【正文】 附錄 2 外文資料翻譯 原文 De?ection Dead Load and Creep De?ection Global vertical de?ections of segmental boxgirder bridges due to the effects of dead load and posttensioning as well as the longterm effect of creep are normally predicted during the design process by the use of a puter analysis program. The de?ections are dependent, to a large extent, on the method of construction of the structure, the age of the segments when posttensioned, and the age of the structure when other loads are applied. It can be expected, therefore, that the actual de?ections of the structure would be different from that predicted during design due to changed assumptions. The de?ections are usually recalculated by the contractor’s engineer, based on the actual construction sequence. Camber Requirements The permanent de?ection of the structure after all creep de?ections have occurred, normally 10 to 15 years after construction, may be objectionable from the perspective of riding fort for the users or for the con?dence of the general public. Even if there is no structural problem with a span with noticeable sag, it will not inspire public con?dence. For these reasons, a camber will normally be cast into the structure so that the permanent de?ection of the bridge is nearly zero. It may be preferable to ignore the camber, if it is otherwise necessary to cast a sag in the structure during onstruction. Global De?ection Due to Live Load Most design codes have a limit on the allowable global de?ection of a bridge span due to the effects of live load. The purpose of this limit is to avoid the noticeable vibration for the user and minimize the effects of 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。 (2) lack of sheathing at segment joints。 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, these bridges are all segmentally cast in place, with mild reinforcement crossing the segment joints. Some guidance for the seismic design of segmental structures is provided in the latest edition of the AASHTO Guide Speci?cations for Design and Construction of Segmental Concrete Bridges [2], which now contains a chapter dedicated to seismic design. The guide allows precastsegmental construction without reinforcement across the joint, but speci?es the following additional require ments for these structures: ? For Seismic Zones C and D [1], either castinplace o