Why steel tube frames?

Kiln dried untreated Douglas Fir 2″x4″ (actual 1.5″x3.5″) lumber is specified to weigh 1.28 lbs per linear foot. This can change with prolonged exposure to humidity and variation in suppliers, species, and other variables.

1″x1″x.065 seam weld ASTM steel tube is .814 lbs per linear foot.

With ends fixed, the calculated deflection for a 4′ section is 0.218″ with a center load of 100 lbs.

For the 2×4 beam, it’s a little different because it’s a rectangle.

Positioned so the farthest fiber is vertical, a 4′ section with the same 100 lb point load in the middle would flex 0.4 inches. FF positioned horizontal you’re looking at somewhere around 2.2 inches of deflection at the load point.

So, for a nominal length, the 1″ square steel tube in any orientation is about as strong as the 2×4 is in it’s strong orientation when given a static load.

Remember that is not a linear relationship, and a longer section of 2×4 oriented FF vertical may flex less than the same length of tube.

Steel weighs less per linear foot.

I would also hold some other assumptions, listed here:

The steel has a higher propensity to bend and not come back to it’s original strength, so +1 for wood

Wood can get wet, mold, splinter, and is far more difficult to control. +1 steel

Steel can rust. +1 wood

Wood requires more “technology” to anchor together properly. +1 steel

Wood is easier for average folks to work with. +1 wood

Steel is easier for me to work with than wood +1 steel. (I can weld steel with the same ease as hot gluing popsicle sticks together)

2 Replies to “Why steel tube frames?”

  1. Great advice. I’ve been trying to decide on this for awhile. Recycled steel is more sustainable although I’m not sure of the accessibility. Where did you obtain your steel tubes? And cost?

    1. It’s likely some or all of the “new” steel you buy is already recycled.

      Fabricating things out of scrap or secondary materials when a specific design is required isn’t really a possibility. Kind of like how a 6 pack of beer bottles tend to be remelted into new ones instead of using a bunch of old random bottles.

      For instance, there’s probably about 300 linear feet of square steel tube stock in this build. That’s just not something you find scrap, and usually scrap steel is also rusty or bent, so it takes even more time to prep it to usable.

      Most places that sell metal have remenant cuts which are sometimes helpful, though.

      There are many steel yards everywhere, of varying customer service. I used https://www.metalsupermarkets.com for my supplier because they’re just down the road, and are really good about selling what I need at a reasonable price point.

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