*This article is about Main Wind-Force Resisting Systems for Structures Other Than Enclosed Buildings and all calculation of MWFRS.*

*Force coefficients (Cf), in Table X apply to flat-sided members. The specified force coefficients are **for towers with structural angles or similar flat-sided members. **Design wind forces (F) for structures with a height/least horizontal dimension of ≤ 4 and f ≥ 1.0 cycle/second can be calculated using the following equation:*

*Af projected area normal to wind except where Cf is given for the surface area.*

*MWFRS for Structures Other Than Enclosed Buildings*

*In the calculation of design wind forces for components and cladding for buildings, the algebraic sum of the pressures acting on opposite faces shall be taken into account.*

*Minimum design wind forces. The wind load used in the design of a main wind-force resisting system shall not be less than 10 lb/ft2 multiplied by the area of the building or structure projected on a **vertical plane normal to the wind direction. The design pressure for components and cladding shall not be *

*less than 10 lb/ft2. The design force for open buildings and other structures shall not be less than 10 lb/ft² **multiplied by the area Af.*

*Table X – Force Coefficients for Trussed Towers, Cf*

*Note:*

*1. ε is the ratio of solid area to gross area of one tower face for the segment under consideration.*

*2. For tower with round members, the design wind force shall be determined using the force coefficient determined in Table X multiplied by the factor (0.51ε² + 0.57), but not >1.0, when determining wind forces on such members.*

*3. Wind forces shall be applied in the directions resulting in maximum member forces and reactions. For tower with square cross-sections, wind forces shall be multiplied by the factor (1.0 + 0.75ε ) but **not >1.2 when the wind is directed along a tower diagonal.*

*4. For triangular section towers, the design wind forces shall be assumed to act normal to a tower face.*

*5. For all wind directions considered, the area Af consistent with the specified force coefficients shall be the solid area of a tower face projected on the plane of that face for the tower segment under consideration.*

*Table XI – Force Coefficients For Open Signs and Lattice Framework, Cf*

*Notes:*

*D = Diameter of a typical round member, m (ft).*

*z = Velocity pressure evaluated at height Z above ground, N/m² (lb/ft²) .*

*Additional Notes:*

*1. Signs with openings comprising 30 percent or more of the gross area are classified as open signs.*

*2. The calculation of design wind forces shall be based upon the area of all exposed members and elements projected on a plane normal to the wind direction. Forces shall be assumed to act parallel to **the wind direction.*

*3. The area Af consistent with these force coefficients is the solid area projected normal to the wind direction.*

*TABLE XII – Force Coefficients for Solid Signs, Cf*

*Notes:*

*ϒ= Ratio of height to width*

*M = Larger dimension of sign, m (ft)*

*N = Smaller dimension of sign, m (ft)*

*Additional Notes:*

*1. Signs with openings comprising less than 30 percent of the gross area are classified as solid **signs. Signs for which the distance from the ground to the bottom edge is less than 0.25 **times **the vertical distance shall be considered to be at the ground level.*

*2. To allow for both normal and oblique wind directions, two cases shall be considered: *

*(i) Resultant force acts normal to the face of the sign on a vertical line passing through the geometric **center.*

*(ii) Resultant force acts normal to the face of a sign at a distance from a vertical line passing through **the geometric center equal to 0.2 times the average width of the sign.*