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Need suggestions for making strongback

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sparky961:
I have a long-term project on the go and the next step requires that I make a "strongback" roughly 12 feet long, with a width between 1-1/2" to just under 2" and height from 3-1/2" to just under 4".  I can deal with some imperfections over the length, say maybe 1/8" deviation over the length, but my main concern is stability.



The obvious answer is to go and buy a 12' long 2x4 at the local building centre.  Although this may be the right choice, I have reservations because the wood I've purchased in the past is practically still growing and often does all sorts of funky things after a few weeks of drying out at home.  If I were to find a really good straight piece, can the stability be ensured by painting or otherwise sealing it?

My second thought was to build a "box beam".  I can use inherently stable wood like plywood, but with a small table saw how do you ensure all your cuts are straight enough, let alone the assembly?  Working with large panels on my Ryobi BT3000 table saw (no infeed/outfeed table or extension) is very unwieldy.

The third and final option I've considered was a piece of 2 x 4 steel tubing.  Stable?  Hell yes.  Overkill?  totally!  Not to mention difficult to mount other things to it that are made of wood, which I will need to do.

So, my question is what's the best (or even a really good) way to move forward?  This seems to have blocked my progress - ie: paralysis through over analysis.

Joe d:
Hi Sparky

I would go with the box-beam, screwed and glued. 
I would expect that your big box outlet would (for a fee, no doubt) cut your plywood for you.

Joe

Lew_Merrick_PE:
There are quite a few rectangular steel tube sections as well as 1.75 X 1.75 X .120 Wall square steel tube sections available.  The 1.75 X 1.75 X .120 Wall tube is 2.7 lb/ft.  Current (as of February) pricing on this in Everett, WA (USA) was $1.15/lb.  My local steel supplier lets me go into the warehouse and cherry pick the pieces I want.

???

mattinker:
You could build the 2 x4 section using4 pieces of  2 x1. We do this in set building when we need larger sections, by gluing (I would screw them from underneath as well) small sections together, the movement is considerably reduced, the centre of your "strong-back" being made of two 2x1s vertically, caped top and bottom by horizontally placed 2x1s. Four reasonably straight pieces can be persuaded to be very straight this way and they remain so even after drying.

Regards, Matthew

S. Heslop:
I'm far from an expert with wood, but to my understanding wood that has the growth rings running flat and fairly parallel to the board tends to be less likely to warp. It might be worth going with planed square edge (PSE) wood, i've had good experiences with it myself. Googling for stuff on wood stability will probably get you better information though.

If going with the box beam then it might be worth using a PSE board clamped to the plywood as a fence to run a handheld circular saw along.

Wood is a mysterious thing though and I think alot of the time it comes down to luck. It's worth designing things using wood to be able to tolerate some warping (and seasonal expansion).

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