# what size 30 ft steel beam

Question:

how deep would 30ft long steel beam?

The rule of thumb in beam design is length divide by 2 in inches as the beam depth. Use that number as starting point of the beam analysis. For a 30 ft span you are looking at W16, 16 inches deep I beam because there is no W15.

Suppose you have a 30ftx30ft garage that you want to be able to park your cars on top and use the bottom as game room or storage. For a concrete slab on metal deck, we will need two beams because the deck can only span 10 to 12 feet. Let’s run the number and see what size beam we need.

8 inch Slab = 8/12*150 = 100 psf
3 inch 20 Ga metal deck = 2.3 psf
Beam weight = 31*30/(30*10) = 3.1 psf
Duct work = 5 psf
Ceiling = 2 psf
Electrical light = 3 psf
Total DL = 115.4 psf

The two beams will be spaced evenly at 10 ft apart for tributary width 10ft.

Beam moment equation:
M = W*L^2/8; w = 155.4*10 = 1554 plf

M = 1554*30^2/8 = 174,825 lb-ft = 174.8kip-ft = 2098 kip-in
S = M/Fy = 2098/.66*50 = 63.6 cubic inches.

A W16x40 with S=64.7 cubic inches will be sufficient for the load. Other beam options W14x48 S=70.3, W12x50 S=64.7, W18x40 S=68.4 will work as well.

The beam weight of 31 lb/ft used in the calculation is less than the 40lb/ft needed for the job, let's double check:
beam weight = 50x30/300 = 5 psf (50lb/ft)

M = 1573*900/8 = 2123.6 k-in

S = 2123.6/33 = 64.35 cubic inches which is less than 64.7 W16x40

To see I beam sections properties