1、 1 Large Span Bridge 1. Suspension Bridge The suspension bridge is currently the only solution in excess of 600 m, and is regarded as competitive for down to 300. The worlds longest bridge at present is the Verrazano Narrows bridge in New York. Another modern example is the Severn Bridge in England.
2、 The components of a suspension bridge are: (a) flexible cables, (b) towers, (c) anchorages, (d) suspenders, (e) deck and ， (f) stiffening trusses. The cable normally consists of parallel wires of high tensile steel individually spun at site and bound into one unit .Each wire is galvanized and the c
3、able is cover with a protective coating. The wire for the cable should be cold-drawn and not of the heat-treated variety. Special attention should be paid to aesthetics in the design of the rowers. The tower is high and is flexible enough to permit their analysis as hinged at both ends. The cable is
4、 anchored securely anchored to very solid anchorage blocks at both ends. The suspenders transfer the load form the deck to the cable. They are made up of high tensile wires and are normally vertical. The deck is usually orthotropic with stiffened steel plate, ribs or troughs， floor beam, etc. Stiffe
5、ning trusses, pinned at the towers, are providing. The stiffening system serves to control aerodynamic movements and to limit the local angle changes in the deck. If the stiffening system is inadequate, torsional oscillations due to wind might result in the collapse of the structure, as illustrated
6、in the tragic failure in 1940 of the first Tacoma Narrows Bridge. The side span to main span ratio varies from 0.17 to 0.50 .The span to depth ratio for the stiffening truss in existing bridge lies between 85 and 100 for spans up to 1,000m and rises rather steeply to 177. The ratio of span to width
7、of deck for existing bridges ranges from 20 to 56. The aerodynamic stability will have be to be investigated thoroughly by detailed analysis as well as wind tunnel tests on models. 2. The cable-stayed bridge During the past decade cable-stayed bridges have found wide application, sespecially in West
8、ern Europe, and to a lesser extent in other parts of the world. The renewal of the cable-stayed system in modern bridge engineering was due to the tendency of bridge engineering in Europe, primarily Germany, to obtain optimum structural performance from material which was in short supply-during the
9、post-war years. Cable-stayed bridges are constructed along a structural system which comprises an 1 orthotropic deck and continuous girders which are supported by stays, i.e. inclined cables passing over or attached to towers located at the main piers. The idea of using cables to support bridge span
10、 bridge span is by no means new, and a number of examples of this type of construction were recorded a long time ago. Unfortunately the system in general met with little success, due to the fact that the statics were not fully understood and that unsuitable materials such as bars and chains were use
11、d to form the inclined supports or stays. Stays made in this manner could not be fully tensioned and in a slack condition allowed large deformations of the deck before they could participate in taking the tensile loads for which they were intended. Wide and successful application of cable-stayed sys
12、tems was realized only recently, with the introduction of high-strength steels, orthotropic decks, development of welding techniques and progress in structural analysis. The development and application of electronic computers opened up new and practically unlimited possibilities for exact solution o
13、f these highly statically indeterminate systems and for precise stoical analysis of their three-dimensional performance. Existing cable-stayed bridges provide useful data regarding design, fabrication, erection and maintenance of the mew system. With the construction of these bridges many basic prob
14、lems encountered in their engineering are shown to have been successfully solved. However, these important data have apparently never before been systematically presented. The application of inclined cable gave a new stimulus to construction of large bridges. The importance of cable-stayed bridges i
15、ncreased rapidly and within only one decade they have become so successful that they have taken their rightful place among classical bridge system. It is interesting to note now how this development which has so revolutionized bridge construction, but which in fact is no new discovery, came about. T
16、he beginning of this system, probably, may be traced back to the time when it was realized that rigid structures could be formed by joining triangles together. Although most of these earlier designs were based on sound principles and assumptions, the girder stiffened by inclined cables suffered vari
17、ous misfortunes which regrettably resulted in abandonment of the system. Nevertheless, the system in itself was not at all unsuitable. The solution of the problem had unfortunately been attempted in the wrong way. The renaissance of the cable-stayed, however, was finally successfully achieved only 1
18、 during the last decade. Modern cable-stayed present a three-dimensional system consisting of stiffening girders, transverse and longitudinal bracings, orthotropic-type deck and supporting parts such as towers in compression and inclined cables in tension. The important characteristics of such a thr
19、ee-dimensional structure is the full participation of the transverse construction in the work of the main longitudinal structure. This means a considerable increase in the moment of inertia of the construction which permits a reduction in the depth of the girders and economy in steel. Long span conc
20、rete bridges are usually of post-tensioned concrete and constructed either as conditions beams types or as free versatile structures. Many methods have been developed for continuous deck construction. If the clearance between the ground and bottom of the deck is small and the soil is firm, the super
21、structure can be built on staging. This method is becoming obsolete. Currently, free-cantilever and movable scaffold systems are increasingly used to save time and improve safety. The movable scaffold system employs movable forms stiffened by steel frames. These forms extend one span length and are
22、supported by steel girders which rest on a pier at one end and can be moved from span to span on a second set of auxiliary steel girders. An economical construction technique known as incremental push-launching method is developed by Baur-Leonhard team. The total continuous deck is subdivided longit
23、udinally into segments of 10 to 30 m length depending on the length of spans and the time available for construction. Each of these segments is constructed immediately behind the abutment of the bridge in steel framed forms, which remain in the same place for concreting all segments .The forms are s
24、o designed as to be capable of being moved transversely or rotated on hinges to facilitate easy stripping after sufficient hardening of concrete. At the head of the first segment, a steel nose consisting of a light truss is attached to facilitate reaching of the first and subsequent piers without in
25、cluding a too large can yielder moment during construction . The second and the following segments are concreted directly on the face of the hardened portion and the longitudinal reinforcement can continue across the construction joint . The pushing is achieved by hydraulic jacks which act against t
26、he abutment .Since the coefficient of friction of Teflon sliding bearings is only about 2 percent, low capacity hydraulic jacks would suffice to move the bridge even over long lengths of several hundred metres . This method can be used for straight and continuously curved bridges up to a span of abo
27、ut 120 m . The free-cantilever system was pioneered by Dyckerhoff and Willmann in Germany .In this system , the superstructure is erected by means of cantilever truck in sections generally of 3.5 m .The cantilever truck ,whose cost is relatively small and which is attached firmly to permanent construction , emits by repeated use the construction of large bridges . The
