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RC GIRDER BRIDGES
Girder bridges, also known as beam bridges, are the most common and simplest type of bridge.
They basically consist of a horizontal beam supported at each end by a pier or other similar structure,
longer girder bridges must be supported along their span by additional structures. Girder bridges are
typically used over short to medium distances. The need for numerous additional supporting
structures below the bridge makes them unsuitable for spanning greater distances.
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.
1. SUPERSTRUCTURE: The superstructure consists of the components that actually span the
obstacle the bridge is intended to cross and includes the following
Parapets (bridge railings), handrails, sidewalk, lighting and some drainage features.
2. SUBSTRUCTURE: 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.
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Girder bridges are the simplest bridge type in structure and consist of steel beams shaped to an I-
section or box section, called a plate girder bridge or a box girder bridge, respectively. Girder bridges
are comprised of deck slabs, on which vehicles and people pass, and of main girders supporting the
deck slabs. Deck slabs include RC deck slabs, steel deck slabs, composite deck slabs, and PC deck
slabs. Bridges where the deck slabs and the main girders work together to resist loads are called
composite girder bridges, and bridges designed to resist loads with the main girders only are called
non-composite girder bridges. In general, effective spans of about 25 to 150 meters are applied.
There are three main types of girder bridge:
1. Box girders: are often used in the construction of roadway flyovers and elevated bridges for
light rail transport. Box girders are more expensive than I-beams and not as easy to build.
However, they do have some important advantages. For example, box girders are better
suited to handling twisting forces.
As such, they are ideal for the construction of curved bridges. Box girders are also more
effective than I-beams over longer spans.
Box girders are more suitable for larger spans and wider decks. Economy and aesthetics
further lead to the evolution of cantilevers in top flanges and inclined webs in external cells
of box girder, As the span and the width increase, the beams and bottom slabs are to be tied
to keep the geometry which in turn leads to evolution of box girder. It can be used for spans
up to 150m depending upon the construction methods.
Box girder bridges may be cast in place using false work supports, removed after
completion, or in sections in case of a segmental bridge. It may also be prefabricated in a
fabrication yard, then transported and installed using cranes.
Another method of box girder bridge construction is the 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.
Fig.2. Cantilever Construction Method for Box Girder Bridge
Fig.3. Incremental Lunching Box Girder Construction Method
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It can cover a range of spans from 25m up to the largest non-suspended concrete
decks built; of the order of 300m.
Single box girders may also carry decks up to 30m wide
Single cell box girder cast-in-situ is used for spans from 40m to 270m.
It is an economical and an aesthetic solution for the over crossings, under crossings,
grade separation structures and viaducts found in the modern highway system.
Interiors of box girder bridges can be used to accommodate service such as gas
pipes, water mains etc.
The maintenance of box girder is easier; interior space is directly accessible without
the use of scaffolding.
It has high structural efficiency
2. Concrete girder: is made of pre-stressed concrete in an I-beam shape. The concrete is also
reinforced with steel rods. This combination is particularly effective in bridge construction.
“The concrete withstands the forces of compression well and the steel rods embedded within
resist the forces of tension.
Fig.5. PS Concrete girder
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1. I-beam girders: are among the most commonly used girders in bridge construction. I-beam
girders are made from rolled steel, and bridges constructed using I-beams are often called
rolled steel girder bridges. I-beam girder bridges are economical, simple to design and
relatively straight forward to build, making them a good option in most cases. However,
according to About Civil.com, “if the bridge contains any curves, the beams become subject
to twisting forces, also known as torque." I-beams, therefore, are best used to construct
bridges that do not have any significant curves.
Fig.6. Rolled beam steel
There are Three Types of Loads Considered in RC girder Bridges Construction specially and
all types of bridges generally:
1. Dead Load: The dead load of a bridge is the bridge itself -- all the parts and materials
that are used in the construction of the bridge. This includes the foundation, beams,
cement, cables, steel or anything else that comprises the parts of the bridge. It's called a
dead load because it doesn't move. It may breathe with the seasons or sway with the
wind, but those movements are almost imperceptible.
2. Live Load: A live load is the moving weight the bridge will hold, such as traffic. It is
based on traffic patterns that include the number of cars, trucks and other vehicles that
will travel across it at any given time. Certain variables, such as snow, may be calculated
into the total live weight for a more accurate estimate. The heaviest possible weight in
the most extreme conditions is also a factor despite the rarity of such an occurrence.
3. Dynamic Load: Dynamic loads are outside forces that cannot be accurately measured
such as wind, vibration and extreme weather. These factors need to be considered in the
construction of a bridge to build "breathing" room into the structure. This breathing
room allows the bridge to move or adjust to the dynamic loads without collapsing or
permanently shifting. As solid as a bridge may seem, it still has the ability to sway when
a strong wind is present.
o When building a bridge, there are other types of loads that need to be considered that are
specific to the terrain in which the foundation will be laid. Environmental factors and
weather patterns are also considered when calculating load-bearing needs. The load
expectation of a bridge will determine the best design for strength and to ensure its
longevity, whether the bridge is to span over large bodies of water or between rising
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o Slab and girder bridges are used when the economical span limit of solid slab bridges is
exceeded. For simply supported spans, this limit is generally found to be nearly 10 meters
and for continuous or balanced cantilever type structures, this limit is 20 to 25 meters, The
deck slab of a slab and Girder Bridge spans transversely over the girders which run
longitudinally spanning between abutment or pier supports. The spacing of the girders
depends on the number of girders to be provided in the deck which again is related to the
cost of materials, shuttering, staging etc.
o Closer beam spacing means lesser thickness of deck slab and consequently savings in
concrete and steel in deck slab but since the number of beams is more in that case, this
increases the quantity of concrete, shuttering and reinforcement for girders and for longer
spans where bearings are required, the number of bearings, therefore, the most economical
arrangement of bridge deck varies from place to place depending on the cost of materials,
shuttering, staging etc., in that locality. It has been observed that three beams deck is
generally found economical than two, four or five beams deck having a carriageway for two
lanes. The girder spacing’s in such cases are usually between 2.25 to 2.75 meters.
Girder Bridge is a one type of structure bridges types, this type called also Beam Bridge consist
of horizontal beam transfer loading for the slab bridge to its piers and columns, this type used in
short and medium spans, and its suitable from the economic way, he construction of a beam bridge is
the simplest of all types of bridges. The design of a beam bridge should be such that it does not bend
under load. For this, the top surface of this bridge is compressed and the bottom surface is under
tension, thus helping the beam remain in a straight line.
STRUCTURAL DESIGN OF REINFORCED CONCRETE BRIDGES Collection by: Skumar
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