Exterior Beam Calculations

EXTERIOR BEAM CALCULATIONS

Beams are usually placed along the longest measurement of a building and classified as exterior (sit on exterior walls of building) or interior (set in the middle of building). Interior beams are usually used when interior columns do not interfer use of the building.
Beams are designated by a letter and two numbers: W 12 x 22 - where the W, M, S, C or MC is for the shape of the beam (W= I-Beam); the first number 12 is the DEPTH of the beam in inches; and the second number 22 is the WEIGHT of the beam per lineal foot.
The total roof load (live and dead) in pounds per square foot, the weight of the truss per lineal foot, the number of truss between the columns, the spacing between the trusses and the length of the trusses must be know to find the size of a beam from standardized tables.

Carefully follow these steps for finding the correct EXTERIOR BEAM:

Step 1 - Enter the following data from your "Structural Layout" or "Truss Calculations Sheets":

TOTAL ROOF LOAD = ___________ lbs./sq.ft.

SPAN (Length of truss) _______ feet

SPACING (between trusses) _______ feet

LENGTH of Beam (between Columns) ______ feet

WEIGHT of End Truss _________ lbs./lin.ft.

WEIGHT of Interior Truss _________ lbs./lin.ft.

No. of Trusses on EXTERIOR BEAM ______ (Do not count trusses sitting on columns)

Step 2 - Find the amount of roof supported by an exterior beam:

Half of the SPAN x "Length of Beam" = AREA of Roof supported by the BEAM in sq.ft.

Step 3 - Find the weight of this ROOF AREA :

Total Roof Load x Area of Roof = Weight of Roof in lbs.

Step 4 - Find the weight of the TRUSSES:

Half of the SPAN x Weight of Truss/lin.ft. x Number of trusses = TOTAL Weight of Trusses
Step 5 - Find the total weight on an EXTERIOR BEAM:

Add "Weight of Roof" + "Weight of Trusses" = TOTAL Weight on Beam in lbs.

Step 6 - Consult the "Structural Steel Tables" to find the correct size:

Convert total weight to KIPS (1KIP=1000 lbs.) then start with the "Length of the Beam" 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:

Beam Designation _____________________

Depth of Beam _______ inches

Weight of Beam ________ lbs./lin.ft.

Max. Allow. Wt. _______________ lbs.

Step 8 - Assign a CODE LETTER to this BEAM: __________ Examples: AA or B1 or EB

Place this letter next to the corresponding exterior beams on your Structural Layout Plan.