英文文章000

Unit 17 Prestressed Concrete BeamBridge Construction Technology预应力混凝土梁桥施工技术内容摘要体外预应力目前已成为预应力体系中的重要组成部分。

但在产生初期及发展过程中，曾面临过许多问题。

本文介绍了体外预应力产生、发展及曾经历代研究和实验。

在过去的时间里，它从实验性应用的状态，发展成为德国的箱梁桥的标准设计方法，这说明体外预应力的研究已取得了阶段性的成果。

Altwipfergrund桥实例表明，仍然有许多改善桥梁体系和预应力体系的可能性。

A. TextNew Technologies of PrestressedConcrete Bredges1 Review of externally prestressed concrete bridgeThe world’s first externally prestressed concrete bridge was built in Aue, Saxony from 1935 to 1937 based on a concept proposed by Dischinger (DRP 727429). The beam bridge had a variable depth, three spans (25.2 m-69.0 m-23.4 m)and the central span consisted of cantilevers with a simply supported beam in the middle. Suspended unbounded tendons running outside the concrete cross-section were used. However corrosion of the tendons and higher than anticipated loss of prestress caused by creep and shrinkage lead to problems with this bridge.After 1945 devlopments in bridge construction in Germany were concentrated on internal, post-tensioned systems. However,external prestressing was used in both Belgium and France. In Germany the corrosion resistance of external tendons was considered to be inadequate at that time, whereas post-tensioned bridges with internal tendons appearedunproblematic and so only this construction method was further developed and applied.Nevertheless, the corrosion protection systems were improved by the middle of the 1980’s. And so external prestressing seemed to be a viable alternative to conventional internal prestressing. This method was first used for special applications ( e.g.strengthening ) and later for standard applications as durability and long-term maintenance considerations became of prime importance. Some pilot projects were constructed of which the valley bridge Ruderting in Bavaria was one. This construction method proved very successful. The good experiences with the external prestressing in bridge construction and the benefits in maintenance wee the main reasons for the introduction of external prestressing as the standard design method for box girder bridges in Germany.2 Technology2.1 Layout of TendonsBasically two different layouts for the external prestressing tendons can be chosen.Examples are presented in section 4.1)Deflected tendonsThe tendons are deflected over the columns as well as in the spans, e.g. at mid-apan(Fig.17-la)). Deflection saddles must be used in this case. The shape position of the deflection saddles must be carefully controlled , therefore often precast concrete elements or steel units are used.2)Straight tendonsThe tendons are placed straight and eccentric within the cross section. Two cross girders or other anchorage systems are necessary in every span to anchor the tendon forces (Fig. 17-1b)).2.2 Construction MethodsSpan-by-span implementation with falsework, scaffold carriage or incremental launching were used in the pilot projects. Dependent upon the construction method one of the following tendon layouts may be chosen:●Straight or deflected●Combination of both layouts●Centric, straight tendons for the construction can bereconstructed into a deflected shape (for example by applying the incremental launching technique)●Furthermore internal and external prestressing in combination ispossible (incremental launching)2.3 External Prestressing systemsAt the moment several external prestressing systems are approved by the construction authorities for use in the market in Germany. In addition almost every producer is developing new systems. So it is expected that the variety of products available in the market will expand.3 Standard Design MethodAs a result of the favorable experiences gained during the pilot projects the highway authority decided to introduce external prestressing as a standard design method for box girder bridges.The authority expects the following advantages, especially with regard to durability and maintenance:●The tendons are easily controllable and verifiable●They are replaceable●They can be subjected to a secondary tensioning(re-stressing)●They have a high quality, industrial corrosion protection●The stress from traffic vibration is small●Partial prestressing can be utilized for economic construction,exhibiting ductile behavior equivalent to a larger quantity ofconventional reinforcing●The web of the superstructure is free of tendons, makingplacing of the concrete easier●The construction site is less weather dependent because nogrouting of tendons is necessaryIn Germany a code for prestressing with external tendons isnot introduced by the building administration. Therefore theroad administration has invited experts to formulateregulations which are based on the experience gained fromthe pilot projects undertaken.4. Examples4.1 Pilot Project RudertingIn 1994,the valley bridge Ruderting, on route 85 from Passauto Cham, was designed as a Bavarian pilot project usingexternal prestressing. This bridge