Question about the strength of Douglas fir

Question

I need your help. I am a rock climber and I use a hangboard (photo) to train finger strength. It's a wooden board mounted on a 3/4" thick piece of plywood, which is screwed into some studs with some 3" deck screws.

I'd like to build a few rock climbing holds out of wood (like these)

and mount them around the edges of my hangboard. My plan is to shape the wood, then use wood glue and a couple of screws to hold the pieces onto the plywood. However, I'm worried about the wood breaking and falling on my butt.

That's why I'm seeking your help. I am wondering if douglas fir is a strong enough wood to use for this purpose. I found it much cheaper than oak at home depot, since they only had oak in decorative pieces for stairways. I'm also looking for any general advice for building some holds that won't break on me.

Thank you for any help!

Answer 1

I'm not an expert in wood species, so maybe someone who knows more can weigh in more authoritatively, but I'd be inclined to use a species like Maple for this purpose. I think you'll find that's what the holds in your picture are made from.

Douglas Fir is strong in the sense that it can bear high compressive axial (in line with the grain) loads. It probably does well with axial tension as well. This makes it good for a lot of purposes in construction, which is what the studs at Home Depot are generally used for.

Where it is less fabulous is in tension in the radial direction (sideways to the grain, roughly speaking). So it would have more of a tendency to fail by splitting than something like maple.

Also note that any screws used for affixing the holds should not be of the countersunk variety. You can see in the holds you've pictured that the screw holes are counterbored, meaning the force of the screw head is directed parallel to the screw body (straight in, loosely speaking) rather than at 45 degrees in each direction, which would work to split the wood. So you would use pan head wood screws or perhaps preferably socket head or button-head socket cap screws to attach the holds.

This latter variety would require a tee-nut or similar to be fitted behind the plywood but would provide superior strength.

Answer 2

I am wondering if douglas fir is a strong enough wood to use for this purpose.

It might be, but it would depend on a few factors. These include how it's used (force relevant to the grain direction), whether the wood is bare or has a finish applied, and whether solid pieces or glued-up sections are utilised — one or two pieces pictured in the Question aren't made from the solid so they are glued assemblies. And when doing this the strength of the glue joints would be absolutely critical to strength because get it wrong and glue joints are weak, get it right and glue joints are as strong or stronger than the wood itself.

But the main thing at the end of the day is likely to be the selection of the stock to begin with. If you take two random pieces of fir from a pile one might have much closer ring spacing than another (where they might be as much as a finger width apart) and the first piece will be significantly stronger than the second piece. This is true of all common softwoods because the light-coloured bands are much softer material than the dark bands. The dark rings are tough and strong, the light wood in between is soft (easily dented with a fingernail) and much much weaker.

Anyway, whether fir might be strong enough I think is a question that can be put aside by looking at the wood chosen for the pieces pictured.

Hardwoods
If fir were a good choice (or even good enough) I think there's a good chance they'd have used it, but they haven't and instead they made them from what looks like maple.

Maple is in many ways an ideal choice for this sort of thing being a very strong close-grained hardwood, relatively easy to get and usually not too expensive. A quick Google search has shown me that oak is also used (red or white I can't tell).

I found it much cheaper than oak at home depot, since they only had oak in decorative pieces for stairways.

You really need to look for other sources of wood in order to be able to reliably buy good stuff. A specialised seller, what might be called a lumber yard where you are (UK: timber merchant), is a much better place to purchase wood.

Not only do they have a better selection (by an order of magnitude!) the wood will likely be far better quality to begin with, handled better so that there will be fewer issues related to poor storage, and still be cheaper than from somewhere like HD. In the US individual lumber yards will have differing prices for various species of wood depending to some extend on what is available locally, but the standard hardwoods including oak and one or more types of maple will generally be relatively inexpensive all across the country.

I'm also looking for any general advice for building some holds that won't break on me.

Because of the SE format this should be asked separately, if necessary splitting your query up into individual Questions on each aspect of shaping and assembly, and then finally mounting.

Answer 3

It isn't so much about the wood as it is about the screw. Almost any wood would be fine as long as you don't have a bad grain. Now the screw is a different issue. https://www.screwdoctor.com/application/home/selection_guide/technical_data.aspx These are the shear values for screws. If you are attaching thru dry wall material the screw isn't in pure shear anymore because the dry wall can't resist the shear and the screw can bend. I think you should attach some 3/4" plywood to the wall and then screw you blocks to it the the values are accurate.
This is the subject of chapter"S" in engineering books so one paragraph isn't all you need to know to be safe. Hope this brief answer helps.