🕑 Reading time: 1 minute
Howrah Bridge has recently completed 75 years of service for the twin Indian cities of Kolkata and Howrah and is considered as an iconic landmark of the city. It remains in a good shape as a structure and is likely to serve the people of Kolkata for many more years to come.
Kolkata has a humid weather and a polluted environment. Other steel structures in the region have been susceptible to deterioration due to corrosion. But Howrah Bridge has needed very little repair and retrofitting and remained in sound structural condition. Undoubtedly, this is attributed to the care and quality control exercised during its construction and the systematic maintenance protocol that has continued over the years.
Howrah Bridge is a balanced cantilever suspension bridge and has held the fancy of the people ever since its construction. Kolkata was built on the river bank away from the mainland. Therefore, the city had to depend on the main railway station at Howrah to enter the city. However, after the construction of the Howrah Bridge, it has become the main link connecting the city to the mainland.
Howrah Bridge has a long span of 457 m and was the third-longest cantilever suspension bridge in the world till the year 1941. Currently, it is the longest cantilever bridge in India and the sixth-longest cantilever bridge in the world.
1. Selection of the Site
The site of the Howrah Bridge was selected in such a way that the existing pontoon bridge could continue to cater to the traffic movement without any hindrance. Due to the presence of some religious structures immediately upstream of the existing pontoon bridge, the site of the Howrah Bridge was selected so that the centreline of the bridge was 500 m away from the structures. The alignment was provided at a right angle to the flow.
The location of the bridge ensured that the main tower foundation kept clear of the waterway. Thus, the possibilities of any scour around the foundation was avoided.
2. Planning Phase of Howrah Bridge
In 1922, a new Howrah Bridge commission was set up and a committee was formed for the planning phase of the project. The committee examined the different types of bridge designs:
- Single-span arch bridge: It was rejected on account of a high lateral thrust.
- Suspension bridge: It was rejected on account of longitudinal pull on anchorages.
- Pier and girder bridge: It was the most economical but was rejected due to likely change in the bathymetry of the river and the obstruction to river traffic.
- Floating bridge: It was rejected on account of its temporary nature and potential of collision with the river traffic.
- Balanced cantilever bridge: It was finally adopted because it allowed the construction of the main girder clear of the river, which carried huge commercial traffic. By cantilevering the span from the anchored span, river traffic was never affected. This option also ensured that the foundations were mainly subject to vertical loads.
3. Materials Used for the Construction of Howrah Bridge
In order to reduce the tonnage of the steelworks, certain specified steel members were used. Table-1 illustrates the yield strength and ultimate strength of steel used in construction of the Howrah Bridge. These members were selected so that the strength and ductility requirements can be addressed during the construction and service stage.
|Steel items||Yield strength (MPa)||Ultimate strength (MPa)|
|1. Plate and sections||355||664|
|2. Plain manganese steel||380||580|
|3. Manganese chromium steel (Imported)||375||575|
|4. Manganese chromium steel (India)||395||614|
A total of 26,500 tons of steel was used in the construction of the bridge. Out of this, 23,000 tons of steel was supplied by Tata Steel at Jamshedpur, India. With the use of higher percentages of copper and chromium, corrosion resistance was imparted into the elements of the structure. Due to higher corrosion resistance, the condition of the bridge is still intact in the area despite a very high degree of humidity and ambient temperature exceeding 40°C.
4. Geology of the Site
A detailed description of the soil strata below the bed at the foundation locations was prepared by boring the soil at the location of the foundation. It showed that both the banks of the river have a clayey layer with stiff clay at about 25 to 30 m below ground level. The location of stiff clay is comparatively at a lower level at the Kolkata side. The Kolkata side also has a sandy silt layer between soft clay and stiff clay, which caused difficulty during construction.
5. Structural Details of Howrah Bridge
Loading standards had to be developed because during the construction of the bridge, there was no standard code available for the design of bridges. The loading standards were developed in such a way that it could cover the nature of traffic under use with scope for further development in future.
High-strength steel was selected for reducing the weight of steel work. With the adoption of a cantilever bridge, unhindered navigation provision was attained. The original concept was to make the central suspended span as hog backed. However, it was changed to parallel chord members in the final design due to aesthetic considerations. Lots of emphasis was given at the design stage to ensure that the structure would remain as an icon for a long time to come.
5.1 Details of Superstructure
The Howrah Bridge is a classic example of a balanced cantilever bridge. The central span of the bridge is 457 m along with a suspended span of 171 m, and cantilever arms of 142 m. The length of the anchor spans is 99 m. The truss formation for the cantilever arm and anchorage arm is a K-type, and that for the suspended span is an N-type. Adoption of a K-type truss helped in the reduction of the slenderness effect of the large vertical loads.
