Civil Engineering Reference
In-Depth Information
these circumstances, either fi ctitious bending moments that give the appropriate stresses
on the extreme fi bres should be added to the table of moments, or the prestress should
be designed to leave residual compressive stresses on the extreme fi bres that are equal
to the tensile stresses due to temperature.
However, there are some circumstances in which these internally balanced tensile
stresses may act as a trigger that sets off cracking due to other, locked-in stresses. For
instance, the bottom fi bre of a continuous deck of short or medium span (less than
about 50 m) at locations either side of an internal pier is particularly susceptible to
cracking during construction, for the following reasons, Figure 6.14:
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Generally, this location is permanently compressed by hogging moments when
in service, and consequently may be virtually devoid of design longitudinal
reinforcing bars or of bonded prestressing cables.
The prestress centroid is generally above the upper limit of the central kern,
and thus there are tensile stresses due to prestress on the bottom fi bre during
construction. For bridges of modest span, before the prestress has suffered its
time-dependent losses and before the deck fi nishes have been added, the self
weight bending moment may not be enough to overcome these tensile stresses.
Immediately adjacent to the pier, the convex shape of the prestressing profi le fi ts
badly to the cusp-shaped bending moment. As design sections are generally located
at the support and at the fi rst tenth point of the span, it is not uncommon to fi nd
that between these two sections the prestress centroid is above the cable zone, and
that the tensile stresses on the bottom fi bre are signifi cantly higher than assumed.
Tensile stresses due to moments caused by differential settlement may well be
present at this location.
In cast-in-situ decks where the webs are thick compared with the slabs, heat of
hydration effects due to the webs cooling more slowly than the slabs are likely to
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Figure 6.14
Zone where bottom fi bre is susceptible to cracking
