The Design of Modern Steel Bridges - TEDI

wind loading on the traffic. An uplift force of 1 kN/m 2 on the loaded bridge on
the total plan area is considered for overturning effects.
In the British Standard BS 5400[2], isotachs of 120 year return mean hourly
wind speeds in the British Isles at 10 m height in open country are provided.
This hourly wind speed at the particular site is then adjusted to obtain the
maximum gust speed Vc, allowing for:
(1) the height of the bridge above ground level
(2) gusting effect over a 3 s period
(3) non-coherence of gusting over the length of the bridge
(4) possible local funnelling or acceleration of wind at bridge site.
When a live load is present on the bridge, Vc is limited to 35 m/s. A factor of
1.1 is taken for sites susceptible to (4) above. The other three parameters are
dealt with by a factor S 2 given by[11]
where
S2 ¼ Z
10
0:17
" # 1=2
1 þ I 2 þ 2 ffiffiffi p
2 pI
Z ¼ height in metres above ground level
L ¼ horizontal length subjected to wind
I ¼ intensity of turbulence
¼ 0:18½1 0:00109ðZ 10ÞŠð10=ZÞ 0:17
l ¼ crosswind width of gust
¼ 50 m for Z > 50 m
¼ 0.375Z þ 31.25, for Z 4 50 m
p ¼ peak force factor
¼½2:56ð10=ZÞ 0:17
1 I2Š=2I: Transverse wind loading is then calculated as
l
L
1
2 rV2 c AeCD
Loads on Bridges 65
l 2
L 2 ð1 e L=l Þ
where r is the air density 1.226 kg/m 3 , V c is the gust speed in m/s, A e is the area
in elevation in m 2 , and C D is the drag coefficient.
In 1997 a new code for wind loading in the UK was published; to conform to
this code, it has been proposed[9] to change the evaluation of the maximum
gust speed V c on the bridge by multiplying the 50 year return mean hourly
wind speed at the location of the bridge given in a map of isotachs for the UK
by a series of factors listed below:
S b ¼ a bridge factor, dependent on the height of the bridge above ground and its
adversely loaded length
K F ¼ a fetch factor, dependent on the height of the bridge above ground and the
distance of the site from sea in the upwind direction
1=2