The suitable techniques of construction of bridge superstructure will vary from site to site,and will depend on the spans and length of the bridge, type of the bridge,materials used and site conditions. For instance, cast-in-site concrete construction could be adopted for short spans up to 40 m, if the river bed is dry for a considerate portion of the year, whereas free cantilever construction with prestressed concrete decking would be appropriate for long spans in rivers with navigational requirements. The current trend is towards the avoidance of staging as much as possible and to use precast or prefabricated components to maximum extent.Also , construction machinery such as cranes and launching girders are coming into wider use . These are greater savings to be effected by paying attention to the method of construction even from the design stage than by attacking permanent materials .
Short Span Bridges
For bridges involving spans up to 40 m , the superstructure may be built on staging supported on the ground . Alternatively , the girders may be precast for the full span length and erected using launching girders or cranes,if the bridge has many equal spans.In the latter procedure , the additional cost on erection equipment should be less than the saving in the cost of formwork and in the labour cost resulting from faster construction .
Long Span Concrete Bridges
Long span concrete bridges are usually of post-tensioned concrete and constructed either as conditions beams types or as free ver cantile 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 developed by Baur-Leonhard team is shown schematically in Figure 22.1.
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 yilever 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 , ermits by repeated use the construction of large bridges . The avoidance of scaffold from below ,the speed of work and the saving in labour cost result in the construction being very economicdal . 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 ar 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 compresed 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 buoyantenough to support a large proportion of the weight of the bridge.
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