Pot rack and worries about wood strength

[Gary Tunak]

Hi, all! I'm fairly new to woodworking, and I'm making a pot rack out of cherry for a couple of my friends who are getting married. This is my first post here since registering about a year ago. Aptly, this project is also a first in many ways: first project with a carcase, and following that first time making something with multiple joints that need to all align with each other, first project with more than just 3 pieces, and first project for someone other than myself.

I have a question about wood strength. I'm always worried if the wood will be able to take the loads put on them. Is there some general rule of thumb or general design principles when it comes to this type of stuff? I know about what joints are strong in what directions of force, what joints are stronger against racking in a frame, stronger against coming apart, good gluing surface area, etc., but I'm worried more about the wood itself completely fracturing, the strength of the material.

While the pots themselves aren't too heavy, I do want it to be absolutely safe and be open to heavier things in the future, like cast iron cookware. Here are a couple of images so you can see how it is supposed to come together.





All the lumber is 3" wide x 1" thick, except where carved of course. The dimensions of the rack are 36" x 16" x 3".
As you can see, the side rails (the ones with the mortises) are going to take the downward force of the "ribs" and any pots and pans hanging from and resting on top of them. I worry that the wood will fail here and the mortises will explode open downward.

I also worry about how to hold this up at all. I bought cross dowels/dowel nuts/barrel nuts that have a diameter of 10mm and are 16mm long, and am planning on installing them at or near to the ends of the side rails, with an eye bolt going in each one (2" long 1/4 20 threaded portion, rated to lift 500 pounds each). So the front and back rails don't actually bear any weight here--it's all on the sides. It seems like it could be awful lot of weight for four little round nuts to carry. However, I don't want to put long bolts though the frame because either I don't want a lot of hardware showing.

And a yet third worry is the skinny portion of the "rib" under each hole for the hooks. I worry that the wood will break there too from the weight of a pot pulling down on such a small surface area and thickness. Here's a picture of the rib, where the edge on the left will become the bottom edge bearing the weight of hanging pots once the rack is assembled:

Yes, the bottom edge is heavily chamfered at 30 degrees so the edge is fairly thin, from 1" down to probably 1/4" over the length of an inch or so. I did this is so that the hooks can freely swing without binding against the faces of the wood.

I am really concerned with how this will work. I've put a lot of time into this already, and it would be pretty dumb if it couldn't even do what it's made for. Do you have any feedback or thoughts? Suggestions? Comforting words? Stories? Cautions?

Thank you in advance!

[Wayne Lomman]

Gary, machine up some extra pieces and make a test mule. Load it up and see what it will take. What you are doing has no set in concrete rules unless you want to get an actual engineer to do calculations.

Another approach if you think it is borderline is to introduce a steel reinforcing rib inside the timber. This can be set in a groove on top or inserted in a way where it is not seen.

Your design and workmanship look great so keep at it. Your dilemma always happens with one off jobs. Cheers

[Gary Tunak]

Thanks for your advice, Wayne, and for slogging through all that. I get wordy with things I'm excited about Unfortunately, I don't think I'll be able to machine up a test piece so quickly and conveniently. I'm doing all of this with hand tools--I just don't have space for the bigger stuff.

Steel ribs are a good idea. This might end up being visible but I think it's okay for the sake of safety. For the concerns with the side rails, I'm sure the rib will have to be along the bottom edge. If I have to go that route, I may as well fully commit to strength and get a long, wide strap that can reinforce the bottom by screwing up into the rails. I'm sure this will be strongest. As for the cross rails, the bottom edge is too thin and not straight to contain a rib neatly, and a rib on the top edge wouldn't address the third concern. But I think it might be okay, really...

Thank you for your compliments--very encouraging to a beginner! I think my future will be filled with one-off jobs, and while I am doing this because I enjoy it, it wouldn't hurt to make some money back off of some production designs, too. Well, as production as you can be with hand tools.

[Malcolm McLeod]

I'm 90% sure that your design has more than adequate strength, and 93.4% sure that 63.5% of statistics are made up as needed.

Slightly more seriously, cherry is my favorite and assuming no preexisting structural flaws in the wood, I'd allow that your ribs with a 1"x1" stub tenon will hold 200lbs - - or 100lbs on each end. I would have no qualms about standing on a 1X3 on edge supported by those cherry tenons, and its been 30 yrs since I was skinny. (7 pots/rib x 10 lbs/pot = 70 lbs :: well within my estimate, and most pots weigh a lot less than 10 lbs)

As for the holes in the ribs, I expect that they would easily support a point load of 20-30 lbs each (and I have a 20" cast iron skillet that doesn't weigh this much). This is more than most people would be willing and able to hoist over their head to hang it from the rack in the first place. I also think that half or fewer of the hangers will actually get used - - there is a practical limit to how many pots you can fit on the rack. I'd be surprised if there was ever more than 70 lbs total placed on the rack, and if it ever did exceed this, my biggest worry would be the failure of the anchor points in the ceiling - not the rack.

In short, skip the steel ribs and press on! And I love the curl in the cherry - - good stuff!

