Retrotechtacular: Understanding The Strength Of Structural Shapes

Strength. Rigidity. Dependability. The ability to bear weight without buckling. These are all things that we look for when we build a mechanical structure. And in today’s Retrotechtacular we take a closer look at the answer to a question: “What’s in A Shape?”

As it turns out, quite a lot. In a wonderful film by the prolific Jam Handy Organization in the 1940’s, we take a scientific look at how shape affects the load bearing capacity of a beam. A single sided piece of metal, angle iron, C-channel, and boxed tubing all made of the same thickness metal are compared to see not just just how much load they can take, but also how they fail.

The concepts are then given practical application in things that we still deal with on a daily basis: Bridges, cars, aircraft, and buildings. Aircraft spars, bridge beams, car frames, and building girders all benefit from the engineering discussed in this time capsule of film.

None of the concepts in this video are suddenly out of date, because while our understanding of engineering has certainly progressed since this film was made, these basic concepts remain the same. As such, they will apply to any structural or mechanical devices that we make, be it 3d printed, CNC routed, welded, glued, vacuum formed, zip tied, duct taped, bailing wired, or hot glued.

Keep your eyes open for a wonderful sights and sounds of a rare Boeing 314 Clipper landing on water and a 1920’s Buffalo Springfield Steam Roller demonstrating how wonderful the film’s sponsor, Chevrolet, makes their automobile frames.

13 thoughts on “Retrotechtacular: Understanding The Strength Of Structural Shapes

  1. Starting at 3:28: In technical terms the 8 lead weights put too much stress on it – stress being the force per area, a factor complicated with bending. The top elements are under tensile stress and the bottom elements are under compressive stress and it is under the compressive stress that the upside down-U buckles. Similar buckling is called Euler buckling – it’s not the same for buckling induced by bending but is worth looking up anyway.

    Strain is the amount the material changes shape. Typical steel elements deflect under tension or compression by 1 millionth of an inch over each inch of length for each 30 pounds per square inch. Again, in bending, it’s much more complicated.

    If a 150 pound person stands on a 1 square inch block that is 10 inches long that’s 150 pounds per square inch *1 millionth of an inch per 30 psi per inch of length all times 10 inches and shortens that block by 50 millionths of an inch.

    That is, the total strain is 50 millionths of an inch and the stress is 150 psi.

    That factor isn’t absolute and varies slightly with different alloys, but look up Young’s Modulus to get that value. It’s usually in millions of pounds per square inch per inch of deflection, which is kind of dumb, but who am I to use realistic values that have sensible ranges? Anyway, take what they have and divide by 1 million and life will be more reasonable.

    One of the major problems for designers of structures is that it is easy to produce stress levels lower than what the material can resist but end up with the ability for the shape to twist to a shape that doesn’t support that – for example a vertical sheet of the same amount of material would have held far more than the closed box beam – except it would immediately twist and bend. In homes this is usually the case with floor beams. To prevent it blocks or diagonals are added to prevent the beams from twisting.

    The second design problem is that while a beam may be strong enough – stress lower than the material can withstand – if the strain is too high the item flexes too much. In homes if the floor beams aren’t stiff enough something like a rigid ceramic floor tile will crack or shatter when the floor flexes and the floor will feel bouncy and walking about will cause furnishings to shake.

      1. The SI units for the Young’s modulus are the same as the units for pressure : Pascals (Newtons of force per square meter area). This is because the modulus is defined as the ratio of stress, which has units of pressure, over strain, which is dimensionless (meters per meter if you want to be pedantic).

        Diamond has a Young’s modulus of about 1000 GPa; A36 structural steel is 200 GPa; Glass is around 50 GPa; HDPE platic is around 1 GPa; Styrofoam is 0.005 GPa.

      2. Anyone competent to make use of the information is competent to convert it themselves.

        Since it was contributed without remuneration, the original contributor doesn’t owe any additional effort to the audience. In other words, if they’re more comfortable using one standard vs another, that’s their right, and if the audience finds it useless, the audience has the right to ignore it.

        In the case where the reader is more comfortable using some other standard, conversion is left as an exercise for that reader, if the reader wishes to bother.

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