INTERIOR TRUSS CALCULATIONS

• Trusses are usually placed to span the shortest measurement of a building and are classified as either end (one at each end of the building) or interior (all within the exterior walls of the building. End trusses usually hold up less weight.
• Trusses are designated by a number, letter(s) and a number: 16K2 - where the 16 is the DEPTH of the truss in inches, the K, KCS, LH, DLH or SLH is the SERIES for simple to super longspans with uniform loads and the 1, 2, 3, 4, 5, etc. is the sequencial number for ordering purposes.
• The total roof load (live and dead) in pounds per square foot and the spacing between trusses in feet and inches must be know to find the size of a truss from standardized tables
• A safety factor of 5 to 15 percent should be considered at the end of each calculation but should avoid duplicaton when calculating beams and columns.

Carefully follow these steps for finding the correct INTERIOR TRUSS:

Step 1 - Determine the DEAD LOAD by describing the materials that make up the roof and find their weights:

ROOFING _________________________________ Weight/sq.ft. ___________ lbs.

INSULATION ______________________________ Weight/sq.ft. ___________ lbs.

METAL DECKING __________________________ Weight/sq.ft. ___________ lbs.

EQUIPMENT _______________________________ Weight/sq.ft. ___________ lbs.

TOTAL DEAD LOAD ___________ lbs.

Step 2 - Determine LIVE LOAD by describing the item that set on the roof and find their weights:

PEOPLE ___________________________________ Weight/sq.ft. ___________ lbs.

SNOW _________________________________ Weight/sq.ft. ___________ lbs.

RAIN ___________________________________ Weight/sq.ft. ___________ lbs.

EQUIPMENT _________________________________ Weight/sq.ft. ___________ lbs.

TOTAL LIVE LOAD ___________ lbs.

Step 3 - Determine the TOTAL ROOF LOAD:

(Total Dead Load + Total Live Load) x Safety Factor (110%) = _____________ lbs/sq.ft.

Step 4 - Determine the following from your "Structural Layout" drawing:

SPAN (Length of truss) _______ feet SPACING (between trusses) _______ feet

Step 5 - Find the proper size of an interior truss:

SPACING x Total Roof Load = TOTAL WEIGHT on one (1) lineal foot of the Truss

Step 6 - Consult the "TRUSS JOIST TABLES" to find the correct size:

Start with the "SPAN of the Truss" and look down the table for the "Load" that is equal to or slightly larger than the TOTAL WEIGHT. There are two figures for each truss size; the upper number is Max. Allow. Wt. including the weight of the truss and the lower number is the SAFE LOAD that can be supported. USE THE LOWER NUMBERS.

Step 7 - Make note of the following:

Truss Model No. _____________________ Depth of Truss _______ inches

Weight of Truss ________ lbs./lin.ft. Max. Allow. Wt. _______________ lbs.

Step 8 - Assign a CODE LETTER to this TRUSS: __________ Examples: B or T2 or IT1

Place this code letter next to each of the corresponding interior trusses on your Structural Plan.