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I'd like to develop an intuition for how the stresses are distributed in a simple wooden structure (in this case, a bunk bed). One option is to spend years building bunk beds, but I'm hoping I can develop some intuition a little faster.

What software could I use to visualize the stresses on and bending of wood in simple geometric structures? I assume I'm looking for some kind of FEA (Finite Element Analysis) system, but it's not clear if any of them are built to do anything this simple.

What software would you recommend for under $100? What if money was no object?

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  • I think the standard answer is "over build it" since that's often cheaper and less error-prone than the software. – BMitch♦ Nov 19 '13 at 19:05
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    Just so you know, this type of analysis is 3rd year rigid frame analysis. You need a solid grounding in Statics, Mechanics of Material and structural analysis, as well as some pretty good matrix algebra and integral calculus techniques. There are some free unix programs around. I recall a product called SDTruss, which did FEA but it was a couple of weeks study just to figure out how to get the input file right. The sign convention was awkward at best. – Chris Cudmore Nov 19 '13 at 19:48
  • Does rigid frame analysis include the effects of members bending? – LoftyGoals Nov 19 '13 at 20:02
  • And, yes, thanks to y'all (seriously -- thank you), I've decided to over build rather than use math, but I still want to play with structures to improve my intuition. – LoftyGoals Nov 19 '13 at 20:03
  • @LoftyGoals Yes, that's why Mechanics of Materials is important. Statics is the study of rigid, statically determinate rigid frames and trusses. (Static = not moving). To solve for indeterminate (over constrained) frames, you need to take into account material bending. – Chris Cudmore Nov 19 '13 at 20:20
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Other than knowing they are out there, the money no object end of the spectrum is not a place I play much, and you can easily exceed $250,000 for software AFAIK.

For practical applications where the right answer is not "hire a structural engineer and let him/her worry about the software" (or become one, if it interests you enough) you need to understand enough of their lingo to use the simplified tables and calculators that are available from various places.

  1. A whole set of calculators from Cornell
  2. A wood beam calculator
  3. Another wood beam calculator

I think all of these (or most) do include a bending term, either as a limit on input, or as an output. Your loads generally need to be self-calculated from (pounds-per-square-foot X square feet supported by beam), though some of the calculators will do that for you. Pay attention to whether you are being given a number with a safety factor built in, or not. Realize that wood is a variable material, and if you use a piece of wood where a flaw is stressed, it may not hold what the calculators suggest based on idealized wood and the assumption that you would have rejected such wood or used it in a manner that would not stress the flaw.

For the specific part of the question where you mention "developing intuition" there is a great deal to be said for building small models (the more accurate the model, the more accurate the intuition gained will be) and stressing them to failure. Grab a couple bags of popsicle sticks and tongue depressors (or both may be labelled as "craft sticks") and make some piles of broken kindling.

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  • Unless the user knows the specific type of calculation that's relevant, tools like those are not likely to be helpful (or may be misleadingly simple). – Hank Nov 20 '13 at 2:28
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    LoftyGoals mentions in final comment having taken 2nd year MechE courses, so I'll trust that he can read up enough to see what's relevant, if he's interested in using them. – Ecnerwal Nov 20 '13 at 2:32
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Also, if you're feeling brave: http://frame3dd.sourceforge.net/ Frame3DD is free open-source software for static and dynamic structural analysis of 2D and 3D frames and trusses.

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  • That certainly sounds like what LoftyGoals is after. Cool. Thanks! – Ecnerwal Nov 20 '13 at 21:23
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It sounds like what you're looking for is a program like Sketchup where you can build a 3D model of your structure and then press an Analyze! button and have it give you feedback about the general structural integrity. There is no such program.

First of all, any software package capable of doing anything more sophisticated than a single beam or so is probably going to be too complicated to use without training or experience.

But more importantly, you need to know the kinds of analysis you want to do for the program to be useful and for you to get the answers your looking for out of it. E.g.: Do you want to check for lateral torsional buckling? Euler buckling? Block shear failure? Have you modeled the bracing points accurately? What kind of loads have you applied and in what direction? What assumptions did you make when modeling contact points with the environment? Is it important that you model the stress in the fasteners?

You can certainly find tools that will do a specific set of calculations, as @Ecnerwal has linked to. But those are very narrowly focused and require that you already know the type of calculation you're interested in. Also they probably require you to simplify the problem to focus on the aspect you're interested in.

Side Note About This Project

I think this King-sized loft bed is an interesting idea and I've been enjoying following your progress, but I am concerned that this may be too big a project to bit off for a new DIYer. A king-sized bed with a mattress + people could be very heavy (I would plan on at least 1000 lbs of weight... preferably more). Plus raising it off the ground adds a host of additional technical challenges and safety risks.

If you do want to see this project through, let me offer these general stability suggestions:

  • Plan for a lot of weight. More than you think is reasonable. Some day your teenage kids are going to climb up there with all their friends and their 20 lb. book bags and your 2 golden retrievers.
  • In addition to the gravity weight, think about how the structure can collapse in a trapezoid shape. For a cube-shaped structure like this, there are 3 possible trapezoid collapses. (Diagonal bracing in all three directions is probably the easiest way.)
  • I would not rely too much (or at all) on the walls for support unless you know what's back there. A standard 2x4 framed wall is not going to be remotely strong enough, even for bracing against sideways collapse.
  • After you've finished, check the joints regularly for several months until you're sure that the piece isn't pulling itself apart.
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  • I guess all those folks who have an entire second story of a house setting on 2x4 stud framed walls are just living in denial, then. – Ecnerwal Nov 20 '13 at 15:29
  • @Ecnerwal: Wood framed houses have a subset of the walls specifically designed as load bearing, with double plates and careful attention to fasteners. Also there will be some walls with diagonal bracing and/or shear walls with specially-nailed plywood panels to provide lateral support. An arbitrary interior wall may or may not be capable of supporting serious weight. I'm not saying it's impossible or even difficult to make a load-bearing wood structure, just that it requires some thought and design. We're talking about substantially more weight than hanging a flat-screen TV, for example. – Hank Nov 21 '13 at 0:40
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Check out Canadian Wood Council calculators: http://www.cwc.ca/index.php/en/resources/electronic-tools

Hope that helps!

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