Wind Loads on Assembled steel frames

[keenaseng]

I am trying to calculate the wind force on square assembled steel frames. I have no problems working it out normal to the face, it is the wind on the diagonal I am having trouble with. I know my drag force coefficient for wind on the diagonal.

Is it the diagonal width of the frame (ie. corner to corner) that is used to calculate the area that the drag force coefficient on the diagonal applies to? Wind force acts perpendicular to the surface it applies to, so i'm having trouble getting my head around the diagonal width.

I am working to Australian standards and the drag force coefficient is simply given to us. But I have found another table that describes wind on the diagonal as multiplying normal by 1+0.75?. I'm not sure what code this is from - does anyone know what the ? represents? I am assuming it is solidity ratio...

 

[apsix]

With regard to AS1170.2 the wind force is always a function of 'b'.
'b' is typically the breadth normal to the wind stream, except in Table E5.
'b'is clearly defined in Tables E3 & E4.

As to the unknown notation in the unknown table, I can't help you.

 

[keenaseng]

Thanks for the response Apsix.

I was working with table E6(A). For square towers for wind onto corner, 'b' would be the diagonal from corner to corner across the tower? I.e if I have a 2m x 2m square tower frame, b would be 2.83m rather than 2m?

 

[keenaseng]

Further to my post above, table E4 in 1170.2 indicates that the drag coefficient is lower for a solid square with corner to wind (where 'b' is defined as corner to corner) than when the wind is perpendicular to the face. This makes sense to me, as corner 'cuts' through the wind.

However, the drag coefficient in table E5 for wind on corner are higher than when wind is perpendicular to face. Why is that? Less shielding to rear of the frame when wind is on the diagonal perhaps?

It doesn't feel right using a larger 'b' (corner to corner) and a larger drag coefficient for wind on the diagonal of lattice frame.