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Table Sketch

I am building a table with four legs which must support a dead weight of 250 kg, and possible peak dynamic load of 1000 kg (A 200 kg person climbs onto and off of it). The legs come straight down from the box, rigidly fixed to an aluminum tubing frame. They flare outward and end in free-rolling wheels. Dimensions are as shown.

The application is very weight-sensitive. Would legs made from 6061-T6 aluminum round tubing, 1.0" OD x ".093 wall be sufficient or should I bite the weight bullet and go with steel tubing?

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    $\begingroup$ No matter what you make the legs from, consider the 470 Nm torque each leg will apply right where it is connected to the table. Crossed wires under tension between opposite legs would greatly reduce this stress. $\endgroup$ – Olin Lathrop Feb 1 '16 at 16:41
  • $\begingroup$ Alternately, ribbed ell brackets (perhaps aesthetically shaped ones) pointed inward toward the center of the table would serve a similar purpose by increasing the lever arm at the table-leg joint. $\endgroup$ – wwarriner Mar 4 '16 at 5:27
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Steel or aluminum can be made equally strong. Generally aluminum can more easily be designed to have a higher strength to weight ratio because it has the a larger area moment of inertia for a given mass.

Like Olin Lanthrop mentioned in the comments, the weakest link will be your connection between the table top and the legs. You can't count on the friction of the surface to hold the legs from moving outward(even in directions the wheels may not roll). If you add cross bracing (steel cables, aluminum rod, etc) your design will be lighter overall. Even a single 1/8" rod between wheels on the bottom would increase the strength considerably.

The primary failure modes that will need to be designed for are:

  1. Buckling of the legs, hard to calculate that shape without FEA. If design fails in testing; increase wall thickness or add cross bracing (think triangles).
  2. Joint failure between the legs and the table top. Make sure that this joint exceeds the strength of the legs and table top; especially if you go without cross bracing.
  3. Buckling of the table top. Probably the easiest to calculate. Make sure to include the moments added by the legs that are cantilevered.
  4. Deflection beyond limits. You may find that even though a given design is strong enough it may not hold the tolerances you wish. Under load this table will bow in the middle. The permissible deflection for a given load may require thicker members or more structure.
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  • $\begingroup$ Thanks for the information and answers. In an effort to keep the question simple, I didn't say the the "table" is a FRP composite box structure with a tubular aluminum framework - basically a very thick beam. The sketch just shows the floor or the box. I like Owen's suggested cross-bracing - will probably add them to the design. $\endgroup$ – robertbranham Feb 4 '16 at 4:25

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