How to convert units.

What is the source of this formula? The factor K cannot be dimensionless.

In the 1995 National Building Code of Canada, the expression for pressure, q is:

q (in kPa) = CV²

when V is in kilometres per hour, C = 50x10^-6

when V is in metres per second, C = 650x10^-6

The constant C is equivalent to ρ/2 where ρ is the density of air in kg/m³.

In Canada, ρ is deemed to be approximately 1.3 kg/m³.

Hi,

I agree that one of your dimensionless parameters can't be dimensionless. The units just dont work out. It is impossible to get to m/s.

I think you should re-visit your formular as the value of K refers to certain conditions in the system and by definition canot be dimensionless. K might be (kg/m)^1/2

Good luck.

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Undergrads are the way of the future.

Every where the relationship between pressure and air speed is:

P= C *ρ/2*V^2 which leads to V= (2*P/(ρ*C))^(1/2)

You see the similarity of the 2 equations. "C" is a form coefficient (dimensionless) [P]= N/m^2 and [ρ]= kg/m^3. 1N=1kg*m/s^2.

A dimensional analysis of your equation will shaw that "K" cannot be dimensionless.

Dear All

Regarding this matter, i already double check that K is dimensional less. I want to attached document in this forum, unfortunately i dont know to do that. Could you all give me an email address and attached to you.

Thank you.

Faizol

Upload to http://ifile.it/ (you don't need to sign up and it is free)

Be sure to copy and paste the exact filepage that it gives you upon uploading - of the form http://ifile.it/abcdefg

Dear All,

Please upload file for wind gust design speed

http://ifile.it/jm5drlc

Thank you.

Dear all,

I think that the value of no-named coefficient 0.637 already allows us to get a result in "m/s", if all other members in the formula are dimensioned as shown.

This method often used in empirical formulas like that.

Sorry for my English.

Best regards

In eqn. U_Z = (P_z /(0.637 x C_f x K))^1/2, if you simply apply and substitute for the units as noted in Section 7. List of Symbols of the PDF file http://ifile.it/ as follows:

Pressure, P in N/m² (not N/mm²),

Shape Coefficient, C_f as a dimensionless,

Coefficient Factor, K as a dimensionless,

Automatically, the resultant Wind Speed, U_z shall be in m/s.

7. LIST OF SYMBOLS

D : transverse dimension of the body normal to the wind direction, (m)

U : design wind speed for specified period of less than 60 seconds, (m/s)

Uz : U at height z

V : design wind speed for specified period longer than 60 seconds, (m/s)

V : mean hourly wind speed, (m/s)

F = U/V gust factor

A : total projected area of all individual members of one frame, (m²)

K : correction factor, depending on aspect ratio

Cf : shape coefficient

Cfs : shape coefficient for a single frame

Cfm : shape coefficient for a total multi-frame structure

Cp : pressure coefficient

pz : wind pressure at height z (N/m²), on closed or fully clad structures (Appendix 1; section 2), or wind pressure at height z on conventional buildings (Appendix 1; section 3), (N/m²)

Pz : total wind force at height z

h : height of building or structure, (m)

n : number of frames

z : height above ground, (m)

a : shielding factor

b : wind direction

1/g an exponent giving variation of the design wind speed with height

l : aspect ratio, i.e. length to width of the aspect of a body normal to wind direction

s : spacing ratio, equals the distance, centre to centre, between the frames divided by the least overall dimension of the frame, beam or girder measured at right angles to the direction of the wind

f : solidity ratio, i.e. ratio of the projected area of the individual members of a frame to the total area enclosed by the frame

N : Newton

m : metre

s second

FAIZOL82

If you express the 0.637 as kg/m³ the dimensions work out correctly as m/s.

0.637 kg/m³ is ~ 1/2 the density of air. Other posts explain why the factor 1/2 is there.

But the actual figures look a bit odd. 0.01 N/mm² (MPa) is a lot pressure for a wind load and calculated velocity by your formula is a brisk 167 m/s (375 mph). But if you use SI units throughout pressure is in N/m² (Pa), and 0.01 N/m² is very low, vel = 0.167 m/s.

Cheers.........Codey