• Login
Design • View • Buy • Online
Skip to main content
Main content follows

Wind Load Calculations of Gemini Sign Letters

Originally Posted: March 8, 2013
By , Staff Writer

You may be required to procure a permit from your local municipality to install sign letters on your building. The municipality will often request wind load data on the structural stability of the sign letters. The following are the calculations for the wind load of Gemini sign letters. These calculations are only for the sign letters and not how you attach the letters to your building.

The following data was calculated in May 2004 using Gemini’s formed plastic sign letters, thermoformed out of C.A.B. Testing and calculations were done by David Schmitt, VP Operations, BSME, MMSE

Assumptions and facts

  • Numbers are based on wind force of 144mph - this equals 82.7 lbs/sq. ft.
  • Sign letter samples used for testing were Helvetica font type (a common style)
  • Mounting hardware for the sign letters was a Combination All Mount (stud through pad) with an injection molded plastic block used to receive the threaded stud.
  • Tensile strength at break for CAB (Cellulose Acetate Butyrate) is 1800 psi
  • Tensile strength of Chemcaulk Silicone Adhesive is 350 psi.
  • Thread area for shear of 10-24 plastic block is 0.048 sq in.
  • Average face area of a building sign letter is .6 x height squared
Maximum depth of sign letters was as follows:
12″ UC 1-1/2″ deep
18″ UC 2″ deep
24″ UC 2-1/2″ deep
36″ UC 3″ deep

The following numbers are the result of actual tests and theoretical information gathered from the manufacturers of the component parts. This chart is designed to show how much force our mounting hardware will withstand when use on a standard building sign letter.

144 Wind Face Load 144 Wind Side Load Block Shear Strength Silicone/Pad Shear Strength
12″ 50 lbs. 11 lbs. 346 3360
18″ 112 lbs. 22 lbs. 518 14700
24″ 198 lbs. 36 lbs. 518 14700
36″ 447 lbs. 65 lbs. 691 19600


For example, a 24″ plastic building sign letter will have 36 lbs. of force pushing on the sign letter’s side with 144 mph winds. A 24″ sign letter will have 198 lbs. of force pushing on the letter’s sign face with 144 mph winds. This 24″ sign letter’s hardware (threaded black) will not fail until the force equals 518 lbs of force. This 24″ sign letter’s pad with silicone sill not fail until the force equals 14700 lbs pf force.

Remember, all Gemini sign letters come with a lifetime guarantee.

Gemini SignLetter Data

Letter Height Sign Face Area Square in. Sign Side Area Square in. Pads Per Letter Pad Area Sq in ea. Total Pad Area Sq in. Plastic Block Thread Total Block Thread Area
12″ 86.4 18 4 2.4 9.6 0.048 0.192
18″ 194.4 36 6 7 42 0.048 0.288
© 2013 All rights reserved. Reproduction with permission.
Recent Sign News