R.C.C. bridges

R.C.C. BRIDGES

 

1. R.C.C. SLAB BRIDGE

See Fig. 2. The basic bridge form is a R.C.C. Slab and hence the name R.C.C. Slab Bridge. It is a single span bridge. The R.C.C. Slab R.C.C. slab of large thickness is laid between two Abutments. Abutments may be built of Masonry or R.C.C. Parapet Wall and Horizontal Hand Rails are also constructed.

Wearing coat of cement concrete is laid over the slab to arrest wearing of the slab due to the traffic movements of carriages like heavy vehicles, trucks, busses, etc.

Slab Bridge is simple to design and construct. The thickness of the slab and the corresponding dead load are quite considerable. But, it is economical up to a span of 9 meters only. The quantity of concrete and steel required are high, because of the large thickness of the slab. R.C.C. slab bridges are used for roadways and railways.

Carriage Way of a Bridge

Carriage way of a bridge is the width of the road meant for the movement of carriages and heavy vehicles. Carriage way is expressed as Traffic Lanes. One traffic lane represents the width of Class A vehicles of 3.75 meters. Bridges should have 2 traffic lanes at least or 4 or 6 traffic lanes in highways and city roads.

 

2. R.C.C. T-BEAM AND SLAB BRIDGE

See Fig. 3. In this type, the Beam and R.C.C. Slab are cast monolithically. The cross-section has the shape of T and hence the name T-Beam and Slab Bridge.

Slab is supported over T-Beams. T-Beams are simply supported over Piers and Abutments. Piers and abutments are made of R.C.C.

T-Beams function as main longitudinal girders. They transmit the load from the slab to the piers and abutments. The roadway is supported on T-Beams. Wearing Coat of cement concrete is laid over the slab to arrest the wearing of the slab due to the traffic movements of heavy vehicles, trucks, buses, etc. The advantages of R.C.C. T-beam and slab bridge are:

• This type of bridge is suitable for spans between 9 to 20 meters.

• It is durable, compact and gives a neat, pleasing and aesthetic appearance.

• It is cheaper in construction than Steel Bridge.

 

3. R.C.C. BOW STRING GIRDER BRIDGE

See Fig. 4. Components of R.C.C. Bow String Girder Bridge are: Arch (Bow), Tie Beam, Abutments, Hangers, etc. In this, the load carrying structure is known as Bow String Girder.

Arch looks like a bow and the horizontal Tie Beam as a string of the bow. Hence, the name is Bow String Girder Bridge.

Cross Girders rest on the tie beam and support the Road Way. The beam is suspended from the arch by Suspenders or Hangers.

In this bridge, horizontal thrust from the arch is resisted by horizontal tie beam. The bow is tied horizontally by tie members. Tie beam is supported on Hinge Bearing at one end to permit rotation. The other end is supported on Roller Bearing to allow for expansion.

Advantages

• This type of bridge is adopted for Arch Bridges having spans of 30 to 40 meters.

• The major portion of the load is taken by the tie beam. Hence, the force on the abutments from the arch will be limited. This results in abutments of less weight.

 

4.  CONTINUOUS R.C.C. BRIDGE

See Fig. 5.

If the river crossing is very wide, then a single span may not be economical. The bridge can have two or more spans using intermediate columns. The intermediate columns are known as Piers or Supports. In this type of bridge, the R.C.C. Deck Slab is made continuous over the Piers and Abutments, hence the name Continuous R.C.C. Bridge.

The R.C.C. Slab may be in the form of Deck Slab or T-Beam section. The intermediate spans between the piers are equal. But, the end spans are 16% to 20% lesser than the intermediate spans.

Advantages

• Because of the continuous arrangement, the deck slab or t-beam is stronger. efficient than separate independent slabs.

 • It is used for large span lengths of more than 30 meters. It is more

 

5. BALANCED CANTILEVER R.C.C. BRIDGE

See Fig. 6. In a Cantilever Beam, the beam is placed on two piers so that one end extends beyond the support. A gap is left between the two free ends or Cantilever Arms. Then, a shorter beam is placed so that it rests on two cantilever arms and fills the gap. The beam that fills the gap is called Central Beam. The whole girder is called a Balanced Cantilever. The cross section of this bridge may be T-Beam or Hollow Girder Type.

The Span of Cantilever portion is about 25% of the Supported Span. The length of the central beam is defined as Articulation. The central beam is designed as a simply supported beam. The Articulation is also treated as support. The important feature of this bridge is the construction of alternate spans with extending cantilevers. The ends of the cantilevers are used as supports for the central beam.

Advantages

• It can be erected without false work.

• Providing continuous spans minimizes the bending moments. Therefore, individual span lengths can be increased. But, this requires unyielding supports. Any yielding of support results in distress in structure. To avoid this, a combination of cantilever and suspended spans can be adopted.

• This type of bridge is suitable for spans in the range of 35 to 70 meters.

• This type of bridges requires comparatively light piers.

• This type of dam is preferred in places where small spans are uneconomical. Also, it can be used in yielding river beds, where foundations are costlier.

 

6. R.C.C. ARCH BRIDGE

An Arch is a curved beam carrying loads by simple compression to the strong end supports. Arch bridges are constructed with R.C.C. or steel.

When an arch is built of R.C.C., the ends of the arch are prevented from moving outwards by a concrete tie.

See Fig. 7. A typical R.C.C. Arch Bridge consists of Arches, Abutments, Piers, Extrados, Intrados, Spandrel Fill, Dirt Walls, Parapet Wall, Foundation for piers and abutments, Hand Rail, Road Surface, etc.

The arches rest on Piers and Abutments. Arch shapes may be Semi-circular, Elliptical, Parabolic, Segmental, Multi-centered, Pointed, etc. The outer surface of the arch is known as Extrados. The inner surface of the arch is known as Intrados. Spandrel Fill is an irregular triangular space as shown. This type of R.C.C. arch bridge is known as Spandrel-Filled Arch. Road Way with wearing coat of concrete is constructed on the top of the arches.

Arch Bridges produce inclined pressures on the supports under vertical loads. Therefore, they require very strong abutments to withstand the inclined end reactions. Consequently, they are used only where it is possible to have rigid foundations.

Advantages

• R.C.C. Arch Bridges can be used for spans from 20 meters to 250 meters.

• Arch bridges are graceful with a neat, pleasing, aesthetic and elegant appearance.

• They are easy to construct.

• They are preferred for deep gorges with rocky abutments. Therefore, bending will not occur in the arch. This results in long life of the bridge.

 

7. RIGID FRAME R.C.C. BRIDGE

See Fig. 8. In this type of bridge, R.C.C. Portal Frames are used for the construction of the bridge. The substructure of the bridge including the Abutments and the Deck Slab are monolithically cast. Road way is laid at the top of the deck slab.

Advantages

• These bridges are suitable for a maximum span of 20 meters.

• They have all the advantages of the Continuous R.C.C. Bridge.

• Bearings are not needed over the abutments.

• Abutments are stable. Hence, the bridge has high stability against lateral forces such as wind and earthquake.