismically induced residual displacements while also reducing assumed in the underlying seismic design philosophy for CIP the damage experienced (14). Combining the use of unbonded bridges, and it helps ensure life safety. Emulative performance post-tensioning and reinforcement, systems can exhibit appre- is commonly used in seismic regions despite the potential for ciable energy dissipation, reduced residual displacements, and Figure 1.3. San Mateo-Hayward Bridge widening Figure 1.2. Potential plastic hinge locations (11). project (12).