Civil Engineering Reference
In-Depth Information
to be below zero. Thus one may fi nd the bottom of the cable zone by plotting a point
that is 3.56 / 9.6 = 0.371 m down from the bottom of the kern. As
a
b
= 0.867 the
bottom of the kern is at an eccentricity of 0.867 + 0.371 = 1.238 m, Figure 5.7 (b).
The cable zone at the quarter point may thus be defi ned as lying between eccentricities
of 0.896 m and 1.238 m.
The cable zone may be plotted in this way at any number of points along the beam,
Figure 5.8. For a simply supported beam, few points are necessary. However, for a
continuous beam, it is normally necessary to plot the cable zone at every tenth point
of each span at least.
5.15 The technology of prestressing
In order to understand the terms used in the following sections, it is necessary to
describe briefl y the technology of prestressing. This subject is covered extensively by
the brochures of the manufacturers of prestressing equipment; in the UK these are
principally CCL, VSL and Freyssinet for cables made of strands, BBR for cables made
of strands or wires, and Macalloy and Dywidag for prestressed bars. However, many
other suppliers exist worldwide. Consequently, only a brief outline of the technology
will be given here.
Strand consists of sub-cables made up of wires that are twisted together. Usually,
each strand consists of seven wires, as may be seen in Figure 5.13 (a). Most commonly,
strand comes in 13 mm nominal size (12.5 mm or 12.9 mm actual size) made up of
wires that are approximately 4 mm in diameter, or 15 mm nominal size (15.2 mm or
15.7 mm actual size) made up of wires that are approximately 5 mm in diameter. Wire
Figure 5.9a
Typical prestress anchors: CCL slab anchor for 6 strands (Image: CCL Stressing
Systems Ltd)
