Re: Suggestions to improve my swing set frame design

On May 29, 1:18 am, "Lew Hodgett" <sails.m...@xxxxxxxxxxx> wrote:
SFWIW, if you truly need an 18 ft open span beam, consider the
following:

1) Build an "I" beam construction consisting of 20 ft long 2x6 top and
bottom flange with a 2x12 web.

2) Since the web is basically dead weight and contributes "Zip"
towards strength, lighten it along the lines of a bar truss design for
a flat roof industrial building.

Start with a 2x12x10 ft timber and cut it into 5 pcs, 2x12x24" long.

Locate a 2x12 at each end of the 2x6x20 pieces, then space 30" to the
next 2x12x24 piece, then repeat.

What you end up with is a 20 ft long structure with 5-24" wood pieces
and 4-30" open spaces alternately spaced.

Assemble with some 3" galvanized deck screws with hex heads if you can
find them.

Drill pilot holes for the deck screws.

You end up with a beam that is "strong like bull" yet light enough
that two people can lift it into place.

Deck screws are too light, and 3" is too short - only 1.5" of
penetration? And how many is "some"? Most engineering designs fail
in the connections and the lack of detailing.

If I'm understanding you, you're trying to give the OP a design for a
full length truss with no intermediate support, but it's not really a
truss. A truss joist has diagonals that are opposed. Where one
diagonal is pushing or pulling, the meeting one is resisting that
force - there is no distance between where those forces are applied.
In your design you are relying on the 2x6s on flat to resist those
forces, there is a distance between where the forces are applied, and
the screws at the ends of the 2x12 blocks will be required to carry
more load than 1.5" of penetration will allow. This will tend to sag
over time. I didn't run the numbers, but it will happen.

A swing set should be designed with a large factor of safety and it
should be designed for the occasional show off adults competition
really going at it. There's also the cyclical nature of the applied
loads, the combined tension and shear load on the screws with the 2x12
design, etc. A #12 wood screw has something like 200 lbs of pullout
resistance in SYP (PT wood) per inch of embedment - 1.5" =~300 lbs. I
would think you'd need a minimum of seven or eight hundred pounds per
fastener for the end conditions with the 2x12 design instead of three
hundred.

It would be better if the OP skipped the 2x12, which has problems with
shrinkage and twisting, and used some 3/4" pressure treated plywood as
the web. Even better would be to use the 3/4" plywood and PT 2x6s
(might be able to get by with smaller 2xs) and build a box beam joined
with construction adhesive and 2.5" epoxy-coated deck screws at 4" -
6" on center.

The top of the beam should be covered so there's no standing water on
top of the beam and water won't enter the edge of the plywood. The
plywood joints should be staggered and - belt and suspenders - screw
and glue plywood scabs to join the ends of the plywood pieces to each
other.

3) Attachments.

I would use loops around the beam rather than drilling holes thru the
beam for bolts for the swing chains.

Having the swings hung from the top of the beam is definitely
preferable with a truss or box beam. I have no idea what sort of
hardware is available for a swing set, so I can't recommend any
specific method of attachment. Another poster mentioned the hardware
is expensive, I'd price that stuff before finalizing a design.

Have fun.

The most important part!

R

.

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