Cable Stayed Bridge


The cable-stayed bridge is related to the cantilever bridge. The cables are in tension, and the deck is in compression. The spans can be constructed as cantilevers until they are joined at the centre. A big difference between cantilever bridges and cable-stayed bridges is that the former usually have a suspended span, and the latter do not.

Cable Stayed Bridge ( Parts )

A cable stayed-bridge lacks the great rigidity of a trussed cantilever, and the continuous beam compensates for this to some extent. Indeed, while a long cable-stayed span is under construction, there may be great concern about possible oscillations, until the cantilevers are joined. For the Pont de Normandie, there was even thought of using active correctors if things threatened to get out of hand. In fact, the construction went smoothly.

The cables are of high tensile steel. In a few examples these are encased in concrete. Towers are often made in concrete, though steel is also used.

Cable Stayed Bridge A

Advantages of cable-stayed bridges

The two halves may be cantilevered out from each side. There is no need for anchorages to sustain strong horizontal forces, because the spans are self-anchoring. They can be cheaper than suspension bridges for a given span. Many asymmetrical designs are possible.

Disadvantages of cable-stayed bridges

In the longer sizes, the cantilevered halves are very susceptible to wind induced oscillation during construction. The cables require careful treatment to protect them from corrosion.


Suspension Bridge


A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 19th century. Simple suspension bridges, which lack vertical suspenders, have a long history in many mountainous parts of the world.

This type of bridge has cables suspended between towers, plus vertical suspender cables that carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc upward for additional clearance. Like other suspension bridge types, this type often is constructed without falsework.

Suspension Bridge A

The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground. The roadway is supported by vertical suspender cables or rods, called hangers. In some circumstances, the towers may sit on a bluff or canyon edge where the road may proceed directly to the main span, otherwise the bridge will usually have two smaller spans, running between either pair of pillars and the highway, which may be supported by suspender cables or may use a truss bridge to make this connection. In the latter case there will be very little arc in the outboard main cables.

Suspension Bridge B

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.

In a suspended deck bridge, cables suspended via towers hold up the road deck. The weight is transferred by the cables to the towers, which in turn transfer the weight to the ground.

Comparison of a catenary and a parabola with the same span and sag

The catenary represents the profile of a simple suspension bridge, or the cable of a suspended-deck suspension bridge on which its deck and hangers have negligible mass compared to its cable. The parabola represents the profile of the cable of a suspended-deck suspension bridge on which its cable and hangers have negligible mass compared to its deck. The profile of the cable of a real suspension bridge with the same span and sag lies between the two curves.

Suspension Bridge Forces

Assuming a negligible weight as compared to the weight of the deck and vehicles being supported, the main cables of a suspension bridge will form a parabola (very similar to a catenary, the form the unloaded cables take before the deck is added). One can see the shape from the constant increase of the gradient of the cable with linear (deck) distance, this increase in gradient at each connection with the deck providing a net upward support force. Combined with the relatively simple constraints placed upon the actual deck, this makes the suspension bridge much simpler to design and analyze than a cable-stayed bridge, where the deck is in compression.


Box Girder Bridges


A box girder bridge is a bridge in which the main beams comprise girders in the shape of a hollow box. The box girder normally comprises either prestressed concrete, structural steel, or a composite of steel and reinforced concrete. The box is typically rectangular or trapezoidal in cross-section. Box girder bridges are commonly used for highway flyovers and for modern elevated structures of light rail transport. Although normally the box girder bridge is a form of beam bridge, box girders may also be used on cable-stayed bridges and other forms.

Construction

If made of concrete, box girder bridges may be cast in place using falsework supports, removed after completion, or in sections if a segmental bridge. Box girders may also be prefabricated in a fabrication yard, then transported and emplaced using cranes.

For steel box girders, the girders are normally fabricated off site and lifted into place by crane, with sections connected by bolting or welding. If a composite concrete bridge deck is used, it is often cast in-place using temporary falsework supported by the steel girder.

