Last updated on August 11th, 2023 at 12:09 pm
Moden highway bridges of moderate and long spans increasingly adopt orthotropic plate decks, The deck consists of a stiffened deck plate over which a thin layer of the asphaltic concrete wearing course is directly laid.
The steel plate is stiffened in two orthogonal directions: longitudinally by closed rib systems and transversely by the floor beams.
Since the stiffness in two orthogonal directions are different the behaviors of the deck is said to be anisotropic.
This type of deck with orthogonally (ortho) placed stiffeners and with anisotropic (tropic) behavior is known as ‘orthotropic plate deck.
Originally developed in Germany in the 1950s, this system has since been used in many bridges worldwide, resulting in substantial savings in materials and cost.
The successful application of orthotropic plate decks is mainly due to advances in mechanized welding.
Essentially, the deck consists of a flat deck plate, stiffened by welded (closed) longitudinal ribe, which span between transverse floor beams, which in tum span between the main girders.
The components are interconnected and together form a complex structural system.
The deck plate acts as a continuous member supporting the concentrated wheel loads placed between ribs and transfers the reactions to the ribs.
The ribs are usually of trapezoidal shape. The deck plate also functions as the top flange of the ribs, the floor beams, and the main longitudinal girders.
The deck is paved with a wearing course to provide a durable and skin resistant surface for vehicular traffic.
Orthotropic plate decks are used for a wide variety of steel bridges such as plate girder bridges, box girder bridges, movable bridges, cable-stayed bridges, and suspension bridges.
The orthotropic plate deck scheme results in reduced weight for the deck, a condition of special importance for long-span bridges.
It also results in shallower sections and leads to economy in the required length of approaches. Other advantages include faster construction due to lighter components.
Give the range of Span in meters for which Truss, Cantilever and Cable-Stayed Bridges are used and found economical.
The following Table gives the range of span in meters for different types of Bridges:
Type Of Bridge Span Range
Truss Bridge 200 – 500 m
Cantilever Bridge 320-549 m
Cable-Stayed Bridge Above 800 m
Under what conditions, Steel Bridges are generally used?
Steel bridges use could be the preferred option in Build Operate Transfer (BOT) projects, where sped-in construction is crucial.
Steel structures may also prove advantageous for urban flyover/elevated road projects as they cause fewer disturbances to traffic through faster construction and possible prefabrication.
Discuss some detail about Howrah Bridge.
Howrah bridge with a main span of 457 m was the third-longest span cantilever bridge in the world at the time of its construction (1943).
The bridge was erected by commencing at the two anchor spans and advancing towards the center with the use of creeper cranes moving along the upper chord.
The closure at the middle was obtained by means of sixteen hydraulic jacks of 800 t capacity each.
The construction was successfully completed with very close precision
What is the difference between Concrete & Steel Bridges?
Steel bridges have been adopted in the past for major bridges on the highways and more commonly on the railways.
The Howrah Bridge at Calcutta is a splendid example of steel construction. In view of the shortage of steel, not many steel bridges have been built in India in the recent past.
General concepts pertaining to steel bridges are discussed in this Chapter.
For a detailed discussion of design and welded fabrication, the reader may refer to more specialized texts. Compared to concrete construction, steel superstructure will be of lighter weight and will facilitate faster construction.
Further, the construction operations at the bridge site can be reduced with steel superstructure by prefabricating parts of the components at a nearby factory.
In the span range of 120 to 140 m, for which prestressed concrete cantilever bridges are being adopted, steel construction can lead to time savings, as in the rail-cum-road bridge across the Brahmaputra river at Jogighopa in Assam.
Steel bridges require greater maintenance attention than concrete bridges. For example, the Forth railway bridge needs continuous painting: it takes three years to complete on a coat and the process is repeated.
Steel bridges could be the preferred option in Build Operate Transfer (BOT) projects, where sped-in construction is crucial.
Steel structures may also prove advantageous for urban flyover elevated road projects as they cause less disturbance to traffic through faster construction and possible prefabrication.
Types of Bridges || Types of Bridges Based on Span, Materials, Structures, Functions || in hindi
List the different types of Steel Bridges, and indicate the span range applicable to each type.
Types Steel bridges can be classified under the following groups
1. Beam bridges :
2. Plate girder bridges
3. Box girder bridges;
4. Truss brides;
5. Arch bridges;
6. Cantilever bridges:
7. Cable-stayed bridges; and
8. Suspension bridges.
Beam bridges are used for culverts, using rolled steel joists as the main supporting member’s Girder bridges are adopted for simply supported spans less than 50 m and for continuous spans up to 260 m.
Truss bridges are suitable for the span range of 40 to 375 m. Arch bridges are competitive for the medium span range of 200 to 500 m.
Cantilever bridges have been built with success with main spans of 320 to 549 m. Cable-stayed bridges are economical when the span is about 200 to 800 m. for long spans above 800 m, suspension bridges provide the most economical solution.
What are the different types of Plate girder bridges? What are its components in the case of Highway & Railway Bridge?
Since the days of early steel bridge construction, there has been a marked preference for the plate girder bridge system, in view of the elegant aesthetics obtainable with this type and also the convenience in maintenance.
Types Plate girder bridges : (a) deck type: and (b) half-through type. Deck type is normally preferred.
The half-through type is adopted when the cost of the additional embankment to raise the rail level is high.
A plate girder highway bridge will consist of the deck slab (normally of reinforced concrete and stringers running longitudinally and resting on the transverse floor become, which in turn rest on the plate girders.
In the case of a railway bridge, the plate girders carry the wooden sleepers over which the steel rails are fastened.
The girder bridges will be braced laterally at the level of the top flange and the bottom flange beside cross bracing to resist the lateral load due to wind.
The cross bracings consist of angles and are provided at the ends and at intervals of about 4 to 5m.
There is usually a choice available between (a) using two widely spaced longitudinal girders, with the cross girder system supporting the dek, and
(b) providing multiple longitudinal girders with small spacing. In the first case, the cross girder system may consist of closely spaced cross girders alone or cross girders supporting a system of longitudinal stringers.
The Circumstances, For the deck type, the distance between the two girders is kept slightly larger than the gauge of the track to reduce the severity of the impact loads on the girders.
In the half-through type of bridge, the railway load is carried at the lower flange.
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