Multi Storey Car Park and Rain Blown In

[waterboy123]

Hello,

I am looking at carrying out the draiange design for a multi storey car park. Does anybody know a good method to estimate the rain that might be blown onto the surface of the car park through the sides. Obviously the middle floors in the car park have a roof but the sides are exposed to the elements.

BS EN 12056-3 2000 suggests wind driven rain 26 degree to the vertical can be calculated as follows:

A = length of roof x (width of roof + (length of gap/2) )

The height of each story is 1.8m so for a 10 x 10 m roof:

A= 10 x (10 + (1.8/2) ) = 109m^2

Clearly this is to much because it's bigger than the plan area of the car park itself.

Any suggestions would be greatly appreciated.

 

[psmart]

Your formula includes the total area of the roof plus half the vertical area of the gap, but all you want is the gap exposure, correct?

I believe the exposure to rain though the gap on one side will be 10 * 1.8 * sin(26) = 7.89 m^2

Note that if the angle were 30 degrees sin(30)=1/2 which is presumably the final term in your equation.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[waterboy123]

psmart - thanks for that, seems like a sensible figure.

 

[waterboy123]

Actual second thoughts - would you not use tan (as opposed to Sin)? Please see attached

 

[psmart]

The tangen would be correct if the top of the triangle is horizontal, as shown in your sketch. This would seem the correct solution if the rainfall is initially vertical, and only turns 26 degrees as it passes the roof.

But I believe the rain will be falling at a fairly constant (26 degree) angle, so the the top of the triangle should be sloped at 26 degrees, making it perpendicular to the rainfall. In this scenario the correct solution would be the sine.

Of course, the difference is pretty small in this case (about 10%), and the exact behavior will depend on whether the wind velocity through the open side of the car park is higher or lower than the prevailing wind above.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

OK, on second thought I'll go with tangent. Even though I believe the rain is falling on a (relatively) constant angle, rainfall depth is always measured relative to a horizontal surface. So using the tangent (as shown in your sketch) seems more accurate.

Another way to look at it: You're calculating the width of a hypothetical roof overhang that would completely shelter the floor from rain at the specified angle.

Peter Smart
HydroCAD Software

http://www.hydrocad.net