[Gary Tunak]

Awesome. Thank you, Malcolm. It sounds like I don't have much to worry about the ribs and their tenons. Still worried about the mortises blowing out though. This is where I'm thinking of some steel strapping or something that would distribute the weight from the ribs across the whole height and length of the rail rather than just under each tenon.

The cherry is beautiful. My friends asked for a "farmhouse" style rack. Not really sure what that means, but Google showed lots of painted and weathered wood, but after planing the cherry, I couldn't resist just leaving it bare and letting them weather it naturally through use.

[Malcolm McLeod]

Still worried about the mortises blowing out though.

Skipped the mortises because it would be the least of my worries. As a precaution, place the barrel nuts as low in the end-rail as possible; try to keep the top of the barrel nut hole below the bottom of the mortises. This will keep the wood loaded in compression as much as possible.

But if you're still not reassured, I'll second Wayne's idea of a model - even if in a weaker, cheaper wood. No need to duplicate every joint; just 2 pieces (rib & end), cut a tenon on rib, mortise in end piece, drill a hook-hole in the middle of rib (worst case loading), support the other ends of the test pieces on blocks, and then load the hole with weight until it fails. Testing is probably easier than redesigning/incorporating reinforcing strap.

I think Wayne's idea of the (hidden) steel reinforcement is very imaginative. I just think it's not necessary in this particular case.

...and maybe just a little BLO to make the curl pop?

Edit: If the mortise is the worry, just cut a piece of 1x3, 6-8" long, cut a mortise in it, support the ends, pass a rope thru the mortice, and load the weight to the rope. Be reassured.

[Gary Tunak]

Yep, I'm thinking the 2" eye bolts are a bit too short now. I'll go get longer ones today at the home improve store. And possibly a metal strap to span the length of the edge, but only if it looks pretty and if so, maybe through bolts, too. It shouldn't be too difficult to do the way I'm imagining, but it's just not as elegant as I initially imagined the project to be, but I'll test it first. When I imagined "test piece," I imagined a prototype. You're right, I don't need to rebuild the whole thing now that I think about it.

Thanks for all the help and ideas.

I was planning on shellac and wax because it sounded durable. I have lacquer, but don't want a glossy finish; and I have Tried & True danish oil, which I think is kind of like BLO, or at least polymerized if not boiled. I'm not too good with finishing yet so it is usually an afterthought for me while I work on my joinery technique and skills.

[David Utterback]

Hey,
Looks like a great project. I would not be concerned about it failing under load. You may wish to consider an alternative finish since grease is likely to accumulate on the surfaces over the years. The shellac, lacquer and wax would not withstand detergents used for cleaning. A penetrating oil looks good on cherry and oil based film finishes would also be durable.

Best of luck!

[Gary Tunak]

Thank you, David. Can you clarify what you mean by a film finish? Polyurethane and varnish, maybe? I thought lacquer was actually very durable.

Actually, I already applied a coat of this thing called Zinsser sanding sealer, which I believe is a shellac, but seems to be more versatile. I'm not exactly sure what it is, but it has great reviews on Amazon. It makes the grain pop, for sure:

Looks like tiger stripes! It's amazing!

I actually changed my plan from wax to follow up with several coats of oil-based polyurethane, and I plan on burnishing it in with very fine steel wool. I'm not sure how I got this idea... must be in the back of my mind from some YouTube video I watched a long time ago. Even though I want to coat it with a film for better protection in the kitchen, I wish it to look more natural and oiled rather than coated.

[David Utterback]

You are correct on the film finishes. It looks nice.

[William Adams]

FWIW, there is a useful publication on this sort of thing:

Wood Handbook: Wood as an Engineering Material

USDA Forest Service Forest Products Laboratory General Technical Report FPL-GTR-190

http://www.nhla.com/assets/1603/wood_handbook.pdf

[Gary Tunak]

William, that is just what I was looking for. 500+ pages... sounds like bedtime reading to me

I'll make sure to share once the project is done. Thanks again, all, for the help and advice! I am going without any reinforcing splines or ribs or any such hardware. I'm trusting your advice on the mortises and holes being able to hold up the cookware. However, I am putting decorative strapping on the corners and it will serve as kind of a wide washer for the bolts I'm using, so it should be strong enough to keep the whole thing suspended.

[Phillip Mitchell]

I don't have anything to add regarding the structural engineering other than that it looks plenty stout to my eyes for its intended purpose. Concerning the finishing, you may want to consider a few coats of a satin wipe-on poly over your seal coat of the Zinsser sealer (which is just de-waxed shellac.) By using the de-waxed sanding sealer as you already did, you can add an oil-based poly on top of it no problem. If you chose a satin version, it won't appear too glossy after just a couple of wiped on coats, but it will go a long way to protecting the wood in a kitchen environment. Plus, it's pretty easy to use wipe-on poly for those of us that aren't very experienced finishers.

[Gary Tunak]

Wow, I don't know what I was ever worried about.



Took my 8 heaviest pots and pans with no problems. The monster cast iron skillet hung from the center hole without so much as a creak. My wife even hit her head on one walking by, and the rack took the rocking like a champ.

Thanks again for all the advice, everyone! I went with an oil-based wipe on poly over the sanding sealer. I think this one is just about ready to give to my friends.