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:
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.
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
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
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 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.