Chapter 4: The Construction Process of Segmental Bridges
to characteristics and requirements, advantages, and disadvantages of each method to prepare for the case study that is presented in Chapter 5.
4.2.1 Cantilevering Method
Before used in construction of concrete bridges, the cantilevering method had already been used in Asia for wooden structures of earliest times, as Podolny and Muller (1982) report. Amongst the major steel structures that were erected with the cantilevering method are the Firth Rail Bridge and the Quebec Bridge that are presented in Section 2.1.5. Erection of concrete bridges with the cantilevering principle led to development of specialized sequences that are discussed further below.
As already introduced in Section 3.6.2, cantilevering for concrete segmental bridges is a construction method where segments, either precast or cast-in-place, are assembled and stressed together subsequently like a chain to form the self-supporting superstructure. Prestressing cables located in the upper part of the segment cross-section support the cantilever. In the variant of the progressive placement method stay cables are often used to support the cantilever prior to closure of the span.
Time-dependent material behavior of the segments under successive load steps requires comprehensive calculations for all construction stages. Every segment will develop strength with increasing age of the concrete. Governing for the structural behavior of the cantilever is that every segment carries and transfers loads from all following segments and construction loads until closure of the span. From these very basic facts in conjunction with geometry and expected loads on the structure the calculation of moments and local stresses, as well as calculation of the deflections that they cause is possible. Optimization of geometry, prestressing, and camber are then performed.
Depending on the specific segment configuration and erection sequence chosen for the cantilevering method the cantilever may never be exactly balanced so that the superstructure needs to be balanced to ensure stability. It is possible to fix the supports at the piers of