Structural calculations
Live loads
Snow 20 psf
People _____psf
Furniture _____psf
Other _____psf
Dead loads
Assume 90 psf
Total ___110__psf
Use A36 steel (36000 lbs)
Beam loading
total load*tributary width
110psf*5.5ft = 605 lbs
Determine Allowable Bending Stress.
2/3*steel yield strength
2/3*36000lbs = 24000lbs/in^3
Determine Maximum moment
loading*length of beam squared divided by 8
M= (605 lbs * 75^2) / 8 = 425,390.6 lbs-ft
Determine Section Modulus Required for Bending.
Max moment divided by allowable bending stress
425,390.6 lbs-ft * 12 in / 24,000 lbs/in^3 = 212.7 in^3
Determine allowable deflection
length of beam divided by 360
75 ft * 12 in / 360 = 2.5 in
Pick a recommended beam in MD Solids.
Determine actual section modulus, and deflection based on your beam.
Section Modulus: 213 in^3
Deflection: -5.21
In real life, there would be a need for more supports throughout the beam. Also these calculations do not include the girders that run the other directions that would help support the 75ft beam. So even though this beam says it would bend too much. It probably wouldn’t be nearly as bad as it seems.
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