Bridges: Vincent Chu Answers
In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?
(i) 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.
(ii) 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.
(iii) For very long span bridge, temporary piers are provided to limit the cantilever moment.