【正文】 f moving load iMPact. When structures are used by pedestrians as well as motorists,the limits are further tightened. Local De?ection Due to Live Load Similar to the limits of global de?ection of bridge spans, there are also limitations on the de?ection of the local elements of the boxgirder cross section. For example, the AASHTO Speci?cations limit the de?ection of cantilever arms due to service live load plus iMPact to 185。??of the cantilever length,except where there is pedestrian use [1]. PostTensioning Layout Exter nal PostTensioning While most concrete bridges cast on falsework or precast beam bridges have utilized posttensioning in ducts which are fully encased in the concrete section, other innovations have been made in precast segmental prevalent in structures constructed using the spanbyspan method, posttensioning has been placed inside the hollow cell of the box girder but not encased in concrete along its length. This is know as external posttensioning. External posttensioning is easily inspected at any time during the life of the structure, eliminates the problems associated with internal tendons, and eliminates the need for using expensive epoxy adhesive between precast segments. The problems associated with internal tendons are (1) misalignment of the tendons at segment joints, which causes spalling。 and (3) tendon pullthrough on spans with tight curvature (see Figure ). External prestressing has been used on many projects in Europe, the United States, and Asia and has performed well. The provision for the addition of posttensioning in the future in order to correct unacceptable creep de?ections or to strengthen the structure for additional dead load, ., future wearing surface, is now required by many codes. Of the positive and negative moment posttensioning, 10% is reasonable. Provisions should be made for access, anchorage attachment, and deviation of these additional tendons. External, unbonded tendons are used so that ungrouted ducts in the concrete are not left open. Seismic Considerations Design Aspects and Design Codes Due to typical vibration characteristics of bridges, it is generally accepted that under seismic loads,some portion of the structure will be allowed to yield, to dissipate energy, and to increase the period of vibration of the system. This yielding is usually achieved by either allowing the columns to yield plastically (monolithic deck/superstructure connection), or by providing a yielding or a soft bearing system [6]. The same principles also apply to segmental structures, ., the segmental superstructure needs to resist the demands imposed by the substructure. Very few implementations of segmental structures are found in seismically active California, where most of the research on earthquakeresistant bridges is conducted in the United States. The Pine Valley Creek Bridge, Parrots Ferry Bridge, and Norwalk/El Segundo Line Overcrossing, all of them being in California, are examples of segmental structures。 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?cantly under high column displacement demands, and would therefore not cause inelastic yielding of the strand, which would otherwise lead to a loss of prestress. The detail of the connection to the superstructure and foundation would require some insight into the dynamic characteristics of such a connection, which entails joint opening and closing providing that dry joints are used between segments. This effect is similar to footing rocking, which i