At a basic level, you probably know that building materials are stronger in certain axes than others. While you could learn all about moments of inertia and beam bending, what the average DIY builder really needs is an intuitive sense of the practical ways you can configure whatever you're building with to make it stronger. Whether you're building a tank or a table, understanding the forces at work will help make it both light and strong.
Recently, I built a kitchen table for a couple in NYC using thin metal for the main support. Here are a few simple tricks for adding strength without adding much weight.
Technically, we're talking about the area moment of inertia and resistance to bending. Practically, we're talking about strength and rigidity. Since most DIY builders are more concerned with building something strong and light than with quantifying resistance to bending, we'll consider the practical approach and leave the math for another time.
Lets look at a simple example. Picture a 2x4 sitting on top of two sawhorses. (If you've never handled a 2x4 before, step away from the computer right now, head to the lumber yard, and try this.) Imagine pushing on the center of that span. Experience tells us that it will be harder to bend the 2x4 when pushing on the narrow side than when pushing on the wide side. Theory tells us that a plane passing through the 2x4 parallel to one of the "4" wide sides contains more material than a plane parallel to one of the "2" sides. At a very basic level, more material means more material to bend, which means a greater resistance to bending—what we call strength.
I used this concept twice recently in the steel column of a table I built for a customer. The column is made of 1/8" thick hot-rolled mild steel. Standing on its own, the 12" wide x 28" tall x 1/8" thick piece that forms the main part of the column would not be very rigid at all. A small amount of force—even just the weight of the tabletop—would cause it to bend and twist. Adding the 4" wide x 1/8" thick pieces on either side—essentially turning it into an I-beam—makes the column much more resistant to bending and adds very little weight compared to other ways of building such a rigid column. Even if I stopped here, the table base would have been strong enough to support the weight of the tabletop and several people standing on it.
With this design however, we went even further. In this table, the cutouts are purely for looks. In larger panels though, this style of cutout can significantly increase the local resistance to bending. The concepts at work are exactly the same as above: More material is added in a plane aligned with an otherwise easily bendable axis of the part. It is important to note that the part as a whole does not need to be made thicker to gain most of this strength - adding the material in just one or in a few places can give significant results.
This concept is found in most sheet metal parts that include large flat spans. Beads or flared holes in the sheet metal add material in planes that are aligned with an axis in which the part would otherwise be weak. That is, they essentially allow a thin part to behave like a thicker part without adding much weight. Where material is removed, as in the case of cutouts as above, or flared holes in sheet metal, weight can actually be reduced.
This concept is not specific to steel; it applies equally well to all materials. Consciously applying it to design decisions will yield significantly lighter and stronger results.
I have to say, the design is beautiful, but could you have done a sloppier job with the welding? The least you could have done was brush down the bead a bit. That's just sloppy work.
Ah yes, the Internet: Land of experts with opinions.
" have to say, the design is beautiful, but could you have done a sloppier job with the welding? The least you could have done was brush down the bead a bit. That's just sloppy work."
Says the guy who has never welded........The author never claims to be a welding master. The article is how to better design a structure not lay a good looking bead.
There are places where a hole can make a part stronger by reducing stress concentrations, but that subtitle is just a tease. It is the added material around these holes that helps, not the hole. The overall structure looks like an I beam, but it functions as a column. The added flanges are not loaded as they would be in a beam.
I'm tempted to re-write the whole article, but would rather go read some J.E. Gordon for clear explanations, and recommend the same for everyone. This article is a hazard to the beginning student.
@BobStuart Just for the record, I don't really have control over the titles and subtitles that go on my posts. The titles are just a hook to draw readers in. You'll note that the content of the article clearly states that the strength comes from the orientation of the material being added.
The gist of the post is about orienting your materials to best resist bending without over building. While you are correct that this table's base looks like it's designed to be loaded as a beam and is actually loaded as a column - i.e. it's materials are oriented best to resist bending in an axis along which it's not actually loaded - I assume you realize that the example is intended to show how to build a strong part from thin materials, not about how specifically to build yourself a post-industrial styled, Manhattan sized, dining room table.
But that said, I'm eager to see your re-write. Your points in the first paragraph are valid and I would probably learn something from your treatment of the topic. Understand that I'm not being sarcastic here - just asking you to put your money where your mouth is.
That table seems to be an unsafe design and should not be used anywhere near young children. It must weigh a lot with that much steel. It probably doesn't require a lot energy to tip over when the tabletop is pushed on the horizontal plane. Yet the weight could easily kill a small child if the tabletop tips over on them or pins them between another object. The holes are a strangulation risk for small children too. While I understand the article is focused on the physics of its structural support - any item intended to be used in a home should always be viewed with safety in mind as a priority above other factors. Having a nice strong, heavy, beautiful table might not be so great if it could kill or injure a child under easily foreseeable conditions. The table also seems to be extremely wasteful of the steel used for support. Certainly two small steel I-beams running under the table top would be sufficient to transfer all load to a single vertical column down to 4 legs with a lower profile and larger footprint, with much greater stability at a fraction of the weight.
Thanks Vin. You shared a great primer. The 2x4 is an intuitive and accessible example. Cool table too.