# Can I calculate beam size if I know length, and weight support requirements?

I am interested in hanging a heavy-bag/swing/aerial yoga bands - but am concerned about hanging from a strong enough beam.
I have tried to read lots about deflections of different material based on span (using span tables) but I am missing some fundamental information that I'm hoping you lot can help me with.

The ceiling rafters are too thin (2x4), and are comprised of a variety joints - I'd prefer not even mess with those. So instead, my thought is to attach 2x6 to the walls, and use joist hangers to support a new set of beams for this project - I just don't know the size of beam that would be needed.

Knowing that I need to span 10', and expect to have a live load (the weight will be swinging) on a single point on the beam, I need to determine the required size to support 200 lbs. Or let's get nutty, say 400 lbs (I assume we can just adjust this variable in the formula once someone helps me understand the formula).

I have read tons of threads that all basically say the same thing - "Go use this calculator!" http://www.awc.org/codes-standards/calculators-software/spancalc The problem is I don't understand the values it wants enough to complete this. The calculator wants me to pick a size & length, which defeats my needs. I know my length, and weight requirements, but not size of the board. (example 2x6, 2x8, 2 - 2x6's bolted together)

I get that the more material that's there, the stronger it will be - but is knowing the info I have enough to use a calculation to determine necessary size?

I'm being recommended 15 other 'similar questions' that I can't seem to piece together an answer for myself, so maybe I need more information before I can determine this, but I'm not clear what other variables those would be based on the information I have read thus far.

Does such an equation exist? Example: Live Load * Distance in inches = X lb/in, where there's then a table of what wood supports by the lb/in that, I can find what would work?

• Try this one which I've mentioned in numerous similar questions: courses.cit.cornell.edu/arch264/calculators/example8.1 but it will require you to express your loads in a different manner. With a "pick a size" calculator, you pick a size and see if it works - if not, you pick a different size. Load capacity goes as inverse length, directly with width, and as the cube of height/depth of the beam. There are practical subtleties you either get from a good calculator or an engineering course (which someone else can take, if you hire them.) Commented Jul 14, 2017 at 1:19
• I actually had tried that calculator as well, but didn't know what some of the values were, and didn't know why some were needed. Can you add some clarity to the variables it's asking for? Commented Jul 14, 2017 at 1:51
• Span - 10ft Spacing - 16 (default - is this necessary? I'm setting up a single beam.) Load of least duration - Live load I guess? Not sure what least duration really is. Wet service conditions - no Species - Hem fir Live load - 200lbs (I assume since it's going to be on a single attachment point, that means all the weight is the live load? Dead Load - 0lbs? If nothings suspended from it when not in use? Dimensions properties - 2012 NDS (default - what's this? Grade - No. 2 allow. live load deflection = span - 360 (default - ?? Allow. total load deflection = span - 240 (default - ??) Commented Jul 14, 2017 at 1:51
• I'll mention, that based on the variables as defined above, I supposedly need a 4x10 which seems nuts - though frankly I don't know. Commented Jul 14, 2017 at 1:57
• You may find this one easier to comprehend. But you'll have to pick a size. woodbin.com/calcs/sagulator Commented Jul 14, 2017 at 2:46

You can do the math by hand or look for a beam calculator online, but it's easier if I just use engineering software I have to size this beam for you.

Use 3 plies of 10' long 2x6 to support 400# located at the middle of the span. With 400# that will always be there, it will deflect 0.18", so less than a quarter of an inch. I sized SPF #2, so you'll be fine using any 2x6 stud you can find at your big box store.

Nail the three plies together with 4-1/2" 30d nails in two staggered rows. Technically this detail is for a built-up column, but just use the same nailing pattern.

If your next question is what if I sized it with 200# instead of 400#, the answer is that you can use 2 plies of 2x6. It will deflect 0.14", and nail them together in the same pattern but with 3" 10d nails. Personally I wouldn't do this though, because kids like to hang on these bags and I think the 400# loading and 3-2x6 makes the most sense.

• I really appreciate you running the numbers for on that. I am disappointed I can't apparently just figure it out simply (as in someone with no engineering background). Again, I do appreciate the detail in your answer though! Commented Dec 6, 2017 at 4:51

EDIT: It was pointed out that this only calculates dead load which a beam with a swing on it will have a live load, so it doesn't work. I'm not going to delete this post because I think the sagulator is a good tool that is worth note, but hopefully, this comment helps others in my situation know why it doesn't work for this need.

@ecnerwal actually provided this in the comments to my question, but I'm posting it here so it shows up as answered.

There's The Sagulator which does what I need without needing to know lots of the other variables that other beam load/deflection calculators ask. The only caveat is you tell it your board size and it tells you if it's acceptable or not (not too hard).

I think for the needs of identifying what will hold the center weight I outlined above this will be my go to for basic stuff, but I would still probably involve an engineer to verify anything that is going to be structural.

• This is a shelf calculator, so is based on a dead-load. Your swing is going to be a live load with significantly higher peak load when in use, as well as having sideways load as the user swings side-to-side. I wouldn't trust a single 10 foot long 2x6 to hold a swing. Since you're risking serious injury if the beam fails, I'd probably go with a pair of 2x8's, or even 2x10's. Commented Jul 18, 2017 at 16:15
• DAMN - you're right. I totally forgot about live vs. dead load which is something beam calculators take into account. ech... ok, I'll see if I can unmark this as an answer because as you pointed out, it's not sufficient for the need. By chance do you know if there's something comparable for beam calculations where in you only need the data I provided, or must you always know all attributes that this one wants - courses.cit.cornell.edu/arch264/calculators/example8.1 Commented Jul 18, 2017 at 21:08