- Suspension Bridge
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 cable 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 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. Stiffening 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 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 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.
- The cable-stayed bridge
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 post-war years.
Cable-stayed bridges are constructed along a structural system which comprises an 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 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 used 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 systems 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 of 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 problems 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 increased 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.
The 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 various 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 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 three-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 concrete 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 superstructure 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 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 longitudinally 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 so 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 including 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 the 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 about 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 avoidance of scaffold from below, the speed of work and the saving in labor cost result in the construction being very economical. The free-cantilever system is ideally suited for launched girders with a large depth above the pier cantilever system is ideally suited for launched girders with a large depth above the pier cantilevering to the middle of the span.
Another technique is the use of the pneumatic caisson .The caisson is a huge cylinder with a bottom edge that can cut into the water bed. When compressed is pumped into it ,the water is forced out .Caissons must be used with extreme care .for one thing, workers can only stay in the compression chamber for short periods of time .For another , if they come up to normal atmospheric pressure too rapidly ,they are subject to the bends ,or caisson disease as it is also called , which is a crippling or even fatal condition caused by excess nitrogen in the blood .When the Eads Bridge across the Mississippi River at St.Louis was under construction between 1867and 1874 , at a time when the danger of working in compressed air was not fully understood ,fourteen deaths was caused by the bends .
When extra strength is necessary in the piers, they sometimes keyed into the bedrock-that is ,they are extended down into the bedrock .This method was used to build the piers for the Golden Gate Bridge in San Francisco ,which is subject to strong tidies and high winds ,and is located in an earthquake zone .The drilling was carried out under water by deep-sea divers .
Where bedrock cannot be reached ,piles are driven into the water bed .Today ,the piles in construction are usually made of prestressed concrete beams .One ingenious technique ,used for the Tappan Zee Bridge across the Hudson River in New York ,is to rest a hollow concrete box on top of a layer of piles .When the box is pumped dry ,it becomes buoyant enough to support a large proportion of the weight of the bridge .
Each type of bridge indeed each individual bridge presents special construction problems. With some truss bridges , the span is floated into position after the piers have been erected and then raised into place by means of jacks or cranes .Arch bridges can be constructed over a false work ,or temporary scaffolding. This method is usually employed with reinforced concrete arch bridges .With steel arches ,however ,a technique has been developed whereby the finished sections are held in place by wires that supply a cantilever support .Cranes move along the top of the arch to place new sections of steel while the tension in the cables increases .
With suspension bridges ,the foundations and the towers are built first .Then a cable is run from the anchorage-concrete block in which the cable is fastened-up to the tower and across to the opposite tower and anchorage .A wheel that unwinds wire from a reel puns along this cable .When the reel reaches the other side ,another wire is placed on it ,and the wheel returns to its original position .When all the wires have been put in place ,another machine moves along the cable to compact and to bind them .Construction begins on the deck when the cables are in place ,with work progressing toward the middle from each end of the structure .
The loads to be considered in the design of substructures and bridge foundations include loads and forces transmitted from the superstructure, and those acting directly on the substructure and foundation.
AASHTO loads .Section 3 of AASHTO specifications summarizes the loads and forces to be considered in the design of bridges (superstructure and substructure). Briefly , these are dead load ,live load , impact or dynamic effect of live load , wind load , and other forces such as longitudinal forces , centrifugal force ,thermal forces , earth pressure , buoyancy , shrinkage and long term creep , rib shortening , erection stresses , ice and current pressure , collision force , and earthquake stresses .Besides these conventional loads that are generally quantified , AASHTO also recognizes indirect load effects such as friction at expansion bearings and stresses associated with differential settlement of bridge components .The LRFD specifications divide loads into two distinct categories : permanent and transient .