For a university assignment, I currently need to determine the wind loads on a steel stair tower with a skeletal frame, but I’m not sure whether my approach is correct.
My method is based on determining wind forces for trusses and is as follows: I first consider just one side of the structure and determine the “enclosed” area and the actual area exposed to the wind in order to find the solidity ratio φ. Then, using φ and the slenderness of the structure, I determine the c_f value. After that, I plug everything into the wind force formula, using the actual area exposed to the wind for A_ref. This gives me a wind force in kN.
This is where I’m unsure. I’m used to wind loads being specified as pressure forces in kN/m². I can’t work out how I can use a point load for the subsequent design of the tower, how the load is transferred as a result, or even where the point load would actually act, since the tower is relatively irregular in shape. Is there a better way to determine the wind loads here? Could I perhaps simply omit A_ref and apply the whole thing as an area load on the side of the structure?
your approach seems sound in principle. For open lattice structures, determine the wind force according to EN 1991-1-4, Section 7.11. Use the actual projected area of the members as the reference area A_ref.
Point load or distributed load?
F_w is the resultant total force acting on the tower face in question. However, it should not be applied as a point load in the design. Instead, distribute it across the members exposed to the wind, in proportion to their projected cross-sectional width b perpendicular to the wind direction.
The sum of these line loads will once again equal F_w. This means that the load is automatically applied at the correct location, namely at the centroid of the projected area. This also applies to irregular geometries.
Area load on the entire tower face? — Implementation in RFEM 6:
The easiest way is to use the “Member Loads from Area Loads” load wizard:
Here, you can simply apply the area load to the entire face and select the “Not filled, members only” option, so that RFEM applies the load only to the projected widths of the members. The procedure can be summarized as follows:
Define the plane using the corner nodes of the tower face.
Under Load Transfer Area, select the “Not filled, members only” option.
Enter the area load.
This procedure is demonstrated in the following section of the FAQ webinar: