Civil Engineering Reference
In-Depth Information
Figure 18.11 continued…
Figure 18.11
continued
may take precedence over weight. However, in a long-span cable-stayed bridge the
cost of the stays, the towers and the foundations are all closely related to self weight
and the pendulum swings back towards minimising the weight of the deck. It is very
surprising to see the number of projects where this has not been understood, with
long-span cable-stayed bridges equipped with crude, simplistic and heavy concrete
decks.
The deck design is very dependent on the live loading code. For the example
illustrated in Figures 18.10 and 18.11, subjected to the British HB + HA loading, the
critical parameter is the variation of moment in the main span,
∆
M
. The variation of
stress on the bottom fi bre of the deck is
M
/
z
b
, where
z
b
is the bottom fi bre section
modulus (
5.3
). In a deck that is fully prestressed to Class 1 or Class 2, it is this variation
of stress on the bottom fi bre that will defi ne the type of deck.
The characteristic of acceptable decks is that the
z
b
must be high compared with the
moment of inertia in order to control the bottom fi bre stresses. This requires adequate
bottom fi bre material, which means that economical beam and slab type decks are
ruled out and one is led to box sections.
This conclusion would be different for live loads that are lighter than the British
loadings, and which are not dominated by a single heavy vehicle, as such loading
gives rise to a smaller
∆
M
. For such decks an acceptable
z
b
may be achieved with
a wider variety of deck types. If partial prestressing is acceptable, the variation of
∆
