Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 351

behavior. Damping devices in the main span are also helpful to improve
the aerodynamic response. From a structural design point of view, separat-
ing the girder's torsional mode away from its first bending mode as far as
possible can be considered the next method in terms of increasing the criti-
cal flutter wind speed. When wind attacks from the sides of a bridge, the
coupling of bending and torsional forces of the girder is a key factor that
leads to the collapse of the bridge. During the preliminary design, several
different formulas can be used to estimate the critical flutter wind speed for
girders that have slablike cross sections as suggested by the Wind-Resistant
Design Specification for Highway Bridges Ministry of Transport of China,
2004, which became mandatory to comply with in China since 2005. For
example, the Van der Put formula (Equation 11.11) considers the ratio of
the first torsion frequency to the first bending frequency.

r
b

   

(11.11)

V 1 ( 0 5 . )  0 72 . b

= + ε − µ ω

cr b

where:
V cr is the critical flutter wind speed, in m/s
ε = f t b is the ratio of the first torsion frequency to the first bending
frequency
f t is the first torsional frequency, in Hz
f b is the first bending frequency, in Hz
r is the ( I m , the mass radius of gyration

I m is the mass inertia per unit length of the girder, in kg  m /m

)

2

m is the mass per unit length of the girder, in kg/m
b is the half width of the deck, in m
μ is ( πρ 2 , the ratio of mass to air density

ρ is the air density, in kg/m 3
ω b is 2π f b , the angular frequency of the first bending

m b )

Equation 11.11 is further simplified by Tongji University as

r
b

µ   

(11.12)

V = 2 5 . 2 b f

cr t

For very long-span cable-stayed bridges, girders and pylons are usually
tied down by anchor cables. This is to increase the wind stability during
construction.
Oscillation of cable stays due to wind and rain in long-span cable-
stayed bridges could be significant and should be considered. Crossties
of cable stays as shown in Figure 11.7 and a stay damping system as
shown in Figure 11.22 are commonly used measures to counter cable
oscillations.