I just completed the tedious task of assembling a large cedar playscape for the kiddies. This weekend, Grandpa (all 275 lbs. of him) sat on the middle swing and made a few passes. I gasped at the deflection in the 4×4 overhead cedar beam that supports the swings (span is approx. 8 ft). I don’t expect Gramps will be swinging very often, and I don’t expect my 3 year old to approach his size soon, but…
I’m thinking of sistering another 4×4 (or other size) to this header to stiffen it up. MY QUESTION is: What do you think is the best approach?
I will try to attach some cross-sectional drawings of my ideas. I am limited (due to protruding hardware) to the middle 50% of the length (i.e., whatever I attach cannot run all the way to each end).
The drawings are supposed to show 3 options:
1 – Attach a 2×4 along the top of the 4×4.
2 – Attach a 4×4 along the top of the 4×4.
3 – Attach a 2×6 (or greater) along the back of the 4×4.
Plan is to use pressure-treated pine.
What will yield the best results? Any suggestions on fasteners?
Many thanks.
Replies
Here's the attachment
Boss Hog is right, depth is what gives you strength. So look at your three options in terms of overall height. A 4x4 with a 2x4 attached to the top on edge gives you approx. 7" overall height. Ditto for a 4x4 stacked on top. The extra width of the 4x4 will be a little stronger than the 2x4, but it will be relatively marginal. Sistering a 2x6 on the back of your 4x4 obly gives you 5.5" overall height, so will not be as strong as the other options. Plus, by making your cross-section assymetric, you'll introduce some twisting loads you may not want.
But they're probably all more than enough.
Adding things on top would give him more depth, but it would only be effective if the pieces were connected together well. I should've been clearer about that in my first post.
If you're happy and you know it see a shrink.
Thanks Boss and Bart. Any thoughts on the best way to fasten?
Does the stress on the fasteners impact the option I choose (on top vs. attached to rear)? I would think if I attach to the rear, the fasters will face shear forces as the combined unit flexes, while a top attachment will pull longitudinally on the fastener....? Just guessing based on basic mechanical intuition. I was thinking of using timberlok screws.
In either case, your fasteners are going to be in shear. That's good, because most fasteners are strongest in shear. But really, you'd have to put a couple of grandpas on the set to start shearing almost any kind of screw or bolt.
Go with whichever option is easiest for you to build, they're all good.
Depth is what gives you strength. Adding a 2X6 to the back will probably do the most good.
God must love stupid people he made so many.
Here are some thoughts on your situation. The strength of your top beam is related, among other things, to a property called moment of inertia, which has to do with the geometry of your beam Assumming that you can securely fix the beams together and that the two pieces of lumber have similar physical properties (maybe), here is an idea of the strength of your original and proposed arrangements. I am only considering the moment if inertia in the vertical direction. I only considered bending and not shear forces. Bending usually governs.
The units of moment of inertia are inches to the forth power. The ratio of one moment of inertia to another will give you the relative strengths of each arrangement. These are approximate numbers for comparison only. Other factors such as span, end connections, orientation of grain, etc must be considered to yield a load which the swing could safely support
Original 4x4 12
4x4+2x4 62
4x4+4x4 100
4x4+2x6 38
The back to back 4x4's are obviously the strongest. To join them, the addition of a real good glue wouldn't hurt in addition to any hardware you put on. I have never used any, but that polyurethane glue that a well know master carpenter uses on a popular Saturday morning TV woodworking show might be worth investigating. I understand that it is waterproof. Take care.
Thank you Mikey. Very interesting.
Without understanding the specifics of your analysis, I'm curious why the 2x4 addition would be stronger relative to the 2x6 addition. Do you assume mounting the two differently?
Thanks again.
The 2x4 on top of the 4x4 yields a beam depth of about 7". The beam depth with the 2x6 on the side of the 4x4 is only about 5 1/2". Depth is the name of the game when it comes to strength in the vertical direction. Put a 2x8 in place of the 2x6 and both built up beams would be pretty close. Put a 2x8 on the the front and back of the 4x4 and you are stronger yet. Cover the top of this "U" shaped built-up beam with a horizontal 2x8 and you really are making a strong beam.
You could sister on a 1 x 4 with "kids only" carved on it. To protect grandfather's feelings, create a diversion by blaming yourself. Maybe something like, "Boy, I wish I had beefed up that beam a little more, so just to be safe, all us grownups are staying off."