I have modeled an electric substation box-type structure which consists of lattice columns connected by struts made with standard shapes. The lattice column longerons are L3X3 angles, laced together with L1-3/4X1-1/4X1/8 members. I have calculated the force coefficient, Cf for latticed structures with flat sided members using the solidity ratio (phi) as given in ASCE 113 (pp. 39-40). I applied one-half of the calculated load to each longeron as a uniform force, since the calculation is for one plane containing two vertical members. The same load was also applied to the "back side" angles.
To verify the application of wind loads using the wind load generator in STAAD, I defined the wind intensity using the same value as the hand calculation and applied the load to the same set of members. One significant difference is that I included the lacing members in the STAAD generation, but this was not done in the hand calculation, since the solidity ratio was calculated with the lacing area included. Therefore, the uniform load placed on the longeron should include the normal forces acting on the lacing.
All said, the STAAD loads are significantly higher than those calculated by hand and applied as uniform loads to the members.
I will provide the spreadsheet calculation of the force coefficients and the STAAD model if they will help explain the difference.
Thank you for your assistance.