huangyhg

how do i determine the force of flowing water
i need to analyze a pipe hanger that will be located along the bottom of a bridge. i need to determine what force the river flow will have on the pipe during a flood. i've been told to use 3 fps as the velocity of flow.
the pipe is 8" and carries water. i've been racking my brain but can't seem to find the correct formula.
thanks for any help.
it is about going back to basics of fluid mechanics.
r=r*q*v
where:
r = reaction
r = density of fluid
q = volume rate of flow
v = velocity of fluid
make sure to be consistent with units. also consider angular velocities because they will produce x and y force components.
also be sure to use the the true outside diameter of the pipe for your calculations, it is often quite different than the nominal diameter. for example: nominal 8 inch ductile iron pipe has an outside diameter of over 9 inches.
pbw2
if you are working on a bridge check the aashto requirements.
aashto 3.18 (standard specs) deals with stream flow forces on bridge elements with applicable formula.
your case is flow force on a cylindrical element.
in the worst case, you might also need to allow for growth on the pipe and hence increased diameter and increased friction factor.
what about ice loading in winter as well as possible snagging from floating trees etc.?
stephena
in terms of force you have the equation-
f=r*(v^2)*a/2
r= mass density
v=velocity of water
a= exposed area
f=force
if you are having a pipe,
a= outerdia * length
if you use units in kg/m/seconds in the rhs expression, you get f in newtons straightaway.
thanks to all who responded.
according to underwater construction using cofferdamsfff">, page 221, by harold v. anderson, the flow pressure is p = ((w*v*v)/2/g)*c
w = density of the water in pcf
v = current velocity in fps
g = gravity = 32.2 ft/sec2
c = a constant (c = ~2.0 for a cofferdam wall; c approaches 1.0 for smooth surfaces with well rounded corners)
for muddy water, w > 62.4 pcf. assume w = 65 pcf
for pbw2's pipe case, with a rounded pipe, assuming c = 1.5, p = 65*3*3/2/32.2*1.5 = 13.6 psf on pipe area normal to flow.
also, stephena makes some good points.
i just returned from a site visit to a temporary bridge i designed which was flooded over by ivan this past weekend. there was approximately 6 feet of water running over the bridge deck. the bridge was 8 ea. steel w36 beams , 74 feet long by 30 feet wide, with a 12" thick timber deck and wooden railings. the contractor measured the creek flow at about 12 fps. the wood hand rails and wood posts were torn off but the beams and timber deck survived without damage. there was minor scour at one of the abutments. there was probably close to 300 psf water pressure hitting the 74' x 4' projected bridge area, not counting floating debris hitting the bridge. the contractor was very lucky.

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