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I have a lot of space and want neat design for a glass table. I have designed a table which will have:

  • legs- 4 oak\beech, 3" x 3"
  • frame on top of the legs - 8' long of 40mm (1 9/16) steel angle
  • tabletop - glass of 1/2", 9' long and 3' wide

I want to have a cut across the legs and push the angle beam into the legs, and have them on the same plane as the wood legs (please see a picture below for example) In each leg, 2 holes will be drilled and will fix the steel beam with 3/8 bolts and washer. In the last 2 pictures you can see the entire table and the cross-section of the table legs. The glass tabletop will be glued across the entire steel beam, so although the glass tabletop weigh 200lbs, there will be a lot of contact point and I see no problem around here.

I believe this should be strong enough for both the base load and to avoid racking. I am not worried on racking on the long side, as the steel angle will function as aprons, and be joined in bolts to the wood leg.

I am worried about racking on the short side, as it is could break (the bolt will be installed only 1" from the end of the leg. I liked @Aloysius Defenestrate idea of drilling a hole and welding a rod\conduit for strength, I am just not sure that this metal rod\conduit will not also bend.

How would you advise (other than a model - great advise!) to hanle with racking on the shorter side?

Thank you

table leg design

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    Please stick to a single question at a time. In short, I think you should edit this question and show the entire design and clearly marked dimensions. The static loads will be fine for the steel and posts, but this thing will sway like a pendulem and tear itself apart (with spectacular failure modes) if you don't design it right, so you should focus on the aspects of the build to do with that. – jdv Apr 28 at 20:06
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    Really, this sort of build is more about multi-media installation art than the craft of woodworking. People who are experts on these sorts of hybrid furniture usually just figure it out by building models and smaller studies so they can work out the design kinks. Jumping right into a 300lb dining room installation might be a challenge. Remember that the failure mode here involves a mass of tempered glass. – jdv Apr 28 at 20:09
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    To concur, this is way too many queries for one Question. But as for the first thing, the strength, there's no reasonable limit to what four 3x3 oak legs could carry! You just need to look at basic kitchen tables, with legs (often in wood much less strong than oak) that are much much slimmer than this (and usually taper too) and they can take the weight of someone standing on them no problem. As an adult I have always weighed more than 200 and I've stood on tables numerous times. Last time I did it I was maybe 225 and the table's legs were 2" or less square, tapering to slimmer at their feet. – Graphus Apr 29 at 7:07
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    OP is encouraged to research here: woodworking.stackexchange.com/search?q=racking – jdv Apr 29 at 14:51
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    @AloysiusDefenestrate, it's not uncommon for pics of a completed project to be posted as edits to the original Q. – Graphus May 1 at 15:50
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In my opinion, your question illustrates the first step in designing any project. You have a basic idea and have attempted to solve it as shown in the sketches. The next step (which your question initiates) is to challenge the design concept for viability both structurally and aesthetically. To that end, my comments:

Structural considerations:

  • The design as shown is VERY unstable. There is no bracing from side to side. In fact the only connection between the legs on each side is the glass. No glue connection of the glass to the steel is going to handle the rotational (tension) stress created between the legs and the glass. Think about trying to drag the table in a direction perpendicular to the long axis. The stress on the glue joint will be substantial and either the glue or the glass will fail (the metal angle is way to strong to fail). It might be better to assume that the glass and leg assembly are independent with the glass resting on glazing shims of some sort. Do not count on the glass to act structurally at all.
  • I assume that the depth of your angle's vertical leg of the long axis aprons is only several inches (2"?). This means that the bolts will be less than 1" on center. This spacing is inadequate for transferring the racking force from the aprons to the legs. The steel should take it, but not the wood. So much force close to the end of the wood legs will make it prone to failure, since there is not much wood left above the connectors to absorb/transfer the force. The end grain may split due to shear force.
  • When moved, the angle will have a tendency to twist or bow. If each leg of the angle is not wide enough, then it cannot resist and the legs will not maintain their vertical orientation.
  • You have not dimensioned the member sizes, but your leg dimensions appear adequate to take the vertical load. However, as mentioned above, the connection between the wood and metal, wood will not easily transfer the stress as shown. In addition, the bolts should have a solid (no gap) connection with both the angle and the wood. Relying on the compression caused by tightening the bolt will not work for long if there is any movement in between the parts. The connection will eventually start racking which will mean the legs will not maintain completely vertical orientation. Things will get looser over time.

