1、附录 2 外文资料翻译 原文 11.7.4 Deection 11.7.4.1 Dead Load and Creep Deection Global vertical deections of segmental box-girder bridges due to the effects of dead load and post-tensioning as well as the long-term effect of creep are normally predicted during the design process by the use of a computer analys
2、is program. The deections are dependent, to a large extent, on the method of construction of the structure, the age of the segments when post-tensioned, and the age of the structure when other loads are applied. It can be expected, therefore, that the actual deections of the structure would be diffe
3、rent from that predicted during design due to changed assumptions. The deections are usually recalculated by the contractors engineer, based on the actual construction sequence. 11.7.4.2 Camber Requirements The permanent deection of the structure after all creep deections have occurred, normally 10
4、to 15 years after construction, may be objectionable from the perspective of riding comfort for the users or for the condence of the general public. Even if there is no structural problem with a span with noticeable sag, it will not inspire public condence. For these reasons, a camber will normally
5、be cast into the structure so that the permanent deection 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. 11.7.4.3 Global Deection Due to Live Load Most design codes have a limit on the allowable
6、 global deection 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. 11.7.4.4 Local
7、 Deection Due to Live Load Similar to the limits of global deection of bridge spans, there are also limitations on the deection of the local elements of the box-girder cross section. For example, the AASHTO Specications limit the deection of cantilever arms due to service live load plus iMPact to of
8、 the cantilever length,except where there is pedestrian use 1. 11.7.5 Post-Tensioning Layout 11.7.5.1 Exter nal Post-Tensioning While most concrete bridges cast on falsework or precast beam bridges have utilized post-tensioning in ducts which are fully encased in the concrete section, other innovati
9、ons have been made in precast segmental construction.Especially prevalent in structures constructed using the span-by-span method, post-tensioning has been placed inside the hollow cell of the box girder but not encased in concrete along its length. This is know as external post-tensioning. External
10、 post-tensioning 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
11、tendons at segment joints, which causes spalling； (2) lack of sheathing at segment joints； and (3) tendon pull-through on spans with tight curvature (see Figure 11.39). External prestressing has been used on many projects in Europe, the United States, and Asia and has performed well. 11.7.5 The prov
12、ision for the addition of post-tensioning in the future in order to correct unacceptable creep deections or to strengthen the structure for additional dead load, i.e., future wearing surface, is now required by many codes. Of the positive and negative moment post-tensioning, 10% is reasonable. Provi
13、sions 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. 11.8 Seismic Considerations 11.8.1 Design Aspects and Design Codes Due to typical vibration characteristics
14、 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/su
15、perstructure connection), or by providing a yielding or a soft bearing system 6. The same principles also apply to segmental structures, i.e., the segmental superstructure needs to resist the demands imposed by the substructure. Very few implementations of segmental struc-tures are found in seismica
16、lly active California, where most of the research on earthquake-resistant 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 bri
17、dges 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 Specications for Design and Construction of Segmental Concrete Bridges 2, which now contains
18、 a chapter dedicated to seismic design. The guide allows precast-segmental construction without reinforcement across the joint, but species the following additional require- ments for these structures: For Seismic Zones C and D 1, either cast-in-place or epoxied joints are required. At least 50% of
19、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
20、cast-in-place structures 5-8. 11.8.2 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 di
21、fferent possibilities follows. 11.8.2.1 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 cast-in-place joints, this continues to be the preferred option for these
22、cases. For short columns and for solid columns, the detailing in this area can be readily adapted from standard Caltrans practice for cast-in-place structures, as shown on Figure 11.40. The joint area is then essentially detailed so it is no different from that of a fully cast-in-place bridge. In pa
23、rticular, 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 conne the main column bars with close
24、ly 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 a
25、nd is not codied. 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 p
26、rescribed in Ref. 2. Of further interest may be a combination of precast and cast-in-place joint as shown in Figure 11.41, which was adapted from Ref. 8. Here, the precast segment serves as a form for the cast-in-place 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
