Civil Engineering Reference
In-Depth Information
Some types of structure are more expressive than others. A beam of constant depth
is singularly inexpressive of the fact that the greatest bending moment is at mid-span.
Various designers, including Morandi, have adopted 'fi sh belly' beams, which do
express the fact that they need greater strength at mid-span, and allow the removal
of redundant web material near the ends, Figure 1.6, despite the fact that they most
probably cost slightly more than a beam of constant depth; the additional complication
of the formwork and the reinforcement is likely to outweigh any savings in the volume
of concrete. The designer was justifi ed in attempting to make the structure more
expressive, and hence more interesting, although the fi nal judgement must be whether
it works aesthetically, as well as expressing engineering values.
An engineer may be justifi ed in choosing a type of bridge that is not the most
economical, but that is more expressive. Clearly, this needs the approbation of the
client and cannot be done as part of a competitive tender where no credit is given for
appearance. For instance, it may be established that the most economical bridge for
a site is a girder bridge on vertical supports. However, the site may be just right for
an arch, Figure 1.7, with the strong foundations required, although the arch would
be more expensive. Such an arch bridge design can only achieve distinction if it is
a rational choice and if the designer then strives for and achieves economy in the
design as described above. It could not be considered a good design if the designer had
imposed an arch on the site, despite its unsuitability; for instance, if the arch required
massive and expensive foundations to resist the thrust, even if these foundations could
not be seen by the public.
This demonstrates that good engineering design has an esoteric component only
to be appreciated by professionals. A good example is the series of valley piers of
the Byker Viaduct in Newcastle, which were fl ared to resist the wind and centrifugal
forces of the twin-track railway it carried. The central part of each pier was cut away
at ground level to save materials and to reduce the impact on walkers in the park.
The size of these cut-outs was made just large enough to allow the precast units of
the bridge deck to fi t through, in an economical and innovative construction method,
Figure 1.8. This fact is only known to those who remember the bridge being built,
but forms part of the intellectual justifi cation for the shape and size of the cut-out.
The fi nal judgement on a design must rest with the combined response of public and
profession.
The choice of a solution that is expressive of the fl ow of forces applies to the design
of members and details of a bridge as well as to the structure as a whole.
The detailed shape of the towers for the two-level Ah Kai Sha cable-stayed bridge
designed by Benaim to cross the Pearl River close to Guangzhou, further illustrates
the relationship between technical and aesthetic decisions, Figure 1.9. The bridge has
Figure 1.6
Fish belly beams: simply supported beams of Maracaibo Bridge
