Civil Engineering Reference
In-Depth Information
Support for the deck, in the absence of cross-beams, is provided by powerful
concrete brackets which are attached by prestress after slip-forming the columns.
All bridges expand and contract with changing temperature and either the deck
has to be separated from the piers by movable bearings, or the piers have to be made
fl exible enough to permit this movement. Here, the deck is fi xed to the piers, which
have been made fl exible by splitting them into two leaves, Figure 18.18.
Each leaf of the tower has a dumbbell shape, which represents the most effi cient
use of material. The dumbbells become solid at the junction with the deck to resist the
concentration of forces that occur in that zone, Figure 18.19.
The towers are situated outside the deck and, as a result, the stay cables all pull
slightly inwards. The combined effect of these pulls is very signifi cant, and the towers
would require larger, more expensive columns if they were not propped apart at the
top. The prop is designed to be made on the deck and winched up into place, and so
has to be as light as possible. For this reason it is made in an 'I' section. It must be stiff
enough to resist buckling under compression and bending under its own weight as
it spans between the columns, but the connections of the prop with the towers must
be suffi ciently small so that they do not attract bending moments under the effect of
lateral loads. The prop is designed with the depth increasing towards mid-span, its
shape expressing these constraints and actions.
Every signifi cant dimension of these towers had a technical, rational justifi cation,
and none were chosen for appearance alone. However, the appearance of the towers
was present in the mind of the designer at all times, and was chosen to express the
forces and actions acting on them.
The splendid Alex Fraser Bridge in Canada, designed by Buckland and Taylor,
Figure 1.10, solved some of the same problems differently. The bridge is narrower
Figure 1.10
Alex Fraser Bridge towers (Photo: Henning J. Woolf/Buckland & Taylor)
