You could choose the ASCE wind definition which has the load intensity generator based on wind speed and other parameters .
If you are not following that code then please refer to your country code for wind load generator .
But if you want to apply your own intensity then you need to follow some fuild mechanic principles .
From general theory of fluid mech. , a plate would experience force if hit by jet fuild is (F= Density x projected area x velocity ) . It is derived from the principle of conservation of momentum . The same thing could be applied here to determine the pressure exerted . But apart from this classical mechanics, one need to follow some other phenomena of wind pressure as a function of wind speed like Bernoulli effect , skin frinction on the sloped part etc.
Once you have derived the resultant and effective wind pressure , you could apply it in the wind intensity , Staad would automatically distribute the panel loa
ding to its surrounding members .
One more point. As your panels are sloped ,use the GROUP option to apply the wind load.