- posted
16 years ago

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

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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

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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.