Either form of bridge may also be installed using the technique of incremental launching. Under this method, gantry cranes are often used to place new segments onto the completed portions of the bridge until the bridge superstructure is completed

Box Girder Bridge

Advantages and disadvantages

Compared to I girders, box girders have a number of key advantages and disadvantages. Box girders offer better resistance to torsion, which is particularly of benefit if the bridge deck is curved in plane. Additionally, larger girders can be constructed, because the presence of two webs allows wider and hence stronger flanges to be used. This in turn allows longer spans. On the other hand, box girders are more expensive to fabricate, and they are more difficult to maintain, because of the need for access to a confined space inside the box.

Box Girder Bridge

Corrosion of the steel cables that provide the post-tensioning for box girder bridges has become a major concern. On December 13, 2009, the Indiana Department of Transportation (INDOT) closed the Cline Avenue (SR-912) bridge over the Indiana Harbor and Ship Canal after a routine inspection revealed significant corrosion of the steel tensioning cables and rebar within the box girders due to water seeping through cracks in the bridge deck. After determining the level of corrosion compromised the bridge’s structural integrity beyond repair, INDOT decided to permanently close and eventually demolish the span.


Basic Bridge Terms


An important first step in understanding the principles and processes of bridge construction is learning basic bridge terminology. Although bridges vary widely in material and design, there are many components that are common to all bridges. In general, these components may be classified either as parts of a bridge superstructure or as parts of a bridge substructure.

SUPERSTRUCTURE

The superstructure consists of the components that actually span the obstacle the bridge is intended to cross and includes the following:

  • Bridge deck
  • Structural members
  • Parapets (bridge railings), handrails, sidewalk, lighting and some drainage features

The deck is the roadway portion of a bridge, including shoulders. Most bridge decks are constructed as reinforced concrete slabs, but timber decks are occasionally used in rural areas and open-grid steel decks are used in some movable bridge designs (bascule bridge). As polymers and fiber technologies improve, Fiber Reinforced Polymer (FRP) decks may be used. Bridge decks are required to conform to the grade of the approach roadway so that there is no bump or dip as a vehicle crosses onto or off of the bridge. The most common causes of premature deck failure are:

  • Insufficient concrete strength from an improper mix design, too much water, improper amounts of air entraining admixtures, segregation, or improper curing
  • Improper concrete placement, such as failure to consolidate the mix as the concrete is placed, pouring the concrete so slowly that the concrete begins the initial set, or not maintaining a placement rate.
  • Insufficient concrete cover due to improper screed settings or incorrect installation of the deck forms and/or reinforcement

A bridge deck is usually supported by structural members. The most common types are:

  • Steel I-beams and girders
  • Precast, prestressed, reinforced concrete bulb T beams
  • Precast, prestressed, reinforced concrete I beams
  • Precast, prestressed, concrete box beams
  • Reinforced concrete slabs

Secondary members called diaphragms are used as cross-braces between the main structural members and are also part of the superstructure. Parapets (bridge railings), handrails, sidewalks, lighting, and drainage features have little to do with the structural strength of a bridge, but are important aesthetic and safety items. The materials and workmanship that go into the construction of these features require the same inspection effort as any other phase of the work.

Componets of Bridge Plate ASUBSTRUCTURE

The substructure consists of all of the parts that support the superstructure. The main components are abutments or end-bents, piers or interior bents, footings, and piling. Abutments support the extreme ends of the bridge and confine the approach embankment, allowing the embankment to be built up to grade with the planned bridge deck.

When a bridge is too long to be supported by abutments alone, piers or interior bents are built to provide intermediate support. Although the terms may be used interchangeably, a pier generally is built as a solid wall, while bents are usually built with columns.

The top part of abutments, piers, and bents is called the cap. The structural members rest on raised, pedestal-like areas on top of the cap called the bridge seats. The devices that are used to connect the structural members to the bridge seats are called shoes or bearings. Abutments, bents, and piers are typically built on spread footings. Spread footings are large blocks of reinforced concrete that provide a solid base for the substructure and anchor the substructure against lateral movements.

Footings also serve to transmit loads borne by the substructure to the underlying foundation material. When the soils beneath a footing are not capable of supporting the weight of the structure above the soil, bearing failure occurs. The foundation shifts or sinks under the load, causing structure movement and damage.

In areas where bearing failure is likely, footings are built on foundation piling . These load-bearing members are driven deep into the ground at footing locations to stabilize the footing foundation. Piling transmits loads from the substructure units down to underlying layers of soil or rock.

Componets of Bridge Plate B