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| The web has two jobs. One is to keep the flanges spaced because if the height changes 10% from 10 to 9", the bending strength based on moment of inertia changes from 1 to .729. Nearly three times the strength change as height change. Why its a good idea when working thin tubing hard that a wood dowel inside keeps the strength up. The bending of the wood doesn't usually add that much strength but it keeps the bend from collapsing the tube.
The other job of the web is to keep the portions of the flanges from moving end to end, like the top flange moving towards the load point while the lower flange moves towards the supports. This puts the web in shear.
On time I had the great idea to make some super sturdy floor joists using a single 2x6 web and two 2x6s flat for each flange. Looked great to my elementary understandings, but just to be sure I asked a local consulting structural engineer. He said, yup, plenty moment of inertia, but you can't stick it together. A nailed flange to web connection would take a 16 or 20 penny spike every inch end to end. Gluing would just shear because there's not enough area being glued and to even try would require perfectly flat planed or jointed surfaces and the worlds best wood glue. So I gave up on that scheme because ordinary 2x6 with a 16 penny nail every inch on both edges is sure to quickly split into 2 1x6 making the web even weaker.
Point is the web can't be arbitrarily thin and develop the full beam strength. And the web does contribute to the bending strength, in proportion to the cube of the distance of the web fiber from that centroid of bending where the contribution to bending strength is zero.
Crowbar is way better at this than I.
Gerald J. | |
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Square vs. rectangle? Tube bridge physics.
