Can anyone give me a few pointers.If i had a wind force acting on wall of given dimensions (say 3m h x 4m w), how easy is it to calculate the wind velocity?
You would have to know also the gust effect factor, directionality factor, importance factor, topographic factor, exposure coefficients and force coefficients. If the wall encloses a space you may also need to consider internal pressure coefficients. These are terms used by ASCE 7, but if you are under a different code or use a different analysis method, you'll still have to consider these effects to some degree to have a meaningful answer.
It sounds like you're asking "My sign blew over, so how fast must the wind have been blowing?" Is that the case?
There are several issues to consider. The two formulas above are for stagnation pressure, and will not necessarily give you the net force on a rectangular surface. The factors suggested by UcfSE would be used to get from a mapped wind velocity as specified by ASCE 7 or similar source to the design loading on a surface. But the building codes aren't set up to be worked in reverse, and I'm not sure how meaningful the results would be when you do this. Probably the best you could do is to state that per a given building code, the structure should have been adequate for a specific wind speed.
I beg to differ hokie.
The formulas presented by Nicam and rb1957 have to be multiplied by the pressure coefficent appropriate for the structure in question to be able to calculate the force on that structure.
The pressure coefficients found in codes would be safe averages, but probably not accurate enough for the reverse calculation suggested.
Well, apsix, if he already knows the design force as he said, any pressure coefficients have been included in factoring the gust dynamic wind pressure to get the design wind pressure. For instance, if he knows that the force on his 3m x 4m wall is 12 kN, the wind pressure is 1.0 kPa, and the wind speed which causes this force is 40.8 m/sec. I agree that wind codes are complex, and differ in different countries, but this is a simple question which deserves a simple answer.
Sorry hokie, I don't follow. You appear to be saying that Cpn or Cd is always equal to 1.
I do agree that the other factors such as directionality factor, importance factor, topographic factor etc are not required.
No, I don't mean that the factors should not be applied. What I tried to say is that after all factors are applied and a design wind pressure is calculated, the same basic physics equation still applies, and you can backfigure an equivalent wind speed. The OP said he knew the wind force.
The 0.5 rho V A^2 equation above corresponds to a drag coefficient of 1.00. My fluids textbook shows the drag coefficient for a flat plate normal to the flow to be 2.0 at a Reynolds number of 100,000. So that equation is off by a factor of 2 for the force on the plate, off by the square root of 2 when reversed for the velocity. (I haven't checked Re to see if it's in that general range or not).
Using this approach will give you a specific velocity. Velocity should actually vary from zero at the ground and increase with height, so you're finding some sort of average velocity, not the velocity at the top of the wall. The building codes are based on 3-second gust at 10m height, which doesn't mean the actual velocity is equal to that at any point on the sign.
Where did the force number come from? A strain gage or breaking of a known component? What direction is the wind assumed to have with respect to the wall? Was the wind steady state or a gust? Was the wall a part of a building or freestanding as a sign/wall? Elevated above grade or down at grade? The answer to these questions will lead to a better answer.
