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4 CHAPTER 2 Research Approach The research approach followed the task list given in the RFP nesota; University of Tennessee at Knoxville; Stanford Uni- by NCHRP. The work was split between the team members versity; University of Washington; and the FHWA. from BergerABAM and the University of Washington, the for- The 13 international contacts included researchers from the mer focusing on the deployment and implementation side of University of Nottingham, University of Rome, University ABC and the latter on the seismic performance substantia- of Kyoto, Tokyo Tech, University of Canterbury, National tion side. Autonomous University of Mexico (UNAM), Technical Uni- versity Frederico Santa Maria, University of Patras, and Uni- versity of Pavia. Literature and Practice Review From the construction side, eight contractors and precast The literature and practice review started with the devel- producers were asked to share their experience on ABC work, opment of a list of bridge owners, researchers, organiza- including, Concrete Technology, Inc.; C.C. Myers, Inc.; Encon tions, contractors, and suppliers who might provide direct United Companies; Flatiron Construction Corp.; Mammoet input or indirect leads to the state of the art of current ABC USA South, Inc.; Mowat Construction; Kiewit Construction; technologies. Parallel to this effort, a set of questionnaires and PCL Construction. was developed for each contact group. The questionnaires Information material on ABC was also gathered from focused on the ABC work done by the institutions and they libraries, research databases, and the internet. The material requested access to design specifications, design guidelines, included standard guidelines or surveys from FHWA, reports, standard plans or details, special construction pro- AASHTO, the Precast/Prestressed Concrete Institute (PCI) visions, or design examples that relate to the use of ABC, Manual, as well as individual research papers and product seismic or non-seismic. For bridge designers and owners, information. The collected information was compiled and the questionnaire was extended to obtain information on used by the research team for this project. specific design procedures that are typically employed for seismic design for ABC and identification of the roadblocks Definition of Seismic Connections to employing ABC. Copies of the questionnaires are pro- and Performance Strategies vided in Appendix I. In total, 43 U.S. and 13 international organizations were contacted. The use of precast element technology for bridges in Twenty-three state DOTs were contacted, including Alaska, moderate-to-high seismic regions must consider the location Arkansas, California, Florida, Georgia, Idaho, Illinois, Indiana, and seismic resistance function of the bridge elements being Louisiana, Massachusetts, Minnesota, Missouri, Montana, connected. In the United States, bridges are designed for seis- Nevada, New York, North Carolina, Oregon, Rhode Island, mic resistance by permitting some inelastic deformation of the South Carolina, Tennessee, Texas, Utah, and Washington. structure. Such inelastic response is typically restricted to Research institutions that were contacted included Univer- the substructure between the ground level and the soffit of the sity of California, San Diego; California State University, Sacra- superstructure. An example of such behavior is a reinforced mento; Multidisciplinary Center for Earthquake Engineering concrete column that may be designed to experience inelas- Research (MCEER); University of California, Berkeley; Iowa tic action in the form of plastic hinges that form at points of State University; University of Texas at Austin; University high moment, which are often at the bottom and top of the of Nevada, Reno; Utah State University; University of Min- column depending on continuity of the connections between
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5 foundation and superstructure. In the recently adopted be capable of sustaining inelastic deformations and dissipating AASHTO Guide Specification for LRFD Seismic Bridge Design kinetic energy input to the bridge system by an earthquake. (2009), the overall concept of seismic behavior is related by Such connections are energy-dissipating (ED) connections. identifying an earthquake resisting system (ERS), which is Connections that are not located where inelastic action is made up of earthquake resisting elements (ERE). In general, expected would typically be designed using capacity protection seismic forces are limited by the formation of a plastic mech- principles where the element and adjacent connection are not anism within the structure when subjected to large infrequent permitted to experience inelastic action. Such elements and earthquakes. connections are termed capacity-protected (CP) connections. There are two seismic design procedures, a force-based Figure 3 illustrates two examples of locations of connec- procedure in the AASHTO LRFD Bridge Design Specifications tions relative to the plastic hinging zones. The figure on the (2010) and a displacement-based procedure in the AASHTO left has ED connections that are in the plastic hinging zones. Guide Specifications for LRFD Seismic Bridge Design (2009). These are commonly encountered because the preferred loca- Both procedures are predicated on the use of inelastic action tions for connecting precast elements are also the preferred to resist large earthquakes. The design earthquake in both locations for plastic hinge zones. The preference for connec- AASHTO specifications has approximately a 1,000-year recur- tions at the ends of members is related to the desire to trans- rence interval. The use of inelastic action and, specifically, the port elements that are compact and do not have pieces that formation of a plastic mechanism, limits the internal forces protrude. The figure on the right shows an option where the that the bridge will experience and provides energy dissipa- precast connections are kept away for the plastic hinging tion to limit seismic response. However, the locations of zones and are, therefore, CP connections. This concept would inelastic action are typically at the areas of connection between be ideal for seismic use of ABC techniques, but is not consis- two members (for example, column and cap beam) because tent with the realities of