huangyhg

hydrostatic pressure on a curved surface
i need a formula to figure the pressure present on a curved form (tank)
the form is a circle 10ft in dia. and 40ft tall. the form is standing vertical on solid concrete. the bottom is flat. the fluid placed in the form has a density of 2.4. there are no additional forces acting on the fluid (only atmospheric, the form has an open top)
i need know the pressure on the form at any point.
density times depth at point of interest.
is this formula correct? density*height=psf or 2.4*40=96psf or 2.4*480=1152psi
the weight of the (at max cap)column placed on the base is 471,238 lbs (150lbs/cuft)
the formula given looks to be low. please verify.
dittowizard
pressure in t/m2 = depth of the point from the water surface in m
dittowizard, the density of 2.4 you mentioned for this fluid doesn't seem right. you did not give any units. are you sure this is not specific gravity in which case the density would be 2.4 * 62.4 pcf (water) = 150 pcf. the pressure then at any depth is 150 * h (measured from the surface). keep in mind though that the pressure varies linearly with depth along the verical face. the resultant force is at the centroid of the pressure prism (not the centroid of the area - fluid on one side) according to my fluids book. if we roll out your form so it's pi * d long or 31.4' and contain this fluid against it, the resultant force would be 150 * 40 * 40/2 * 31.4 = 3.768 x 10^6 lbs acting at 2/3 * 40 = 26.8' below the surface.
maybe someone could do some qa and check this.
briansch
briansch - no you're wrong. the question looks like pressure in a pipe, not force on a flat plate. the pressure in a pipe 40 ft high full of fluid at 150 pcf is 6,000 psf at the bottom of the pipe and varies linearly as you go up to zero at the top.
dottowizard - if you're placing a concrete column 40 ft high you can reduce the maximum internal pressure according to aci formula which accounts for the concrete at the bottom starting to set before you get the full 40 ft placed. depends on pour rate and temperature.
thanks krd. maybe making it more complicated than it is.
dittowizard, what are you building?
by the way dittowizard, it is doubtfull if you can get forms to allow a 40' concrete placement. in many cases you will have to allow for the lower lifts to set up somewhat prior to continued placement. most forms (ready-made steel forms) are rated based on the framing (kicker) system that is employed. other arrangements should be checked by a registered engineer.
thank you for the reply. this information will help. we do mostly concrete work, however, this is going to be a concrete tank. someone asked the original question of me and i was not sure of the answer due to the curved surface.
brainsch was correct when he stated,"are you sure this is not specific gravity in which case the density would be 2.4 * 62.4 pcf (water) = 150 pcf."
krd was correct when he stated,"the question looks like pressure in a pipe, not force on a flat plate."
it looks like denisty(pcf)*height(f)=pressure(psf) is the formula.(?)
example: a fluid colum with a density of 90(pcf) and 4(f) tall will have an outward pressure on a curved surface at the lowest point of 360(psf)
i think that tall (i.e. - grain elevators, 100+ ft tall) cylindrical concrete structures are often formed by slip forming. in this method, the forms are slowly (a few feet per day) jacked up as the additional concrete and steel is added. the idea is that the forms are only on the order of ~10 ft tall, and the concrete solidifies as you go. think of it as a continuous, rather than a batch manufacturing process.
having a mechanical background, this is just about all the level of detail i know, but i've watched a couple of these projects in construction.
the hydrostatic pressure is equal to the density of the fluid times the height of the column, the pressure at any point does not depend on the volume but on the height and on the density. example: height of vessel equals 30 ft. and the fluid is water (i.e. density = 62.4 lb/cu.ft.) thus pressure at bottom of vessel is 62,4 x 30 = 18,720 lb/sq.ft

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