Interior Beam Calculations

INTERIOR 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 INTERIOR BEAM:

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

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

SPAN A (Length of truss A) _______ feet

SPAN B (Length of truss B) _______ feet

SPACING (between trusses) _______ feet

LENGTH of Beam (between Columns) ______ feet

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

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

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

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

Step 2 - Find the amount of roof supported by an INTERIOR BEAM:

Half of the LENGTH of the Beam x Half of the LENGTH of the Truss = AREA of Roof supported by the COLUMN 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 LENGTH of an Interior Truss A to one side of the BEAM x Weight of Truss A lbs./lin.ft. x the Number of Trusses) plus (Half of the LENGTH of an Interior Truss B to the other side of the BEAM x Weight of Truss B lbs./lin.ft. x the Number of Trusses) = Total Weight of Trusses in lbs.

Step 5 - Find the total weight on an INTERIOR BEAM:

Add the "Weight of the Roof" to "Weight of the 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.

Step 7 - Make note of the following:

Beam Designation __________________

Beam Size _______ inches

Weight of Beam ________ lbs./lin.ft.

Allow. Wt. _______________ lbs.

Step 8 - Assign a CODE LETTER to this BEAM: _______ Examples: AAA or C1 or CC

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