Proper Design For Bridge Superstructure

Bridge Superstructure

Various types of bridge superstructures are Arches, Masonry, C.C, R.C.C Girder and deck slab, Solid Slab, R.C.C T-Beam Slab, R.C.C Box Beam, Voided Slab, P.S.C Two Girder, Three Girder, Multi-Girder, Box Girder, Simply supported continuous Cantilever, Balance Cantilever, Hammer Head, Bow string girder, composite construction, cable stayed, suspension.

Selection of Proper Superstructure

Generally the following criteria should be followed for selection of superstructure depending on span length.

1. Spans upto 10m. R.C.C solid slab.

2. Spans- 10 to 15m R.C.C solid slab /Ribbed slab,

3. Spans – 20m R.C.C Multi-girder slab system.

4. Spans – 30m P.S.C Girder/Box type superstructure.

5. Span – 30m to 60m P.S.C Box girder.

For spans more than 60 m the discussions should be held with Superintending Engineer, Designs Circle regarding selection of the type of superstructure. 2-girder system for two-lane superstructure should not be proposed unless other alternatives are considered unfeasible.

For spans up to 10m solid slab superstructures are found most suitable. As the span increases beyond 10m the thickness of solid slab poses difficulties during concreting. Lot of construction joints are created in the structure if proper program of concreting is not prepared and insisted upon. It is, thus, desirable to go for ribbed slab or multi-girder system of deck slab.

Spans between 10 m to 15 m could be conveniently covered in this manner.

Spans between 15m to 20m, multi-girder system would be desirable. Two-girder system should be avoided as far as possible. In case of single lane bridges two-girder system is natural choice. But this system should not be preferred in severe exposure conditions. There is a school of thought that damage to one girder makes the entire structure unstable and unsafe and hence multi girder system is to be preferred.

For spans between 20m and 30m R.C.C box type superstructure is considered suitable. Use of R.C.C girder and slab system might result in excessive deflections under live load. Box girder is a more desirable shape for the superstructure.

Beyond 30 m span, it is necessary to go for P.S.C. This enables us to somewhat restrict the deck height to the desired level. For spans greater than 60m discussions should be held with Superintending Engineer, Designs Circle for deciding the type of bridge superstructure. The parameters influencing selection of superstructure need to be studied.