【正文】 r epoxied joints are required. ? At least 50% of the prestress force should be provided by internal tendons. ? The internal tendons alone should be able to carry 130% of the dead load. For other seismic design and detailing issues, the reader is referred to the design literature provided by the California Department of Transportation, Caltrans, for castinplace structures [58]. Deck/Superstructure Connection Regardless of the design approach adopted (ductility through plastic hinging of the column or through bearings), the deck/superstructure connection is a critical element in the seismic resistant system. A brief description of the different possibilities follows. Monolithic Deck/Superstructure Connection For the longitudinal direction, plastic hinging will form at the top and bottom of the columns. Since most of the testing has been conducted on castinplace joints, this continues to be the preferred option for these cases. For short columns and for solid columns, the detailing in this area can be readily adapted from standard Caltrans practice for castinplace structures, as shown on Figure . The joint area is then essentially detailed so it is no different from that of a fully castinplace bridge. In particular, a Caltrans requirement for positive moment reinforcement over the pier can be detailed with prestressing strand, as shown below. For large spans and tall columns, hollow column sections would be more appropriate. In these cases, care should be taken to con?ne the main column bars with closely spaced ties, and joint shear reinforcement should be provided according to Reference [3 or 7]. The use of fully precast pier segments in segmental superstructures would probably require special approval of the regulating government agency, since such a solution has not yet been tested for bridges and is not codi?ed. Nevertheless, based upon ?rst principles, and with the help of strut– tiemodels, it is possible to design systems that would work in practice [6]. The segmental superstructure should be designed to resist at least 130% of the column nominal moment using the strength reduction factors prescribed in Ref. [2]. Of further interest may be a bination of precast and castinplace joint as shown in Figure , which was adapted from Ref. [8]. Here, the precast segment serves as a form for the castinplace portion that ?lls up the remainder of the solid pier cap. Other ideas can also be derived from the building industry where some model testing has been performed. Of particular interest for bridges could be a system that works by leaving dowels in the columns and supplying the precast segment with matching formed holes, which are grouted after the segment is slipped over the reinforcement [9] Typically, for spans up to 45 m erected with the spanbyspan method, the superstructure will be supported on bearings. For action in the longitudinal direction, elastomeric or isolation bearings are preferred to a ?xedend/expansionend arrangement, since these better distribute the load between the bearings. Furthermore, these bearings will increase the period of the structure, which results in an overall lower induced force level (bene ? cial for higherfrequency structures), and isolation bearings will provide some structural damping as the transverse direction, the bearings may be able to transfer load between super and substructure by shear deformation。 however, for the cases where this is not possible, shear keys can be provided as is shown in Figure . It should be noted that in regions of high seismicity,for structures with tall piers or soft substructures, the bearing demands may bee excessive and a monolithic deck– superstructure connection may bee the structureonbearings approach, the force level for the superstructure can be readily， determined, since once the bearing demands are obtained from the analysis, they can be applied to the superstructure and substructure. The superstructure should resist the resulting forces at ultimate (using the applicable code forcereduction factors), whereas the substructure can be allowed to yield plastically if necessary. Expansion Hinges From the seismic point of view, it is desirable to reduce the number of expansion hinges (EH) to a minimum. If EHs are needed, the most bene?cial location from the seismic point of view is at midspan. This can be explained by observing Figure , where the superstructure bending midspan and for an EH at quarterspan. For the latter, it can be seen that the moment at the face The location of expansion hinges within a span, and its characteristics, depends also on the stiffness of the substructure and the type of connection of the superstructure to the piers. presents general guidelines intended to assist in the selection of location of expansion hinges. Precast Segmental Piers Precast segmental piers are usually hollow cross section to save weight. From research in other areas it can be extrapolated that the precast segments of the pier would be joined by means of unbonded prestressing tendons anchored in the footing. The advantage of unbonded over bonded tendons is that for the former, the prestress force would not increase signi?ca