Outdoor Stage and Banner Wind Load

The wind force on an outdoor banner or scrim, the moment it tries to tip the structure with, and the ballast that holds it down. Velocity pressure rises with the SQUARE of wind speed, which is the fact that catches people out: a 40 mph gust does not carry twice the load of a 20 mph one, it carries four times. The drag coefficient for a flat panel normal to the wind is around 1.3, and a solid banner is exactly that, with no shape to shed the load. The overturning check is a moment balance about the downwind base edge, where the wind force acts at the banner's centroid height and the only thing resisting it is the ballast acting at half the base width. Because the lever arms are so different -- a centroid twelve feet up against a four-foot resisting arm -- the required ballast comes out several times the wind force itself, and the numbers get large fast. A 20 by 8 ft banner with its centroid 12 ft above an 8 ft base, in a 40 mph gust at a 1.5 safety factor, sees 4.10 psf and 852 lb of force, a 10,224 ft-lb overturning moment, and needs 3,834 lb of ballast: nearly two tons to hold one banner in a gust an ordinary summer thunderstorm produces. Drop the design wind to 30 mph and the requirement falls to 2,157 lb, because the square law works in both directions; raise it to 55 mph and it climbs to 7,248 lb. The design wind speed is the most expensive number on the drawing, and the honest way to reduce it is to plan to drop the banner rather than to assume the storm will be small. The safety factor and the wind speed at which the structure comes down are decisions ANSI E1.21 requires be made IN ADVANCE, written into an operations management plan, with someone watching an anemometer and holding the authority to stop the show. A screen, never an engineered result; ANSI E1.21, the structure manufacturer's data, and a qualified engineer govern.

velocity_pressure_psf = 0.00256 x wind_speed_mph^2; wind_force_lb = velocity_pressure_psf x drag_coefficient x area_sqft; overturning_moment_ftlb = wind_force_lb x centroid_height_ft; required_ballast_lb = overturning_moment_ftlb x safety_factor / (base_width_ft / 2).

Velocity pressure q = 0.00256 V^2 with V in mph, force = q x Cd x area, and an overturning-moment balance about the downwind base edge against ballast acting at half the base width. ANSI E1.21, Entertainment Technology -- Temporary Structures Used for Technical Production of Outdoor Entertainment Events, is cited by name for the requirement that the safety factor and the take-down wind speed be set IN ADVANCE in an operations management plan. A screen, never an engineered result; a qualified engineer governs.

Velocity pressure and the moment balance are public physics. The banner dimensions, design wind speed, drag coefficient, base width, and safety factor are the production's own values; ANSI E1.21 is cited by name, not reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: banner_height_ft, banner_width_ft, centroid_height_ft, wind_speed_mph, drag_coefficient, base_width_ft, safety_factor, available_ballast_lb, velocity_pressure_psf, wind_force_lb, overturning_moment_ftlb, required_ballast_lb

• Velocity pressure q = 0.00256 V^2 with V in mph; the load goes as the SQUARE of the speedpublic wind engineering
• Drag coefficient around 1.3 for a flat panel normal to the wind; a solid banner has no shape to shed loadpublic wind engineering
• Decided in advance ANSI E1.21 requires the safety factor and the take-down wind speed be set in advance in an operations management plan, with an anemometer watched and someone holding stop authorityANSI E1.21