19/09/2021

Bridge Engineering

Bridges:

Bridges form an important part of the transportation rules. They are constructed across the rivers to carry highways and railways. In a country roads and railways often follow the shortest route and therefore, their alignment is determined much before the construction of bridges. As a result, the site of a bridge does not allow much freedom of choice. Bridge engineering is a field of civil engineering related to the construction and designing of bridges.

List of Contents

Bridge terminology:

Abutment: The abutment is the terminal support of the bridge. It is built on either side of a river valley where the road or railway joins the bridge.
Piers: In a multi-span bridge pillars-like supports are built between the abutments. These supports are known as the ”piers”. The piers are built mostly of concrete, plain, or reinforced. They are commonly faced with dimension stones. The rock selected for this purpose must be able to withstand the constant impact of running water.
Multispan bridge: When a bridge is supported by abutments at both sides of a valley and there are no piers in between, it is called ”one span bridge”. While a bridge, which is supported by abutments at both the ends, as well supported by multiple piers/pillars in between the two abutments, is known as a ”multi-span bridge”. Multi-span bridges are built in the middle or lower reaches of rivers where the valleys are generally very wide.
Suspension bridges: The bridges which are suspended with the help of cables (wire/rope) and steel towers across a deep and narrow river valley, is called a ”suspension bridge”. In a suspension bridge, the ends of the babel are firmly anchored in the hard rock or massive concrete wall on either side of the valley. The suspension bridges are usually built on the mountain rivers because: (i) their valleys being narrow and deep, the height of the piers would be out of proportion as compared to the length of the bridge, and (ii) the scouring of the pier foundation in the narrow stream channel would be very rapid.

Stability of bridges:

The most important element of a bridge is its stability. Civil engineers primarily need to focus on the stability and durability of the bridges. There are several factors, which ensure the stability of a bridge, among which chief factors are as follows: (i) Lateral Forces, (ii) Earthquake forces, and (iii) Scouring action of rivers.

Lateral Forces: The pressure of wind and the running water is the main lateral force, which operates on bridges. These forces have a tendency to push the structure on the lee side, where the overloading of the foundation is caused. If the foundation of piers is not keyed sufficiently deep, the stability of the bridge may become critical. But in modern bridge construction where the foundation of piers are generally kept deep, the lateral forces are of little importance.
Earthquake Forces: The earthquake, undoubtedly, a key factor influencing the stability of the bridges. The acceleration produced by earthquakes depends on the intensity of shock and on the nature of the ground. The chances of damage are maximum in those bridges which are situated close to an active fault or founded on loose earth materials. Therefore while constructing bridges in earthquake-prone regions, suitable precautionary measures should be taken.
Scouring Action of Rivers: The scouring action of a river is very nearly proportional to the square of its velocity. However, this is complicated by various factors such as roughness of the river bed, uniformity of the channel, nature of rocks in the river bed, and the amount of suspended matter in the flowing water. Where the river flows straight, the maximum velocity is found halfway between the banks but at bends, the water currents of maximum velocity lie near the concave bank.

The rivers whose channels are composed of alluvial material may give a false impression of the level of the river bed during low water. During flood when the velocity of the flowing water is increased, the river bed is vigorously scoured and deepened, and when the flood recedes, it is filled up again to the level when the river was low. The piers should be founded at such a depth where they remain safe from erosion during high water periods.

The piers of the bridge constructed in a river channel, obstruct a part o the channel. As a result, there will be an increase in the velocity around the piers, which causes enhanced scouring.

Foundation of Bridges:

The deadweight of the bridge, the load of the traffic, and the pressures of the wind and the flowing water are ultimately transmitted to the foundation of the piers and abutments. Therefore, the design and construction of bridges are governed largely by (i) nature of rocks, (ii) structure of rocks, (iii) faults, and (iv) type of river channel.

(i). Nature of Rocks: The rocks over which the piers and abutments are to be founded must be strong and durable. They should be free from closely spaced joints, fissures, shear zones, solution channels, and other zones of weakness. Poorly cemented, thinly bedded, and soft sedimentary rocks should be avoided. The rocks having joints and fractures may be consolidated by grouting.

For placing the abutments of a bridge, the valley walls are thoroughly examined. The valley walls where the strata dip into the river channel form unstable slopes because they have the tendency to slide into the river channel. Such unstable valley slopes should be avoided.

(ii). Structure of Rocks: If a bridge is aligned across the strike of the country rocks, different types of rock beds having varying strength and composition are met with, along the foundation. In such cases, a close examination of the foundation rock under each pier and abutment should be done by putting boreholes. If thinly bedded soft rocks such as shales are exposed in the river bed, the water currents would easily cut deep grooves parallel to the bedding and hence would undermine the foundation of piers.

(iii) Faults: A fault, if it is running across the bridge alignment, is a source of many troubles. The highly crushed and weathered rocks, which exist in the fault zones, make the foundation treatment extremely expensive. It is, therefore advised that the possibility of avoiding the fault by shifting the bridge alignment upstream or downstream may seriously be considered.

(iv). Type of River Channel: In alluvial channels, the thickness of loose sands and gravels may be so great that it is not economical to reach the bedrock for placing the piers. In such cases, pile foundation is used. The piles are generally given through the alluvial material to the bedrock. Friction piles are used where the bedrock is not available up to a great depth.

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Writer at Geo Studies Organization; https://geostudies.org