# swingset design

• posted

I'm planning to build a post and beam swingset out of 6x6 pressure treated lumber. I want to have 2 swings and one tire swing. The top beam will be 16 feet, the posts 12 footers probably 4 feet into the ground making it 8 feet high. The challenge is the tire swing. I have two possible designs:

one

----- cantilevered with the middle post centered somewhere around 8 feet. The tire swing would mount at the end of the cantilever with the swings inbetween the two beams. This allows both swings and the tire swing to be used at one time. I prefer this design, but my concern is the amount of force on the connection point with the middle post and beam.

two

----- post and beam (not cantilevered). The tire swing would go in the middle with a swing on each side. In this design, the two swings would need to be set up on hooks while the tire swing is used. The advantage is the elimination of the severe stress on the middle beam with the cantilevered approach.

so, being the curious type, I snooped around a little and found some momemt of intertia and modulus of rupture calculations, like this...

moment of inertia, l = (W x H^3)/12 and strength, inch lbs = modulus of rupture x moment of intertia

so for me.... l = 5.5^4/12 = 76.3 strength = 76.3 X 8000 = 686,700 inch lbs

ok, so I'm a computer programmer, not an engineer. I have no idea what this number means to me. Doesn't the modulus of rupture depend on the span? Also this applies to the beam and I'm guessing my bigger concern is the post with approach #1.

Also, I originally wanted to use mortis and tenon joints mainly because I've always wanted to do that, but (especially with approach #1), I thought it might be better to drill 4 holes straight down thru the beam into the post and pound rebar or something similar down thru beam into the post.

The real question here is how do I calculate whether the middle post/beam connection will hold in design #1. I'm kinda assuming the second approach won't be a problem.

Any help on this would be appreciated.

• posted

Multiply the weight of you kid (in pounds) by the number of inches from the end of the cantilevered beam to about the center of the first beam (lets say

96 inches) and you get the moment on that point. If your kid weighs any more than about 6800 pounds, you have a problem. If your kid is the type to really swing out a ways, then lets say that anything over about 3000 pounds is going to be an issue. Most very active 3000 pound kids would recognize the bending and take it a little easier on the equipment. The mortice and tennon joints would be ok, though you may end up derating the wood a little. The bolts would be ok. In general, it is better to put a hole in the center of a beam than on the outside of it because the stiffness is generated mostly from the outside edges.

Michael

• posted

For your design reference I suggest you check this text. It may be out of print, but it is an exceptional reference for working with lumber.

"Timber Design and Construction Handbook", McGraw-Hill, Timber Engineering Company, 1956, ISBN

07-064606-6 Library of Congress Catalog Card Number 56-10879

The Timber Engineering Company is (was) an affiliate of the National Lumber Manufacturers Association if that is any help.

The text discusses joints, bolting, etc. In addition, it has the formulas for the beams that you will be using.

Good luck, Jim Y

• posted

did you write that without cracking a smile :)

seriously, I like that center of wood idea. One big one right down the middle. But I think I need to go back and see if I can calculate a sheering force on the middle post. My kids an alot, but I doubt they'll get that big - I think the beam is safe.

• posted

"the MoE of wood perpendicular to grain is about 1/50 the value of MoE parallel to grain"

so, looks like I'm still ok...

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