Classification of Bridges ppt
Classification of Bridges
Bridge can be classified in different ways depending upon:-
- High Flood Level
- Position of bridge floor
- Inter-Span Relation
- Nature of superstructure action
- Alignment of the Superstructure w.r.t obstacle
- Material
- Span length
- Type of connection
According to road level relative to the H.F.L of the river below
Non-Submersible Bridge – H.F.L always below the superstructure
Submersible Bridge– H.F.L always above the superstructure
High Level Bridge or Non-Submersible Bridge
- It is a bridge, which does not allow the high floor waters to pass over them.
- All the floodwater is allowed to pass through its vents.
- In other words, it carries the roadway above the Highest Flood Level (HFL) of the channel.
Submersible Bridge
- A submersible bridge is a structure that allows floodwater to pass over bridge submerging the communication route.
- Its formation level should be so fixed as neither to cause interruption to traffic during floods for more than three days at a time nor for more than six times in a year.
According to Position of Bridge Floor relative to Superstructure
Deck Bridges.:-Bridges whose floorings are supported or suspended above the level of bearings.
Through Bridges:- These are bridges whose floorings are supported or suspended at the bottom of the superstructure.
According to Inter-Span Relations
Simple Bridges :-They include all type of beam, girder or truss bridges supported at both ends only. It is suitable for spans up to 8m only.
Cantilever Bridges:- Bridges that are fixed at one end and free at the other end and are used for spans from 8-20m.
Continuous Bridges:-Bridges that continue over two or more spans They are provided for large spans and where unyielding foundations are available.
According to Nature of Superstructure Action
- Arch Bridges
- Portal Frame Bridges
- Truss Bridges
- Balanced Cantilever Bridges
- Suspension Bridges
Balanced Cantilever Bridge
- A cantilever bridge is a built using cantilevers, that projects horizontally into bridge structures space, supported on only one end.
- For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete.
- Balanced Cantilever Bridge: One portion of a span is suspended from or rests over or is hinged with the other portion of span
Suspension Bridge
Suspension bridge is better suited for crossing over a very large boat/ship navigation.
Golden Gate Bridge
The Golden Gate Bridge is a suspension bridge spanning the Golden Gate, the opening of the San Francisco Bay into the Pacific Ocean.
It has been declared one of the modern Wonders of the World by the American Society of Civil Engineers.
The Frommers travel guide considers the Golden Gate Bridge “possibly the most beautiful, certainly the most photographed, bridge in the world“.
Light and strong, suspension bridges can span distances from 2,000 to 7,000 feet far longer than any other kind of bridge.
They are ideal for covering busy waterways. Suspension bridges tend to be the most expensive to build.
Main Elements are:- Cables, Decking, Suspenders, Supporting Tower, Anchorages
Cable is the main load carrying member, curvature of which changes as the load moves over the deck.
Traffic load of the decking is transferred to the cable thru’ the suspenders.
Decking is supported by stiffening girders.
Load Transfer Mechanism
Suspension cables pass over a smooth frictionless pulley and anchorage on either sides.
Tension on either side of the cable is equal.
If it passes over a saddle on a pulley, horizontal components of the tension on two sides is same since the cable cannot have a movement relative to saddle.
Most of the weight or load of the bridge is transferred by the cables to the anchorage systems.
These are embedded in either solid rock or huge concrete blocks.
Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.
Structural Analysis
- The main forces in a suspension bridge of any type are tension in the cables and compression in the pillars.
- Since almost all the force on the pillars is vertically downwards and they are also stabilized by the main cables, the pillars can be made quite slender, as on the Severn Bridge, on the Wales-England border.
- Cables support loads over long spans such as suspension bridges and only force in them is direct tension.
- They are too flexible to carry moments.
- Analysis involves straight forward application of equilibrium equations to various free bodies.
- Cable under a given loading takes the shape of a funicular polygon which represents to some scale the BMD of a simple beam under the same loading.
- If the hangers are large, the load transmitted to the cables can be approximated as UDL for which the cables assume the shape of a parabola, similar to BMD of a simple beam under UDL.
Advantages over other bridge types
- A suspension bridge can be made out of simple materials such as wood and common wire rope.
- Longer main spans are achievable than with any other type of bridge.
- Less material may be required than other bridge types, even at spans they can achieve, leading to a reduced construction cost.
- Except for installation of the initial temporary cables, little or no access from below is required during construction, for example allowing a waterway to remain open while the bridge is built above.
- May be better to withstand earthquake movements than heavier and more rigid bridges
Disadvantages compared with other bridge types
- Considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck vibrating under high winds.
- The relatively low deck stiffness compared to other (non-suspension) types of bridges makes it more difficult to carry heavy rail traffic where high concentrated live loads occur.
- Some access below may be required during construction, to lift the initial cables or to lift deck units. This access can often be avoided in cable-stayed bridge construction