Measuring Seam Strength in a Monolithic Airform
May 25, 2005
by David B. South
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| Don Garrison prepares to test Airform fabric. Testing is performed in a hot room of 140 degrees Fahrenheit. |
Inside Bruco's belly is where a lot of the action takes place here at Monolithic. Plant Manager, Donald Garrison puts Airform fabric to the test before any fabric is used to create a Monolithic Airform. Testing allows us to know the fabrics ability to withstand tension, pressure and seam strength.
The Formula
The tension in the fabric in a pipe or cylinder (vertical stem wall) is calculated using the formula: Tension = Radius (measured in inches) X Pressure (measured in psi).
The formula for a sphere is exactly the same only divided by 2. In other words the same pressure for the same radius creates twice the tension in the pipe as it does in a sphere.
Tension is the force that tears an Airform as the pressure increases. The tension increases for the same pressure as the radius increases. As we usually build portions of a sphere the actual measurement is the radius of curvature (Rc). For oblate ellipses we need the Rc for the topmost section of the Airform.
Seam Testing
We seam test the fabric we use. At present we are generally using Mehler fabric. For the larger domes we primarily use Type III (31 oz per square yard) and Type IV (40 oz per square yard). To test the ultimate strength of the Airform, testing is performed in a hot room of 140 degrees Fahrenheit. We use high heat because we do not want the seam failing on a hot "sun-shiny" day.
The Type III seam will hold to 280 lbs per inch ultimate at 140 degrees. The Type IV seam will hold to 380 lbs per inch ultimate at 140 degrees.
We use a 5 times safety margin. The tension on the seam should not exceed 56 lbs per inch on the Type III nor 76 lbs per inch on the Type IV. We do not want to Fabricate Airforms with a Rc larger than 130 feet for the Type III nor 170 feet for the Type IV.
There are several other reasons why not to exceed 170 foot Rc in a smooth dome. A two hundred foot diameter dome will have a Rc of 125 for a height of 50 foot ? within the margins for the Type III fabric. If the height is lowered to 40 foot the Rc goes to 145 which is too high for the Type III and must go to the Type IV. Obviously the cost will increase both for the Airform and probably the dome.
Related Links:
- How to Order an Airform
- Airform Handling and Repair
- Airform Dynamics
- Inflating an Airform and Regulating Air Pressure
- Bruco -- A Very Busy Caterpillar
- New Welder Produces Superior Airforms
- Low Pressure Air Forms
- How to Attach an Airform
- What to do when your airform blisters
- OMEGAball: the final ball, the greatest game
Surplus Monolithic Dome and EcoShell Airforms for Sale- Monolithic Dome Shapes
- Dome Variation/Configurations
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