In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

Incrementally launched bridge construction Itz Valley Bridge near Coburg

In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

Introduction

The incremental launching method is one of the highly mechanised erection methods used in bridge construction. The method consists of manufacturing the superstructure of a bridge by sections in a prefabrication area behind one of the abutments; each new unit is concreted directly against the preceding one and after it has hardened the resultant structure is moved forward by the length of one unit. This principle has already been used for many years in the construction of steel bridges. This is hardly surprising, in view of the equal strength of steel in tension and compression since, provided the design is suitable, the alternating stresses which occur when the bridge is slid forwards can be accepted without difficulty. This is not so with concrete, which can withstand only small tensile stresses without damage. Special measures are therefore necessary to enable concrete bridges to be slid forward by steps; the skilful use of prestressing is the most important of these measures.

Incrementally launched bridge construction Itz Valley Bridge near Coburg

Incrementally launched bridge construction Itz Valley Bridge near Coburg

In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

  1. During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.6-0.65 times span length is provided at the leading edge to reduce the cantilever moment. Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose.
  2. The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central prestress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively. Later when the whole superstructure is completely launched, continuity prestressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central prestress.
  3. For very long span bridge, temporary piers are provided to limit the cantilever moment.