CIVIL ENGINEERING FORMULAS

BRIDGE AND SUSPENSION-CABLE FORMULAS 273

whereninteger, 1 for fundamental mode of vibration, 2 for second mode,...
lspan of cable, ft (m)
wload on cable, kip/ft (kN/m)
gacceleration due to gravity 32.2 ft/s^2
Tcable tension, kip (N)

The spreaders of a cable truss impose the condition that under a given load
the change in sag of the cables must be equal. Nevertheless, the changes in ten-
sion of the two cables may not be equal. If the ratio of sag to span f/lis small
(less than about 0.1), for a parabolic cable, the change in tension is given
approximately by

(10.100)

where fchange in sag
Across-sectional area of cable
Emodulus of elasticity of cable steel

RAINWATER ACCUMULATION
AND DRAINAGE ON BRIDGES

Rainwater accumulation and drainage are important considerations in highway
bridge design. The runoff rate of rainwater from a bridge during a rainstorm is
given by:*
(10.101)

whereQpeak runoff rate (ft^3 /s)
Crunoff coefficient
iaverage rainfall intensity (in/h)
Adrainage area (acres)
k1.00083

The runoff coefficient, C, ranges from 0.70 to 0.95 for pavements made of
asphalt, concrete, or brick. Specific values are available in Tonias—Bridge Engi-
neering, McGraw-Hill.
Most bridge designers base the rainfall intensity on a once-in-10-year storm
lasting for 5 min. Historic rainfall intensities can be determined from local munic-
ipal records available from the city or state in which a bridge will be located.
To drain the water from a highway bridge the sheet flow of the rainwater
must be studied. Determine the deck width for handling the rainwater runoff to
the drain scuppers by using:*

(10.102)

where W width of deck used in analysis

WShoulder Width

1

3

(Traffic Lane)

QkCiA

H

16

3

AEf

l^2

f

*Tonias—Bridge Engineering, McGraw-Hill.