Howrah Bridge Study

STEEL STRUCTURESbuilding construction

a case study

HOWRAH BRIDGE

abhishek behera

HOWRAH BRIDGEcalcutta

The following are some of the advantages of steel bridges that have contributed to their :

• They could carry heavier loads over longer spans with minimum dead weight, leading to smaller foundations.

• Steel has the advantage where speed of construction is vital, as many elements can be prefabricated and erected at site.

• In urban environment with traffic congestion and limited working space, steel bridges can be constructed with minimum disruption to the community.

• Greater efficiency than concrete structures is invariably achieved in resisting seismic forces and blast loading.

• The life of steel bridges is longer than that of concrete bridges.

• Due to shallow construction depth, steel bridges offer slender appearance, which make them aesthetically attractive. The reduced depth also contributes to the reduced cost of embankments.

• All these frequently leads to low life cycle costs in steel bridges

Classification based on the position of carriagewayThe bridges may be

• Deck Type Bridge - The carriageway rests on the top of the main load carrying members. In the deck type plate girder bridge, the roadway or railway is placed on the top flanges. In the deck type truss girder bridge, the roadway or railway is placed at the top chord level

• Through Type Bridge - The carriageway rests at the bottom level of the main load carrying members. In the through type plate girder bridge, the roadway or railway is placed at the level of bottom flanges. In the through type truss girder bridge, the roadway or railway is placed at the bottom chord level. The bracing of the top flange or lateral support of the top chord under compression is also required.

• Semi through Type Bridge - The deck lies in between the top and the bottom of the main load carrying members. The bracing of the top flange or top chord under compression is not done and part of the load carrying system project above the floor level. The lateral restraint in the system is obtained usually by the U-frame action of the verticals and cross beam acting together.


Classification based on the main structural system

Many different types of structural systems are used in bridges depending upon the span,carriageway width and types of traffic. Classification, according to make up of main loadcarrying system, is as follows:

(i) Girder bridges - Flexure or bending between vertical supports is the main structural action in this type. Girder bridges may be either solid web girders or truss girders or box girders. Plate girder bridges are adopted for simply supported spans less than 50 m and box girders for continuous spans up to 250 m. Truss bridges are suitable for the span range of 30 m to 375 m. Cantilever bridges have been built with success with main spans of 300m to 550m.

(ii) Rigid frame bridges - In this type, the longitudinal girders are made structurally continuous with the vertical or inclined supporting member by means of moment carrying joints. Flexure with some axial force is the main forces in the members in this type. Rigid frame bridges are suitable in the span range of 25m to 200m.

(iii) Arch bridges - The loads are transferred to the foundations by arches acting as the main structural element. Axial compression in arch rib is the main force, combined with some bending. Arch bridges are competitive in span range of 200m to 500m.

(iv) Cable stayed bridges - Cables in the vertical or near vertical planes support the main longitudinal girders. These cables are hung from one or more tall towers, and are usually anchored at the bottom to the girders. Cable stayed bridges are economical when the span is about 150m to 700m.

(v) Suspension bridges - The bridge deck is suspended from cables stretched over the gap to be bridged, anchored to the ground at two ends and passing over tall towers erected at or near the two edges of the gap. Currently, the suspension bridge is best solution for long span bridges.

Arched bridge

Cable stayed bridge

Suspension bridge


LOADS ON BRIDGES

The following are the various loads to be considered for the purpose of computingstresses, wherever they are applicable.

• Dead load• Live load• Impact load• Longitudinal force• Thermal force• Wind load• Seismic load• Racking force• Forces due to curvature.• Forces on parapets• Frictional resistance of

expansion bearings• Erection forces

Normal span ranges of bridge system


The Rabridra Setu or more famously called the Howrah Bridge over the Hooghly is primarily a cantilever truss bridge, built in the year 1943. The bridge is completely made up of steel.

By the 19th century, Calcutta and Howrah had grown to be the most important economic and cultural centres, and a need for bridging the Hooghly River arose.

A proposal to build a bridge over the Hooghly was made by the government of Bengal, but the following observations were made :

• The foundations for a bridge at Calcutta would be at a considerable depth because of the depth of the mud there.• the structure would cause considerable hindrance to the shipping on the river• The only feasible site was at a great distance.

