Bridge building can be traced back several -thousand years. In primitive times, man used fallen trees, arches made of a rock fall and suspension. bridges made of vines and creepers to cross streams and ravines. These are the three natural forms of bridging methods, namely, the beam, the arch and the suspension structure. Despite the technological advances of the recent past, these three forms, either singularly or in combination, remain the basis of all bridge construction.
The vast majority of arch bridges were built with stone masonry. A few were also built of iron. Some examples of these will be presented during the lecture. Modern arch bridges are built with reinforced concrete and therefore only die construction of reinforced concrete arch bridges will be considered in this paper.
Suspension bridges are used to form exceptionally large Spans. They are a highly specialised form of construction and far more expensive than beam bridges. Their use is therefore limited to a very small number of applications where their ability to span large distances is paramount.
Beam bridges are by far the most widely used form of bridge construction. They are applicable to small span bridges as well as. large spans of around 150m in prestressed precast concrete and spans of around 200m in steel box girder form. In certain circumstances larger spans have been achieved with these forms of construction. Traffic requirements in Hong Kong (and some other parts of the world) have made grade separation necessary at junctions. Generally, such grade separations require the construction of small to medium span bridges or flyovers for which beam type bridges are the most appropriate. The techniques of construction of this form of bridges will therefore be considered in detail. Since concrete is the most popular material for bridge construction in Hong Kong, emphasis will be given to concrete bridges.
Common bridge construction methods
|
Type of bridge according to method of construction |
Span (m) |
Length of bridge (m) |
|
|
1 |
Prefabricated steel or precast concrete beams lifted into
place over the full span
(i) standard beams (ii) non-standard beams (iii) steel trusses |
10-40 40-250 50-100 |
No limit |
| 2 | Prefabricated steel or precast concrete beams progressively placed by gantry girder method |
30-60
|
300-600 |
| 3 | Precast concrete or prefabricated steel deck pushed out (constant radius deck only) |
25-60
|
300-1200 |
| 4 | Prefabricated or precast elements or in situ concrete structures supported by falsework founded directly on the ground |
5-125
|
5-125 |
| 5 | In situ concrete deck constructed on stepping formwork |
20-60
|
300-600 |
| 6 | In situ or precast conciete segments constructed in cantilever and progressively post-tensioned |
50-150
|
No limit |
| 7 | Prefabricated steel or precast concrete segments stayed by cables |
75-800
|
No limit |
| 8 | Prefabricated steel or precast concrete segments suspended from cables |
Span between towers 400-1500
|
No limit |
กก