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The bridge is a type of structure that extends horizontally between support piers but do you know the latest type of bridge and its components commonly used in Civil Engineering? Please read this blog for details.
A bridge has five key components as below:
The superstructure of a bridge directly supports the traffic and provides a smooth and uninterrupted route over natural/man-made blockades like rivers, railway tracks, creeks, roads etc., by conveying the loads and forces approaching over it to the footing through the bearings and substructure.
Expansion joints are provided at the end of the deck and accommodate the movement of the deck due to temperature, creep, shrinkage, etc. Expansion joints make the deck joint leak-proof, protect the edges of slab/girder and permit smooth passage of loads from one span to another by connecting the gap.
Bearings are important components of a bridge which while permitting longitudinal and/or transverse rotations and/ or movements of the superstructure with respect to the substructure and effectively transmit loads and forces from the superstructure to the substructure.
Substructure includes those components of a bridge that are above the foundation but excludes bearings and superstructure. Substructure can be built of brick/ stone masonry, plain/ reinforced/ prestressed concrete and steel. The shape of columns or piers and abutments generally, should be such as to cause the least impediment to the flow of water.
Foundations are components of the bridge that are below the substructure and placed below the level of the soil. Foundation transmits all the loads coming from the substructure to the underlying soil.
There are 10 modern Bridge forms we commonly see in the bridge design
The arch bridge is a curve-shaped bridge where the vertical load is not directly applied straight down, but as an alternative, loads are carried along the curve of the arch. By doing this, the load is transferred symmetrically to the arch shape and reduces the high amount of compression. These supports are known as abutments. Abutments transmit loads of the bridge to the foundation below the soil. The arch bridges are generally made of stone, concrete or steel.
Solid Slab bridges are horizontal beams reinforced at each end by substructure components. Beams connect crosswise a single span or multi spans with the transitional supports as bridge piers. In olden times, the beams were mere logs placed across simple structures. In modern design, beams are made of various small beams from timber beams to huge steel boxes.
The vertical weight on the bridge converts a shear and or flexural weight on the beam which is transmitted down its load longitudinally to the substructures on any side of the piers.
Voided Slabs bridges are precast prestressed slab bridges. Voided slab bridges are considered to reduce the self-weight of the bridge. The voids are typically circular with the depth of voids usually limited to sixty per cent of the depth of the slab so that the slab endures to behave like a single plate.
If the limit of void depth is exceeded, the slab may act more or less like a cellular floor. The voids might either be designed for the full extent length or otherwise, these may be designed in the central extent length so that a compact part is present adjacent to the supports (at pier location), where shear controls the design.
T-beam bridges are structurally simple to construct and maintain, hence they are favoured over other types of bridges, specifically those connecting a short span. A typical T-beam and slab Bridge comprises the longitudinal girder, non-stop deck slab between the T-beams and girders to support lateral firmness to the bridge deck.
The longitudinal girders are generally spaced out at intervals of 2 to 2.5 m and cross girders are placed at 4 to 5 m Intervals.
In modern design, the suspension bridges are designed as a cost-effective solution for a situation where it is hard to place bridge piers in the middle of the river or stream. A German- American engineer John Augustus Roebling came up with the idea of a web truss to either side of the bridge and developed the design of a Brooklyn bridge connecting Brooklyn to Manhattan in New York.
The two piers at either end support the significant weight of the bridge as compression thrusts down the bridge. The cables or chains carry the tension force of the bridge. These cables also work as a supporting truss system underneath the bridge deck to harden the deck and decrease the ripple and sway.
In case of a Suspension Bridge, Road Restraint System also plays a very important role because these bridges are expensive assets and also generally provide a high-speed corridor, which means the safety of all road users is paramount. Please see our blog to know "Why Road Restraint system is so important?"
The cable-stayed bridge looks similar to a suspended bridge due to similar towers and cables arrangement, but the cable-stayed bridge is different from the suspension bridge. In a cable-stayed bridge, its cables grip the deck by linking it directly to the towers as an alternative via suspender cables.
This means the weight of the deck is maintained by diagonal cables in tension connecting to one or more vertical towers or piers. The towers or piers transmit the cable to the foundations through vertical compression. The tensile forces in the cables make sure that the deck is placed into horizontal compression.
The construction of cable-stayed bridges typically follows the cantilever technique. Here the construction commences with the dropping of caissons and the assembly of towers or piers and anchorages. After constructing the tower, cable and a unit of the deck are built in each direction.
Each unit of the deck is prestressed before commencing the next unit. The process then continues until the deck sections tie at the midway and then they joined together. The ends are then anchored at the abutments.
Extradosed bridges generally have the same appearance as cable-stayed bridges, but extradosed bridges come with very short towers or piers. Short piers require a shallow angle of cable stays. In an extradosed bridge, the deck is directly rested on part of the tower. In this case, the deck behaves as a continuous beam.
The cables from the tower interconnect with the deck only further out, and at a shallow angle so that their tension behaves to compress the bridge deck horizontally than to support it vertically. The hybrid nature of extradosed bridges leads to significant complexity but it significantly reduces the construction cost.
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In the Box girder bridge, the main beams contain girders in the form of a hollow box. The box girder usually includes either prestressed concrete, reinforced concrete and structural steel. The box is characteristically rectangular or trapezoidal in sections.
Steel box girders and concrete-steel composite box girders are designed for long spans bridges, where the mass of the bridge is aimed to minimize. Box girder bridges are generally used for road flyovers or modern raised constructions for railways.
The box girder bridge sections are developed in single-cell or multi-cell profiles. A multi-cell box bridge provides greater torsional toughness. Trapezoidal box girder bridges are usually used in curved bridges due to the gigantic torsional stiffness resulting from the closed cross-section.
Multi-cell Box girder decks are cast-in-situ and can be constructed to meet any desired alignment in the plan such as straight, skew, and curved section bridges for highway or railway networks.
A truss bridge is comprised of a load-bearing superstructure that is made of a truss unit. The truss units are connected usually forming triangular units. These triangular units are interconnected forming tension, compression, or occasionally both to carry dynamic loads.
The truss is used because it is a very firm structure and it transmits the load from a single point to a considerably wider area. Truss bridges began to gain popularity very early in the history of bridges and were economical to construct. Engineers investigated different systems of truss bridges to discover better forms and the ones that will solve a problem.
As a result, we see many forms of truss bridges today.
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