ASCE 7 velocity pressure numerical constant

[a7x1984]

Refer to Chapter C27 (Commentary) page 550 in ASCE 7-10

"The constant 0.00256 reflects the mass density of air for the standard atmosphere, that is, temp of 59 F and sea level pressure of 29.92 in Hg, and dimensions associated with wind speed in mi/h."

When deriving the constant, ASCE has chosen to take an average value for the mass density (0.0765 lbm/ft^3) opposed to the apparent maximum of 0.0822 lbm/ft^3. Any idea which reference on pgs 555-556 get into that detail? I would think the oldest reference would be most likely.




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[wannabeSE]

The formula appears to be a derivation of [½]mv² for velocity head or kinetic energy. A basic fluids or physics text should include a discussion on Bernoulli Equation and the conservation of energy.

 

[a7x1984]

wannabe: I understand the use of the relationship, and agree with you on its use. Do you really think a basic physics text would explain (or even address) the use of average air density for wind pressure computations? That seems like a decision that ASCE has made based on empirical evaluations, opposed to some universal accepted norm - that is why I suspect it is in one of the references indicated in that chapter.

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[wannabeSE]

I missed the part in our post about using the average density of air at sea level rather than the maximum. It seems like a reasonable and perhaps conservative number. The maximum density is only 7% greater than the average used. At 1000 ft elevation, the maximum pressure is 4% greater than the average used. I would sure hate to see ASCE start requiring temperature, altitude and barometric pressure data to calculate the basic velocity pressure.

If you want to reduce the wind pressure because the project is located at high altitude (high temperature), the commentary does state this is possible. I never tried to sharpen the pencil that much.

 

[a7x1984]

Good Lord. I would never want to do that. Maybe one day when we are building skyscrapers in a more literal meaning - to the sky. Besides, pressure varies linearly with the density, unlike varying to the square of the velocity. Further, the velocity is a function of elevation. I would guess that it may not do much good to reduce the density, as the loading pressure at high altitudes would be dominated by the velocity component.

Maybe I will tabulate something to see how it varies to some high elevations...maybe...

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