How much weight can a 2" x 6" wood plank support? Engineering or construction expertise please.?

Question

There is a 2" x 6" wood plank attached horizontally to two vertical 4" x 4" posts. The posts are spaced 3 feet apart. The plank is attached to each post with 2 bolts/nuts on each post. The plank is located 8 feet high on the posts from the ground. The plank extends four feet from the 2nd post. So the plank is a total of 7 feet long. The posts are anchored in the ground with concrete.

If a tire swing is attached to the end of the plank, how much weight can it safely support? thanks!

Answer 1

Doing a simple bending analysis i can tell you that if it does break it will break at the beam nearest the swing.

To aleviate a lot of stress at this point you could add an angle brace (knee brace) at the support nearest the swing coming up 45 degrees to the plank.

Put the added support piece in last as you want it to become a support to limit bending, not a new pivot point.

If you have the specific wood type I could try to give you a force of rupture for the plank.  But i've only ever done these  calculations for steel members so it would be more difficult for me than someone with more experience working with strength properties of wood.  ( i saw a few terms dealing with wood that i had not seen before)

Answer 2

It depends on things like the species and grade of wood. There is a web site to calculate it here:

http://www.awc.org/calculators/span/calc...

Since you are only supporting the beam from one end, you need to adjust the calculations accordingly.

Answer 3

Without doing any calculations, I'd say probably not enough.  Deflection and stress in the beam are probably within acceptable limits; what you have to worry about is torsion in the 'cantilevered' section - the overhanging 2X6 twisting and breaking.  

2X6's are only about 4 bucks at the home center, so why not use 2 or 3 bolted together?  A squarer cross section would resist torsion a lot better.  

'When it doubt, build it stout!'