Civil Engineering Reference
In-Depth Information
of the main span. At this span ratio, the sagging bending moments due to live loads
are about the same at the centre of the main span, and close to the end of the side
span. However, as the side span is shorter than half the main span, the two or three
uppermost stays coalesce to form a concentrated back stay, requiring a substantial
counterweight or tension pile capacity to resist the uplift. If the side span is further
shortened, this holding down force will increase. If the side span is lengthened, the live
load bending moment may exceed that in the main span.
However, the aim of this topic is to give engineers the tools necessary to question
conventional wisdom. Consequently it is suggested that these fi gures are considered
to be a starting point, but that they should not be taken as gospel. There exists in
fact a continuum between extradosed bridges with low towers and stiff decks, and
conventionally proportioned cable-stayed bridges.
Clearly, the ratio of main to side span is only valid if the side spans are fulfi lling
a useful function in extending the length of the bridge. At the extreme, when only a
single long span is required, an arch bridge may be a better option. However, there
are intermediate situations when it may be appropriate to shorten the side spans and
provide the necessary tie downs, as in the Barrios de Luna Bridge, Figure 18.5
,
which
had a 440 m main span and 102 m side spans.
Some cable-stayed bridges have intermediate supports in their side spans. These
provide no benefi t to the bridge in its completed state (except possibly when the cables
are in the harp arrangement,
18.4.4
); on the contrary, they are expensive particularly
as the deck has to be tied down to them. They can be useful during the construction
of the bridge, particularly if the side spans are launched into place, but their cost must
then be justifi ed as temporary works.
Figure 18.5
Barrios de Luna Bridge (Adapted from image in Walther, R., Houriet, B., Isler, M.
and Moïa, P.,
Cable Stayed Bridges
, p. 47)
