Roof Diaphragm Calculations

Diaphragm Calculations

• Roof diaphragm calculations tend to shift based on what aspect of the diaphragm contractors are examining and what the primary purposes of the diaphragm are. Most calculations are used to find the limits of the materials at hand. For example, torsional irregularities are calculated to make sure maximum drift between diaphragms at one end of the building is not more than 1.2 times the average drift of both ends. This is an important consideration for buildings that are many stories tall, but involves complex calculations that are best made via engineering programs and calculators.

Shear Strength and PLFs

• Shear strength is a very important factor in diaphragms, showing how strong the diaphragm is based on its materials, using PLFs (pounds per linear foot) to show the strength limits of the diaphragm. While you could calculate PLFs if you had enough information about the shape, size and materials of the diaphragm, it is more effective to find a PLF chart. These charts show the shear strength of diaphragms based on size, fill and design, letting you compare multiple strengths at the same time.

Shear Stiffness

• Horizontal diaphragm stiffness is also an important calculation to make when considering the flexibility and durability of the diaphragm support. For example, a diaphragm that is 8 by 24 feet with 11 feet of frame spacing may have a shear diaphragm stiffness of 43,200 pounds per inch. Entire building diaphragm shear stiffness can be calculated through the equation ch = 2cp (cos2θ)(b’/b)(a/sf). Cp is the panel shear stiffness, which is multiplied by 2. Theta is the angle of the roof slope in degrees, while b' is the roof span length from ridge to eave, b is the test panel length, a is the test assembly frame spacing, and sf is the frame spacing of the building.

Deflection

• Deflection is a measurement of how flexible the diaphragm is. It approaches stiffness from the other direction, calculating how many inches the diaphragm can bend under stress. Too much deflection shows a diaphragm that is unstable or cannot perform its load bearing job correctly. Deflection amounts should not reach more than 3 inches.