Aesthetic considerations:

  • The glue joint you propose between the glass and the metal angle will be visible through the glass. Applying enough glue uniformly to present an attractive appearance will be very difficult.
  • The exposed holes for the bolts may be necessary to tighten or unfasten the connections, but is not an, in my opinion, a very elegant woodworking detail. A flush cut dowel would look better, but I would have serious concerns about its strength in this case.
  • You should consider the material for your angle. Looking at your sketch, my first impression was that this was a standard steel angle. Look carefully at the uniformity of the surface. If you desire a polish grade surface, you may find that standard rolled steel requires a lot of preparation work to achieve a desired level of subsurface levelness. Painting a standard steel angle will call attention to the imperfections.

Next Steps:

  • Take the design you have and evaluate it in view of any comments that you receive, {here and elsewhere). Then start over again from scratch. Criticize, your new design and then compare it with previous ones. Rinse and repeat. The design process is iterative and you may go through many variations before you come up with the one that works. Do look at other designs that have been attempted. All solid tables will have certain characteristics in common, for a good reason. Certain design features (such as aprons on four sides) are simply necessary to function.
  • As commenters here have mentioned, you may have to build some mock-ups to test your design before going for the real thing. This is good advice, especially for a unique design such as yours.
  • Short axis aprons are probably essential, although they need not be at the leg corners if the connections between perpendicular angles is solid (welded).
  • Consider welding the bolts through the angles for solid connection rather than relying on tightening the bolts. You could use steel bars instead of bolts and then grind/polish the exposed edges for a flush appearance with the wood legs.
  • Consider vertical rods welded to the underside of the angles extending down into the wood leg to help resist racking between the top frame and the legs.
  • Make certain that your glass top is thick enough to support itself with some weight on the top surface (1/4" plate may not be strong enough).
  • Consider other shapes for the legs to enhance the overall appearance. I am not criticizing the design you currently have, only trying to open up more possibilities. Any wood should be strong enough to support the purely vertical load so the legs could taper in any manner you desire.

As you refine your design, feel free to come back for more comments.

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  • Thank you very much for your inputs and time put for the answer. The glass top is intended to be 12mm or 15mm- so the glass alone will be very heavy, and the table will not move from the place. The steel angle is intended to be 40mm wide (1.5"). I thought additonal drilling with forstner bit, and mixing sawdust with glue to cover the bolts - do you think it will will look aesthetic? – NewDrill May 1 at 16:09
  • In addition, taking into account inputs received from the community, I thought of adding a steel angle vertically inside the leg to take a lot of stress and avoid racking, say 15-20 inches deep. Any thoughts how to drill so deep vertically? – NewDrill May 1 at 16:12
  • To your point about the angle down into the leg... it would be much easier to slice up the leg and reassemble it with the angle inside. A strategically placed bolt or two would ensure that you aren't relying entirely on adhesives. If you choose to putty over the bolts (and that wouldn't be my first choice), consider thread glue and be sure you've got a handle on wood moisture content. (Wood flexes as emc varies, so bolts that are tight in June might be loose in December.) – Aloysius Defenestrate May 1 at 16:32
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    @NewDrill A quick online tells me that your 12mm glass should weigh 6.5 lbs per sq. ft. I leave it to you to translate to metric :). This is 117 lbs. for a 3'x6' table. Not so bad. You may want to talk to a structural engineer to get his opinion on the sizes and design of your steel top frame. (This should possibly cause you at least the cost of a beer or two). – Ashlar May 1 at 17:39
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    @NewDrill You can also buy extended length spade drill bits which could drill down 12" or so. – Ashlar May 1 at 17:41
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I don't think the wood joints are strong enough. If somebody leans on the table pushing it sideways, the leg is translating that via a lever action onto the joint with a tremendous amount of force. Think about how much torque a 1 metre long spanner can provide.

Experiment with scrap timber to confirm, but I suspect you need the joint to start 100mm down from the top of the leg as a minimum. 40mm just isn't enough. Also use glue instead of (or in addition to) bolts. A glue joint has a larger surface area than any bolt which will help.

You could perhaps run a steel rod down inside the leg, but I don't have enough experience with steel to know how strong the welded joint would be. Another option might be using 100mm to 150mm steel angle instead of 40mm (and cut the steel down to 40mm away from the leg)... though it might be difficult to find such a large steel angle without being 10mm thick.

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  • Thank you for your insight. I do not beliebe that that having a steel plate would be sufficient, as 3mm flat steel easily bend (even by a child's hand!), and it can also bend or move the breaking point downwards to the wood. Maybe if I drill into the wood from above and have an angle steel inside the wood leg - this will hold pretty well. You will need a lot of torque to bend this. How would you suggest to drill this into the leg? 4mm drill bit can only go down as deep as 2-3 inches, and I am thinking to have steel angle 20-30 deep. – NewDrill May 1 at 7:33

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