Launching Systems for Segmental Bridges

Vinay Gupta, Principal Consultant, Tandon Consultants Pvt. Ltd. New Delhi.

Introduction

India has witnessed a large growth in the field of construction, with construction sector comprising 40% to 50% of India’s capital expenditure on the projects in various sectors. While the growth has been equally high in the field of real estate and infrastructure projects, a much larger growth has been witnessed in the field of segmental bridge construction. This is due to the distinct advantage of speed and aesthetics that this technology provides.

There are various types of segmental bridges, to name a few:

1. Precast segmental superstructure, simply supported or continuous, internally prestressed or Externally prestressed, epoxy jointed or dry jointed.
2. Balanced cantilever construction using cast-in-situ or precast segments
3. Spliced girder superstructure constructed using concrete stitch or epoxy joint, etc.

Until year 2005, Indian codes did not cover design and construction of segmental superstructures. Now, a new document IRC: SPs: 65-2005 prepared by the author under the aegis of the IRC code committee, has been published. This document covers design and construction aspects of segmental bridges.

In totality, there are many forms of segmental construction. However, the most popularly known form of segmental construction is precast segmental superstructure, constructed by span-by span method of construction. In this technique, precast box girder segments, aggregating a total length equal to approximately one span at a time are assembled and prestressed. Thereafter, segments of similar total length are assembled for each subsequent span. The assembled length can either be between two consecutive piers, each time or 1 ¼ span then ¾ span + ¼ span then ¾ span + ¼, and so on. At each stage, prestressing has to be carried out, in order to make the constructed structure self- supporting. It may be noted that precast segmental superstructure, so constructed, has to necessarily be prestressed, as untensioned reinforcement cannot continue through the joints of precast segments.

Assembling of precast segments is facilitated through either under slung assembly truss or over head assembly truss.