A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
7. In incremental launching method of bridge construction, what are
the measures adopted to enhance sufficient resistance of the
superstructure during the launching process? (BC2)
(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.
8. Should special design be catered for in bridge piers upon jacking
up of superstructure for installation of bearings in incremental
launching method? (BC2)
After the completion of launching process, the superstructure has to be
lifted up to allow for installation of bearings. This is usually achieved by
means of jacks to raise 5-10mm successively at each pier. In fact, it is
anticipated that no special design is necessary for this operation because
the effect of differential settlements at support should already be checked
in bridge design. Level readings should be checked to ensure that it does
not deviate from the designed figure.
9. Can the use of temporary nose in incremental launching method
reduce the cantilever moment of superstructure to the value of inner
support moment? (BC2)
When the superstructure is pushed forward, a temporary nose is usually
adopted at the front end of the superstructure to reduce the cantilever
moment for which the central prestress is designed. The length of
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