Originally Posted by
Tom Veatch
Assumptions:
1) 6' 4x4 clear Pine beam loaded with equal vertical loads 1' from each end and simply supported at each end.
2) Actual cross section dimensions of the beam is 3.5"x3.5"
3) 1/2" hole drilled vertically through the beam at each load application point.
From "Standard Handbook For Mechanical Engineers", Seventh Edition (AKA "Marks Handbook"), the fiber stress at the proportional limit for clear shortleaf pine is 7700 psi. Applying a safety factor of 4 gives an allowable stress of 1925 psi.
These values and assumptions used in a very simple (f=Mc/I) bending analysis of the beam indicates the allowable stresses in the beam will be reached with a total static load on the swing of 1965 lbs (983 lbs at each attachment point).
This does not consider crushing strength of the beam at the joint with the vertical columns or at the swing attachment points, nor does it consider any dynamic loading on the beam or the columns. It does not address the column strength of the vertical supports. It only addresses the bending strength of the beam supporting a static load in the swing. Beam deflections are not addressed.
Knots, cracks, splits or other deviations from a clear, straight grained shortleaf pine beam will reduce the load bearing ability of the beam. The applied safety factor of 4 is an attempt to account for those deviations.
All in all, since it's doubtful the swing itself will support almost a ton of dead load, I'd say that you'll break the swing before you break the beam.