The bridge was not built. The famous Floating Pontoon Bridge was built in 1874 by Sir Bradford Leslie.

The Floating Pontoon Bridge was built using timber on pontoon. The bridge was opened to let river traffic through. The bridge had hinged shore spans since Hooghly River is subjected to tides. During high tides these became very steep. It was also feared that the floating bridge could affect river currents and cause silting problems. With time the bridge became too inadequate for the ever increasing traffic.

The old pontoon bridge

Construction of the New Howrah Bridge was started on 1937. The Cantilever Era was prevailing at that time, and engineers felts that Cantilever bridges were more rigid than suspension bridge. This bridge is one of the finest Cantilever bridges in the world - left to India by the British engineers.

Considering various aspects like navigational, hydraulics, tidal conditions of the river and the projected traffic conditions, Rendel Palmer & Tritton came up with a design for a Cantilever bridge of 1500 feet, with a 71 feet wide roadway and two 15 feet wide cantilever footways.

The contract was awarded to Cleveland Bridge & Engineering Co. Ltd of Darlington with a strong recommendation that they use Indian-made steel, which they agreed to do. Out of the total 26,500 tons of steel used, Tata Iron and Steel Company supplied 23,500 tons of steel and fabrication was done by Braithwaite, Burn and Jessop Co. at four different shops in Calcutta.

The two huge caissons which was sunk (on the first stage of construction) is still the biggest ever sunk caisson on land.

To keep the water out at depth of around the foundations so that construction can be done, around 500 people were employed on the compressed air operation. The air pressure maintained was about 40 lbs per square inch (2.8 bar).. By the end of 1940 the erection of the cantilever arms was commenced and was completed in mid-summer of 1941.

The two halves of the suspended span, each long and weighing 2,000 tons, were built in December 1941. 16 hydraulic jacks, each of 800 ton capacity were pressed in to service for joining the two halves of the suspended span.

After completing the steel work of the deck and concreting of roadway. the New Howrah Bridge was finally opened to traffic on February 1943.


Howrah bridge under construction


1500 feet 325 feet325 feet

280 feet

• Central span 1500 ft between centres of main towers• Anchor arm 325ft each• Cantilever arm 468ft each• Suspended span 564ft• Main towers are 280ft high above the monoliths and 76 ft apart at the top• Bridge deck width 71 ft with two footpaths of 15 ft either side

15 ft wide footpath on either sides of the bridge

71ft wide bridge deck or road –deserted on a bandh in Calcutta

Other features of the Bridge

• All members of the super structure comprise built up riveted sections with a combination of high tensile and mild steel

• Between towers bridge deck hangs from panel points in the lower chord of the main trusses with a series of hungers (39 pairs)

• Road way beyond the tower is supported on ground leaving anchor arm free from deck loads

• Bridge deck comprises 71 ft carriage way and 15 ft footway projecting either side of the trusses and braced by a longitudinal fascia girder.

More about the Bridge

• The deck system consists of cross girders hung between pairs of hungers with pinned connection.

• Six rows of longitudinal stringer girders span between cross girder.

• Floor joists supported transversely on top of stringers.

• They support a continuous pressed steel troughing system.

• Over which deck concrete is laid out.


Joint System of Bridge (Expansion Joints)

• Longitudinal expansion and lateral sway movement of the deck are taken care of by expansion and articulation joints.

• There are two main expansion joints, one at each interface between the suspended span and the cantilever arms.

• There are expansion joints at the towers and at the interface of steel and concrete structures at both approach.

Articulation Joints

• There are total 8 articulation joints.• 3 at each of the cantilever arms.• 2 in the suspended portions.• They divide the bridge into segments with vertical pin

connection between them to facilitate rotational movements of the deck.


Camber and Traffic clearance

• Bridge deck has longitudinal ruling gradient of 1 in 40 from either end• They are joined by a vertical curve of radius 4000 ft.• Cross gradient of deck is 1 in 48 between kerbs and central 4.9mtr.

is level to provide tramway housing channel in between troughing.

Foundation

• The main tower is founded with single monoliths which are 55.31 x 24.8m in plan with 21 chambers

• Monoliths at Calcutta and Howrah side are founded 31.41 m and 26.53 m in below ground level respectively.

• Minimum headroom in carriageway is 5.8 m• Freeboard for river traffic is 8.8 m

Documents

Howrah Bridge Study