Help me design a metal bracket

August 7, 2012 5:59 PM

I want to know the minimum thickness for a steel bracket that attaches a table leg to a table top.

I'm designing a table like this.

I'm in the process of designing the steel bracket that joins each leg to the top. The bracket will be held in place with 3/8" steel bolts going into inserts (three of them on each face, with the inserts and bolts penetrating 1/2" into the wood).

The bracket is 16 inches wide and its legs are 4" and 2" long. One bracket-leg is shorter so it can be hidden between the leg and tabletop.

The table legs are 27 inches tall and 2 inches thick. The top is 2 inches thick and weighs about 200 pounds, if that matters.

The questions are:
1. How thick does the steel bracket need to be?
2. Brake-bent sheet material, or extruded angle steel?

I want to be sure that either the bolts or the wood fails before the bracket. The worst-case loads I can think of are:

- Heavy objects falling on the table (e.g., in an earthquake, a few hundred pounds of roof might fall down, and I'd like to be safe beneath it in that scenario)
- Whatever future children might do (e.g., running into it at top speed)
- Careless movers whacking a leg as they go through a doorway

The bracket will be mortised into the leg, so the thicker the metal is, the thinner the wood will be. I don't want to just use the thickest steel I can find because it'll be more work to get it mortised in and will possibly reduce the strength of the wood.

posted by jewzilla to science & nature (9 answers total)

Bent sheet metal. We just built a bunch of brackets like this at work--pretty sure our metalworker used 3/16ths, and these are designed for holding 20-50# of lighting fixtures. If you bump it up to 1/4th" you ought to be more than fine. Ultimately, the brackets, in ideal usage, wouldn't be under load except in a side-load situation. The legs would bear the load of the top the majority of the time.

Thoughts on bolts: Are you thinking lag bolts or are you going to be putting some sort of insert into the wood that the bolts can screw into? A regular bolt and wood isn't gonna work--the threads aren't deep enough.
posted by mollymayhem at 6:14 PM on August 7, 2012

Thanks, mollymayhem. I'm using steel inserts like these: http://www.rockler.com/product.cfm?page=362
posted by jewzilla at 6:19 PM on August 7, 2012

What I would do is drill down into the leg through the top and counter bore into it. Then crew a lag bolt into the leg and into the counter bore and plug the bore with matching wood (or contrasting). This will hold better than a bracket, IMHO.

Alternately, I would consider making ersatz mortise and tenon joints by using biscuits from the legs into the top. This will be strong for compressive loads, but will likely have some problems with racking, so for that I would use small brackets (1/8" thick, 1" long per side) on both sides of each leg to keep the racking to a minimum.
posted by plinth at 6:20 PM on August 7, 2012 [1 favorite]

Are you talking about an L-shaped piece of metal? Nothing about that supports any load. It just makes sure the leg doesn't topple sideways if the table is pushed from the small edge.
posted by odinsdream at 6:40 PM on August 7, 2012 [1 favorite]

In case it helps you think about loading, I think that the worst loads your brackets will see is when some big guy pushes hard on the edge of the tabletop (like to scoot the table across the floor). Assuming that the leg is stuck to the floor by old gum or something, that's a fair bit of force on that joint. You want a thick enough piece of metal to not bend under that much leverage; vertical loads are irrelevant. The wood won't break under any loading you can give it; those short bolts are what will tear out.
posted by Forktine at 6:56 PM on August 7, 2012 [1 favorite]

Agreeing with Forktine, it's sideways pressure that you need to worry about, and the weak point will be the inserts in the top which will tear out. This is not a very good way to attach legs to a table; the benefits are that it takes relatively little skill, could be cheaply mass-produced, and can be disassembled, but strength is not one of its virtues.

What species of wood are you using? The holding power of those inserts will vary quite a bit with the hardness of the wood.

Solid wood planks expand and contract across the grain with changes in humidity, but a hot-rolled angle iron does not. This means you'll need to have the holes drilled oversize (or milled as slots) towards the outer ends of the brackets so that seasonal movement doesn't crack the wood or shear the bolts.

Making the bracket (ad thus the mortise) thicker won't weaken the wood. If I had to use this sort of design, I'd probably choose 1/4" angle iron. It would be better would be to skip the angle iron altogether and bolt through the top like Plinth suggested using 4" or so lag bolts. Better still is to add a stretcher between the legs to strengthen them against racking, at which point you can practically attach the top with bubble gum because the strength of that joint is so much less important.
posted by jon1270 at 3:19 AM on August 8, 2012

Another thought: with 2" planks, there's no need to use inserts unless you intend to take this apart and put it back together frequently. 1.5" lags would probably be stronger.
posted by jon1270 at 4:11 AM on August 8, 2012 [1 favorite]

This is in black walnut. The top is a solid slab and there is absolutely no way I am marring the top by putting a lag bolt through it. I would think that the inserts would not be ripped out by a torquing force, which is what we're talking about. They are very well supported on the sides, as they're half an inch into the wood. Thanks for the suggestions on thickness...I'll see if I can do 1/4".
posted by jewzilla at 10:53 PM on August 8, 2012

I can't blame you for not wanting to bolt through the top, but walnut is soft as hardwoods go. 1/2" penetration isn't all that much, and a 27" lever is a heck of a force multiplier. I have no doubt that the table will withstand hundreds of pounds of vertical loading, but you're going to have to insist that your movers be careful when going through doorways. It's good that the OD of the inserts is fairly large.

A ~16" wide plank of black walnut might change width by around 1/8" between summer and winter, depending on where you live, whether you've got central AC, and the permeability of the finish used. Also, a plank that finishes out at 2" thick will probably have been sawn at 10/4 (2.5" thick) and is unlikely to be kiln-dried, so the moisture content may still be high at the time of construction. It could shrink quite a bit during the first several months it is indoors. Slotted holes at the ends of your bracket are probably fairly important.

I'd also suggest you stagger the holes towards the edges of each leg of the bracket, rather than having them all in a straight line.
posted by jon1270 at 4:22 AM on August 9, 2012