Civil Engineering Reference
In-Depth Information
that the cantilevers were drooping below the predicted alignment, possibly due to a
mathematical error. It was calculated that if all the epoxy resin joints were 0.5 mm
thicker at the intrados than at the extrados the droop would be cured. Experiments
were carried out on fi llers to the epoxy resin that increased its viscosity by the requisite
amount. This modifi ed resin was applied over the lower half of the segments, curing
the droop.
It is also possible to pack part of the joint with layers of fi breglass to achieve an
increase in thickness from the normal 0.5 mm to a maximum of 3-4 mm. The fi breglass
layers are tapered out to avoid sudden steps in the resin thickness, Figure 14.20.
Although feasible, this method seriously slows erection, and greatly increases the risks
of voids in the resin, allowing grout transmission from one duct to another as the
tendons are pressure grouted.
Varying the joint thickness has been managed by using shims sized to transmit an
acceptable stress to the concrete during erection. These shims may be made of hardened
resin, which will not affect the fi nal distribution of forces in the section. However, such
shims stop the fl uid resin from being compressed by the temporary prestress, which
may compromise good adhesion to the concrete. The author has no experience of this
method of steering, but correctly designed it may be useful. Shims made of steel must
not be used as they have a much higher stiffness than resin, and in service the prestress
compression in the deck will transfer to the shims, causing local high compressive
stresses with risks of spalling or splitting of the concrete.
Figure 14.20
Byker Viaduct: preparing fi breglass steering packs (Photo: Robert Benaim)