handling and transporting heavy pre- these are the locations of maximum moment. cast elements. Figure 2 illustrates a pier of a bridge where ABC techniques A third type of element and connection may be used, typi- have been used to connect both superstructure and substruc- cally when seeking to provide internal articulation and permit ture elements. The connections are shown as lines. Addition- displacements with minimal force induced. An example of this ally, the plastic hinge locations for this bridge are indicated. It type of element would be an internal pin connection. Such can be seen that some of the connection interfaces are adjacent connections are termed deformation elements (DE). Another to or in the plastic hinge zones and some are away from such type of DE element is the seismic isolation bearing. Such zones. This is an important distinction for the use of ABC tech- bearings provide both internal system articulation, typically niques in seismic regions of the country. If a connection is between the super- and substructure, and they may also pro- made in a plastic hinging location, then the connection must vide significant energy dissipation. Seismic isolation is a rel- atively mature technology and, thus, is not focused on in this report. That is not to imply that seismic isolation cannot be effective with ABC, it certainly can. The three types of seismic performance strategies are summarized in Table 1. The extent to which the connection possesses strength and deformation capabilities determines its usefulness for different types of seismic resistance and the seismic zone for which it could be considered. In many cases, the strength and deforma- bility are associated with rotational behavior and, thus, refer to moment and rotation. Figure 2. Potential precast element connection Figure 3. Energy-dissipating (ED) versus capacity- locations relative to plastic hinge locations. protected (CP) connections.
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6 Table 1. Seismic performance strategies for connections. Seismic Performance Strategy Performance Behavior Capacity-Protected (CP) CP elements provide a cyclic strength that is higher than the strength of the Connections adjacent bridge members, allowing the connection to remain essentially elastic with minimal or no damage. As a result, the inelastic deformations are forced to occur in the adjacent elements. Energy-Dissipating (ED) ED elements provide a cyclic strength that is lower than that of the adjacent Connections members, thereby causing the inelastic deformation to occur in the connection, but high enough to dissipate enough energy to contribute usefully to the system damping. The deformation capacity is high enough to satisfy the demands associated with the seismic zone in which the bridge is built. The connection may suffer damage, but the consequent strength loss must be acceptable in all degrees of freedom, including both the primary one in which the inelastic deformation occurs and others in which minimal deformation is expected. Deformation Elements (DE) DE have little or no strength in the degree of freedom in which the deformation occurs. The deformation capacity is high enough to satisfy the demands associated with the seismic zone in which the bridge is built. The connection protects the adjacent bridge members by concentrating seismic deformation within the connection region but typically provides negligible energy dissipation. The deformation may be free (e.g., a pin), elastic (e.g., an elastomeric pad), or inelastic. Seismic isolation bearings may provide both large deformation capabilities and significant energy dissipation. Ideally, the strength would not degrade with cycling, naturally by virtue of the dimensions of the members. For although in reality some degradation is almost inevitable. The example, in a bridge bent that includes a dropped cap beam strength of the connection is evaluated relative to that of the and cast-in-place (CIP) diaphragm, the combined cap beam- adjacent members because their relative strengths control diaphragm is typically much larger than the column. Thus, the the location of the damage. inelastic deformation is likely forced into the column. If the The cyclic deformation capacity measures the ability of lower stage of the beam and the column are precast, and then the connection to undergo cycles of deformation without jeo- connected by grouting bars into sleeves or ducts within the pardizing the strength or performance in some other degree lower stage, the connection between the two stages of cap beam of freedom. For example, a connection with good rotational occurs in a beam-column joint region, which must be designed deformability could undergo many cycles of rotation without as capacity protected. For energy-dissipating and deformable loss of shear strength. The deformability is evaluated relative connections, the required deformation capacity depends on the to the expected deformation demand in a high seismic zone, seismic zone. The distinction between an energy-dissipating assuming rigid behavior in the connected elements. connection and a deformable one is not precise, and depends Figure 4 presents the three seismic performance strategies on the level of energy dissipation that is considered necessary from Table 1 in terms of their cyclic strength and deformation to limit peak displacements during an earthquake. capabilities and how they are applicable to different seismic The word "connection" has been used numerous times, but zones. A CP connection may be used in any seismic zone, it has not been defined. A clear definition is required to under- including high seismic zones, as long as deformation capacity stand how a connection, particularly an ED or deformable- and energy dissipation are provided somewhere else within the elements connection, relates to the seismic demands placed on bridge system. Note that such capacity protection may occur the system of connected elements. Figure 5 illustrates the High Capacity Protected to Adjacent Energy Dissipating Energy Dissipating Members Strength Moderate Not Permitted Relative (Moderate Seismic) (High Seismic) Deformable Deformable Low Not Permitted (Low Seismic) (High Seismic) Low Moderate High Deformability Figure 4. Seismic performance of connection elements in relation to cyclic strength and deformability and their application for moderate and high seismic zones.