【文章內(nèi)容簡介】 ly 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 is well known to be bene?cial to the response of a structure in an earthquake. This is due to the period shift and the damping of the soil. The latter effect is clearly not available to the precast columns, but the period shift is. Details need to be developed for the bearing areas at the end of the columns, as well as the provision for clearance of the tendons to move relative to the pier during the the upper column segment is designed to be connected monolithically to the superstructure, yielding of the reinforcement should be expected. In this case, the expected plastic hinge length should be detailed ductile, using closely spaced ties [3,5]. Casting and Erection Casting There are obvious major differences in casting and erection when working with castinplace cantilever in travelers or in handling precast segments. There are also mon features, which must be kept in mind in the design stages to keep the projects simple and thereby economic and ef?cient,such as ? Keeping the length of segments equal and segments straight, even in curved bridges。 ? Maintaining constant cross section dimensions as much as possible。 ? Minimizing the number of diaphragms and stiffeners, and avoiding dowels through form work. CastinPlace Cantilevers The conventional form traveler supports the weight of the fresh concrete of the new segment by means of longitudinal beams or frames extending out in cantilever from the last segment. These beams are tied down to the previous segment. A counterweight is used when launching the traveler forward. The main beams are subjected to some de?ections, which may produce cracks in the joint between the old and new segments. Jacking of the form during casting is sometimes needed to avoid these cracks. The weight of a traveler is about 60% of the weight of the rate of construction is typically one segment per traveler per week. Precast concrete anchor blocks are used to speed up posttensioning operations. In cold climates, curing can be accelerated by various heating most critical practical problem of castinplace construction is de?ection control. There are ?ve categories of de?ections during and after construction: ? De?ection of traveler frame under the weight of the concrete segment。 ? De?ection of the concrete cantilever arm during construction under the weight of segment plus posttensioning。 ? De?ection of cantilever arms after construction and before continuity。 ? Short and longterm de?ections of the continuous structure。 ? Short and longterm pier shortenings and foundation sum of the various de?ection values for the successive sections of the deck allows the construc tion of a camber diagram to be added to the theoretical pro?le of the bridge. A construction camber for setting the elevation of the traveler at each joint must also be developed. Precast Segments Opposite to the precast girder concept where the bridge is cut longitudinally in the precast segmental methods, the bridge is cut transversally, each slice being a segment. Segments are cast in a casting yard one at a time. Furthermore, the new segment is cast against the previously cast segment so that the faces in contact match perfectly. This is the matchcast principle. When the segments are reassembled at the bridge site, they will take the same relative position with regard to the adjacent segments that they had when they were cast. Accuracy of segment geometry is an absolute priority, and adequate surveying methods must be used to ensure followup of the casting of the segments is a prerequisite for the application of glued joints, achieved by covering the end face of one or both of the meeting segments with epoxy at the erection. The epoxy serves as a lubricant during the assembly of the segments, and it ensures a watertight joint in the?nished structure. Full watertightness is needed for corrosion protection of internal tendons (tendons inside the concrete). The tensile strength of the epoxy material is higher than that of the concrete, but, even so, the strength of the epoxy is not considered in the structural behavior of the joint. The required shear capacity is generally provided by shear keys, single or multiple, in bination with longitudinal the introduction of external posttensioning, where the tendons are installed in PE ducts,outside the concrete but inside the box girder, the joints are relieved of the traditional requirement of watertightness and are left dry. The introduction of external tendons in connection with dry joints greatly enhanced the ef?ciency of precasting. Casting Methods There are two methods for casting segments. The ?rst one is the longline method, where all the segments are cast in their correct position on a casting bed that reproduces the span. The second me