SCI precast prestressed plank units are the simplest of all of the basic
bridge construction elements. They find widespread use in short to medium span bridges and it is noteworthy that some plank bridges
are achieving spans in excess of 20m spans.
For the smaller spans up to 12m the planks are solid. Above this span
the planks are voided with circular voids to reduce self weight and increase structural efficiency.
The planks may be either composite or non-composite with their deck
wearing surface depending upon the design of the bridge.
Seven standard depths are readily available ranging from 240mm to
700mm. Current design philosophy employs a mix of prestressed and
non tensioned reinforcement, although the most economical designs use
prestressing strands only. These are placed horizontally on a 50mm x
50mm grid, 25mm each side of plank centreline and debonded locally as
required at each end to control concrete stresses to within acceptable
Two strands are always placed near the top of the section to support the
stirrups within the planks and to assist in control of concrete stresses at time of
transfer of prestress. Concrete of grade S50 at 28 days and strength at
transfer of prestress 35 Mpa is commonly specified.
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