has 6spans (36 m﹢4×45m﹢36 m), and crosses the valley Hasebachtal at a height ofapproximately 43 m. The straight routing and light curvatureof the gradient offered the optimal repuirements for the useof external prestressing.A detailed invitation to tender was prepaed by the highwayadministration in Passau inCo-operation with the author. A box girder was chosen for thesuperstructure, with a height of 2.7 m and a regular span of 45.0m (slenderness =1/16.7). Partial prestressing was planned.No construction method was specified in the design. The 252 m long bridge could be built with falsework or with incremental launching. Therefore, both possibilities were pre-investigated and included in the design. The prestressing system was not specified. However, all prestressing systems should have been approved by the appropriate regulatory authority.The draft by the highway administration was advertised Europe-wide. Alternative bids were allowed under the following conditions:●No change in spans and overall length●Slenderness of superstructure h/L≦1/16●The tendons must be exchangeable under the full trafficload of bridge●Announcement of failure through crack formation●Possibility of an increase in traffic load by 20%through structuralprovisions for 4 additional tendons●No transverse prestressingInitally, 9 bidders submitted 9 main offers and 12 secondary offers. On analyzing the offers, a special offer from the contractor Bilfinger & Berger Bau AG was the most economical and so obtained the contract.This special offer chose straight tendons for prestressing the2.60 m high box girder. The range of the spans and under the deckslab in the range of the columns. The anchoring of these tendons was performed in the opening of the span cross girders, which were placed 8 m in front and behind he column cross girders. The remaining tendons were placed at the centrold of the cross section area and are only anchored in the final state in the end cross girders.Due to the slight sag of the bridge, The continuous, Coupled tendons had to be deflected at two saddles. The construction of the superstructure followed in three sections on falsework.4.2 Valley Bridge TrockauThe renovation of the 60 years old highway A9, the most important connection between Bavaria and the newly-formed German states, necessitates this new bridge building. The bidding for the 6 lane highway bridge was performed by the local highway authority in the year 1998. Thereby a composite bridge with 9 spans(48.0﹢58.0﹢60.0﹢85.0﹢100.0﹢85.0﹢60.0﹢58.0﹢48.0m) wasplaned .However concrete box girders with external prestressing were proposed in some redesign. A working group-consisting of Max B.gl, Neumarkt, and Walter Bau-AG, Niederlassung Nurnberg-was instructed to build the bridge with only external prestressing, as it was proposed in their redesign. Deflected tendons were chosen as well asStraight tendons in the column areas and middle field areas. Scaffoldcarriage was used as construction system. The superstructure consists of two separated single box girders with a variable depth between 3.5 and 6.5 m in the area of the large central span.4.3 Hybrid Construction: Valley Bridge AltwipfergundThe reduction of dead load lead to this new construction method, which is called hybrid construction. The aim is to take advantage of the different materials. The concrete is used for the deck and the bottom slab. Prestressing is introduced in sections with tensile stresses. The webs have only to connect the two point cross section and to transfer the transverse forces. For this a folded steel plate is used. With this geometry the web has nearly no longitudinal stiffness, but because of the higher transverse stiffness the transverse forces can be transferred.One problem that occurred with this construction is, that the deformations caused by the transverse forces are not negligible. These deformations strongly depend on the imperfections of the steel web. Therefore a experiment on one section of the web (height: 4.5 m, length: 9.0 m) for th e valley bridge Altwipfergrund is performed at the Lehrstrhl fur Massivbau of the Technischen Universitat MUnchen. The aim of the test is to verify the load carrying behavior of these folded webs.5 ConclusionsExternal prestressing gets more and more important for the design of new bridges. During the last years it has evolved from the status of pilot applications (Ruderting) to a standard design method for box girders in Germany (Trockau) .And as the example Altwipfergrund shows, there are still possibilities to improve the bridge systems and the prestressing systems.New Words and ExpressionsPrestressed concrete 预应力混凝土Cantilever n.悬臂Simply 简支Tendon n.刚束Cross-section 横截面Corrosion n.锈蚀Loss of prestress 预应力损失Creep n.徐变Shrinkage N收缩Post-tensioned method 后张法External prestressing 体外预应力Unit 18 Bridge Rehabilitationand Consolidating桥梁维修与加固内容概要：纽约州交通部面临着许多桥梁属于结构性或功能性的缺陷，而其中很大一部分是由于桥面破坏或限载而被归于缺陷类型。