In very simple terms, integral bridges may be defined as the bridges without any joints. Precisely, an integral bridge is the type of bridge that does not have any joint between spans or between spans and abutments. On account of this, the integral bridge does not essentially consist of an expansion joint to accommodate the enlargements due to temperature increase.
An example of an integral bridge is the typical masonry arch bridge. Since such a bridge does not have any joint, the deflections or movement due to subsequent expansion or contraction as well as braking load is accounted by the end walls of the abutment. The history of integral bridges dates back to 1959 in the United Kingdom.
IMPORTANCE OF INTEGRAL BRIDGES
Some of the main importance of integral bridges can be listed as follows:
1. Integral bridges greatly reduce maintenance needs than a conventional bridge.
2. Such bridges offer greater durability than other jointed bridges.
3. Integral bridges can serve as an economical highway bridge in the long run.
TYPES OF INTEGRAL BRIDGES
Based on the type of abutments, integral bridges can be categorized into the following:
i. Integral Bridge with Frame Abutment
Integral bridges with frame abutments are similar to a portal frame structure. It may be defined as the type of integral bridge in which the axial forces, shear forces and moments are transferred directly from the deck of the bridge to the supporting structure. The backfill is also retained by the frames of abutment similar to the retaining wall. In such bridges, embedded wall footing and spread footing is preferred as the foundation.
During subsequent thermal expansion or contraction, the beams on the deck react. The reaction results in a horizontal displacement. Thus, the abutments must be constructed with some flexibility to prevent the displacement on foundations. Due to this reason, stiff reinforced concrete abutment must not be used. Instead, sheet piles may be used as a foundation.
ii. Integral Bridge with Bank Pad Abutments
Integral bridges with bank pad abutment are similar to the stiff portal type construction. Such type of bridges consists of the end supports that are fully integrated with the deck beams. The supports are flexible to rotate and slide. During thermal expansion or contraction, these supports can freely slide and can freely rotate due to the bending moment from the deck beams. Such type of movements i.e. sliding and rotation can considerably affect the bearing capacity of the soil underneath.
Thus, to account for this, the serviceability limit states are kept lower than the normal static values. The backfill is filled behind the end screen wall formed by the bank pad abutment. It must be noted that the width of the end screen wall must be kept the same as that of the pavement constructed above it.
iii. Integral Bridge with Flexible Support Abutments
Integral bridge with flexible support abutments is constructed with piles as the foundation. Generally, post holes are created around the pile and up to the depth of the pile. The reason for creating the hole is to allow the space for the movement of the pile due to thermal expansion or contraction.
Hence, the hole must provide sufficient space for the horizontal pile movement eliminating the soil-foundation interaction. Such a hole is commonly constructed using precast concrete rings for larger size pipes and polythene for smaller size pipes. The pipes must be essentially checked for buckling from time to time. For this purpose, a duct for inspection is created in the hole.
iv. Integral Bridge with Semi-Integral End Screen Abutments
It consists of deck beams and the end screen walls integral with each other. However, the end screen walls, in this case, do not provide support to the deck beams. An arrangement of bearings is provided to resist the horizontal displacement. In such bridge, the support is constructed separately eliminating soil-
ADVANTAGES OF INTEGRAL BRIDGES
Some of the advantages of integral bridges can be listed as follows:
1. Integral bridges are cheaper in comparison to the conventional bridges. The maintenance cost is also relatively less.
2. The construction process of such bridges is simple as well as rapid.
3. The absence of expansion joints makes the surface smoother enhancing the riding quality.
4. When integral bridges are constructed in place of an old bridge, the same foundation may be used. Thus, the cost of construction can be greatly reduced.
5. The drainage system may be easily provided behind the integral abutments thereby preventing leakage.
DISADVANTAGES OF INTEGRAL BRIDGE
Some of the disadvantages of integral bridges can be listed as follows:
1. It is not suitable in areas with temperature fluctuations or variations greater than 51mm.
2. When the pile foundation is used, there is a high possibility of formation of plastic hinges under high stresses of expansion and contraction.
3. Such bridges are not suitable in regions where the subsoil is weak.
4. The span of integral bridges is limited. In case of steel girder integral bridges, the length is limited to 40m and up to 50m for concrete girder bridges.