The Design of Modern Steel Bridges - TEDI

where r is the air density ¼ 1.226 kg/m 3 , V c is the gust speed in m/s, A p is the
plan area in m 2 , and C L is the lift coefficient.
For bridges with deck superelevation up to 1 , C L is 0.4 for b/d ratio up to 7
and 0.15 for b/d ratio greater than 16, with a linear variation in between; for
superelevation between 1 and 5 , C L is 0.75.
A combination of full transverse and vertical loading due to wind is considered,
but full longitudinal loading is considered in combination with half the
transverse and vertical loading.
The American bridge loading proposals[8] are similar to, but probably not as
detailed as, the British loading. The transverse wind loading on the full area in
elevation is stipulated as
Z0:2V 2 30CD lb=ft
600
2
where Z is the height in feet of the bridge deck surface above ground or water
level but not less than 30 ft, V 30 is the 100 year return fastest mile wind speed
in miles per hour at the 30 ft height and can be obtained from the map of
isotachs or preferably from local wind data, and CD is the drag coefficient and
is specified as 1.5 for plate or box girder bridges and 2.3 for truss bridges
unless a lower value is justified by wind tunnel tests. When a live load is
present on the bridge, a vertical surface of 10 ft height, less the area shielded by
solid parapets, is included in the total area in elevation, C D is specified as 1.2
on this part, and V 30 is limited to 55 mile/h. The vertical wind load on the plan
area of the bridge is stipulated as
Z0:2V 2 30CL lb=ft
600
2
Loads on Bridges 67
where CL is the lift coefficient specified as 1.0.
The above two proposed formulae for wind load are based on the following
assumptions:
(1) The maximum gust speed is approximately 1.6 times the mean hourly
wind speed, but because of the incoherence of gusts along the whole
length of the bridge the gust speed is reduced to 1.41 times the mean
hourly speed.
(2) At heights above 30 ft the gust speed is assumed to increase according to
the 1/10th power of height.
(3) The mean hourly wind speed is 0.8 times V 30, the fastest mile wind speed
at 30 ft, in which form the wind speed is recorded in the USA.
(4) The maximum gust speed, unlike the mean hourly speed, is relatively
insensitive to terrain condition.
At 30 ft height and V 30 ¼ 100 mile/h, the new formulae give very similar
wind loads to the current AASHTO values, but for other heights and locations
the new values will obviously be different. At Z ¼ 30 ft and V 30 ¼ 80 mile/h,