Cable-supported decks - The Design of Prestressed Concrete Bridges - page 560

Figure 18.29 (b). If the deck is stiff it will be subjected to lateral bending moments
and shearing forces. If the deck is infi nitely fl exible, it will defl ect sideways until the
horizontal sag in the cables has been adjusted so that once again the horizontal forces
applied to the deck are in equilibrium. As this will increase the force in the less-loaded
cable, it must reduce its vertical sag to maintain equilibrium, Figure 18.29 (c). The
resulting overall movement of the deck is a combination of a vertical defl ection, a
lateral defl ection and a rotation about its longitudinal axis. Although this behaviour is
non-linear, the horizontal and vertical defl ections of the cables may easily be calculated
by hand using the techniques described in 18.5.2 above.
18.7.3 Dynamic performance
It should be noted that very shallow suspension bridges are extremely fl exible, with
defl ections under full pedestrian loading that may attain span/150. It is likely that the
sizing of the cables will be governed more by the need to control this defl ection than to
limit their tensile stresses. The dynamic behaviour of such fl exible decks must clearly
be investigated. It is well known that random pedestrian footfalls can cause vertical
oscillations in a fl exible deck. It follows that in bridges where eccentric vertical loads
cause lateral defl ections, eccentric vertical pedestrian footfalls are likely also to cause
lateral oscillations. Examples of such decks are those carried by tall, fl exible central
columns or decks carried by a single suspension cable, Figure 18.30, as well as decks
carried by cables curved in the horizontal plane as described above. Some studies
have also suggested that the small horizontal component of footfalls may cause lateral
oscillations in particularly susceptible bridges even when they are not predisposed to
such movements by their geometry.
The bridge may also suffer from wind-induced oscillations. In a shallow suspension
bridge where the stiffness of the cables may be large compared with that of the
deck, or where the deck has no stiffness, it is unlikely that the classical theory of
suspension bridge stability remains applicable. The wind tunnel testing of these bridges
is essential.
In order to control oscillations due to pedestrian or wind excitation, it may be
necessary to equip the bridge with dampers ab initio , or to make provision for
retrofi tting dampers in marginal cases.
Figure 18.30 Examples of structures which defl ect sideways under eccentric loads