C. J. Burgoyne
Dept of Civil Engineering, Imperial College of Science
and Technology, London, UK
The design of continuous
prestressed concrete bridges gives the engineer considerable freedom when
compared with the design of statically determine structures. By varying the
secondary (or parasitic) moments, the relative magnitudes of the bending
moments at mid-span and pier positions can be varied, thus allowing structures
to be designed in which the line of thrust of the cable is outside the section,
while the cable itself lies safely within the section.
The penalty to be paid for this
freedom is greater complexity in design. It is necessary to determine the line
of thrust of the actual cable that is used, or to choose a cable profile that
is concordant. Either way, a knowledge and thorough understanding of the
secondary moments in the structure are required.
Virtually all the standard texts on
prestressed concrete design dwell on the problems associated with the
calculation of the line of thrust and secondary moments associated with a given
cable profile, but do not consider in detail the wider implications for the
designer. However, one Author has considered other aspects of the problem. Low
determined limits on the cable forces for the internal spans of a multi-span
structure, based on considerations of potential crack patterns.
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