View Question
Q: Sizing I or H beam to support 2"x12" lumber floor joists at a 25' span ( Answered ,   0 Comments )
 Question
 Subject: Sizing I or H beam to support 2"x12" lumber floor joists at a 25' span Category: Science > Physics Asked by: bad67deuce-ga List Price: \$40.00 Posted: 02 Apr 2006 08:35 PDT Expires: 02 May 2006 08:35 PDT Question ID: 714614
 ```I am building an addition onto my house. The area in question has a 25' x 32' dimension. I would like to reduce the 32' span with an unspecified I or H beam to carry the load of the floor. The beam would run parallel to the 25' length and be set 13' from one end of the 32' span. There would be 2" x 12" lumber spaced 16" on center spanning the 13' and the 19' distances. 3/4"decking will be used, and 3/4" hardwood flooring on top of that. This area will be used a kitchen on one half of the room, and a living room on the other half. What I or H beam would be adequate to support this layout? Thank you```
 ```Hello bad67deuce, you did a very good job describing your problem. Here is the info we need to do the calculations: Live Load = 40 psf Deflection limitation = L/360 = (25 x 12)/360 = 0.83 inch 2 x 12 weight = 4.1 # per ft 3/4 plywood = 2.13 # per sq ft 3/4 hardwood flooring = 3 3 per sq ft Dead load calculation: Quantity of 2 x 12 = (25 x 12)/16 (minus 1) = 18 Tributary area (area which beam supports) = 13/2 + 19/2= 16 ft x 25 ft Weight of 2 x 12 = (18 x 4.1 x 16)/25 = 47.2 #/ft Weight of flooring & plywood = (2.13 + 3) x 16 = 82.1 # per ft Total dead load = 47.2 + 82.1 = 129.3 # per ft Live load = 40 psf x 16 = 640 # per ft Total load (w) = 129.3 + 640 = 769.3 # per ft The beam formulas for this loading are: M (maximum bending moment) = wl^2/8 D (deflection @ center of span) = 5wl^4/384 EI Where E is a constant for steel + 30,000,000 psi And I is the moment of inertia Solving for M: M = 769.3 x 25^2 / 8 = 60,102 ft lb = 721,224 in lb The allowable bending stress for structural steel (s) = 0.55 x 36,000 psi = 19,800 psi The section modulus of the required beam (S) = M/s = 721,224/19,800 = 36.42 in^3 Now we must calculate the required I (moment of inertia): Solving for I in the above formula for deflection we get: I = 5wl^4/384 ED = (5 x 769.3 x 25^4 / 384 x 30,000,000 x 0.83) x 1728 NOTE: 1728 is a conversion factor to get the proper units for I I = 272 in^4 Now we can look for a beam with these minimum properties. A good choice would be a 16 inch deep Wide Flange beam weighing 26 # per ft (W16x26) S = 38.3 in^3 I = 300 in^4 Of course, you can use a heavier beam if there is one more available. Please ask for a clarification if there is anything you don't understand and I will try to explain. Good luck with your project, Redhoss``` Request for Answer Clarification by bad67deuce-ga on 03 Apr 2006 14:05 PDT ```Looks great Redhoss. So I need 16" I beam, wow that's much bigger than I was thinking. I'm glad I asked this question. The only thing that I did'nt see was the beam thickness?? And also, in your opinion is this the best material to use for this job, or is there something better that you can suggest?? Thank you Redhoss, I will be rating this answer as a 5 star when I receive your follow up.``` Request for Answer Clarification by bad67deuce-ga on 03 Apr 2006 14:09 PDT ```One more thing. If your calculations were for lets say 3/8" thick I beam at 16" deep. Could we go with 1/2" web thickness to reduce the 16" dimension. If I could use a shallower depth beam this would increase my headroom. Thanks again, your the man Redhoss.``` Clarification of Answer by redhoss-ga on 03 Apr 2006 16:37 PDT ```There are many beam that would satisfy the I and S minimum values we calculated. I selected what would be the best choice for price. When it comes to structural members pounds per foot determines how many dollars you will spend. The W16x26 which I chose has these dimensions: Depth = 15.65" Flange width = 5.5" Flange thickness = 0.345" Web thickness = 0.250" If you drop down to a 14 inch depth beam, you would need a W14x30, going on down to a 12 inch requires a W12x36, 10 inch requires a W10x54, and so on. You can see that after 12 inches the weight goes up drastically. So, if you want to spend a few more dollars to gain headroom the W12x36 might be the best bet. And yes, I do think that a steel beam is the best material choice.```
 bad67deuce-ga rated this answer: `Redhoss is a total professional. Thank you`