The width of the deck is 21 m, including a footpath of 4.5 m on either side. The carriageway is equivalent to six lanes as per the current code Indian Road Congress (IRC: 6-2017) although, it was planned way ahead of publication of IRC codes. The cross-girders are supported on the hangers from the main structure of the bridge, and stringers span between the cross-girders. A headroom of 5.8 m was provided for the vehicular movement.
The deck system consists of a grillage of cross-girders and stringers. The deck has eight articulation joints with a slab seal expansion joint. The two main expansion joints were provided with a double slab seal type joint at the junction of the suspended span and cantilever arm.
Two vertical pins on the central axis of the bridge were provided at each main tower to allow rotation during erection. A sliding friction type expansion provision in the longitudinal direction had been made at the junction of the cantilever arm and suspended span and at the main piers. There are total 26 pin-joints, which ensure the behavior as a perfect truss. The link member between the cantilever arm and suspended arm is pinned at both the ends and the vertical member. Thus, the rotation during expansion was achieved.
5.2 Prestressing of Steel Work
The members of trusses were fabricated to such lengths that under dead load and live load, the trusses would resemble their correct geometric shape and all members would be straight. Therefore, during erection, individual members were prestressed into temporarily strained or bent position.
5.3 Towers and Main Bearing
The two main towers each consist of two posts with a system of K-bracing between them and a portal opening to accommodate the carriageway. The saddles are located at the top of the posts, with pin connections for the upper chord and diagonals for the main trusses.
The main bearing at the towers were inserted between the lower chords of the anchor and cantilever arms. These bearings were supported on the pedestal extensions projected out from the main posts.
The posts are rectangular in cross-section and consist of three main vertical webs with transverse vertical diaphragms dividing them into eight compartments.
The lower lateral bracing members are attached by large gussets to the main bearings. The transverse members between each pair of bearings transmit the lateral and longitudinal forces to the main towers.
5.4 Wind Frames
The wind frames at anchorages were provided on the central axis of the bridge. Frames were designed to take the lateral wind reaction of the superstructure without interfering with the longitudinal and vertical movements resulting from changes in the length of the superstructure. Each wind frame consists of a pair of braced A-frames.
6. Foundation Details of Howrah Bridge
During construction of Howrah bridge, the standard penetration test was not developed, which is the current practice used to understand the strength of soil. Therefore, in order to understand the strength of the soil, a preliminary test of the bearing capacity of the clay was carried out using cast-iron cylinder.
The cast-iron cylinder with 32 mm thick metal and 2 m internal diameter was used. At the Howrah side, it was loaded and forced down up to 26.5 m below ground level, where stiff clay was encountered. The shaft of cylinder was then dewatered, and the clay was inspected in its natural state.
The cylinder was then filled with concrete and loaded to 212 t/m2 without any movement. The load was increased to 233 t/m2, and the movement was observed. The Kolkata side cylinder was sunk up to 31.6 m, where stiff clay was encountered, and loaded to 183 t/m2 without any sign of movement. The load was increased to 269 t/m2 when movement was observed. This testing of load gave an idea about the strength of the clay strata.
6.1 Pier Foundation
The total structural steel used in the superstructure is 26,500 tons. The other self-weight of the superstructure comes from the reinforced cement concrete (RCC) deck of the roadway span, which has a total thickness of 130 mm, including the wearing course.
The analysis showed that the maximum load that may come on the foundation from the superstructure is 30,000 tons downward compression at the main pier locations and 13,500 tons upward tension at the anchorage points. The two piers and two anchorages at each end will take half of the above load each.
To resist the maximum load, 24 m × 55 m monolith with internal connecting crisscross walls was constructed. These walls went down to 26.5 m at the Howrah side and 31.5 m at the Kolkata side, penetrating into a stiff clay layer.
6.2 Anchorage Block
The governing conditions for the anchorages differed from those for the main piers. The maximum load on the foundations occurred during the construction period. Thereafter, the downward load was reduced by the uplift of the superstructure.
The specifications required that the dead weight of each anchorage should exceed the maximum possible uplift by not less than 50%. Since the uplift from the superstructure was located at two points at a distance of 23.2 m apart underneath the main trusses and the foundations had necessarily to be taken deep to reach the clay, a single large monolith was neither necessary nor economical.
Therefore, it was decided that each anchorage should consist of two separately sunk monoliths, each 8 m x 16 m, with two dredging shafts, which would afterwards be tied together by means of a tie beam.
The anchorage monoliths were similar to the main monoliths and were of RCC construction with a structural steel curb.
The Howrah Bridge is the longest cantilever bridge of India, located in Kolkata city, West Bengal. It is also the sixth longest cantilever bridge in the world.
It was constructed to connect two major Indian cities of West Bengal i.e. Kolkata and Howrah city.
The Howrah bridge has a 457 m long span.
It was constructed in 1935.
It is a balanced cantilever suspension bridge.
The height of the Howrah bridge is 8.8 m.