The 'C' shapes on the inner face of the flanges are a sort of traditional way of attaching the end to an extruded box. I.e., if you wanted to cap the end, you would prepare a plate with four drilled holes in appropriate places, and run a sheet metal screw through each hole into the 'C', which would capture the screw. The detail is often used in low to medium quantity for aluminum boxes to contain electronics.

When those extrusions were made, the biggest extrusion press in the world was in Arkansas, and could make any shape that would fit in a 13 inch diameter circle, 5000 lbs of product per stroke.

I think I see axial wrinkles or witness marks of some kind in the webs, which may hint that the parts were made in a smaller extrusion press. The traditional way is to extrude a part like that is with the eventual web curled in a partial circle, and to unroll the freshly extruded part (which comes out in a pretty soft temper) in a press brake, flattening a little at a time. The wrinkles suggest that the web may have been extruded as a zigzag/ serpentine shape, which might reduce the skill level required to flatten the web, at the expense of some expensive tooling, perhaps even a set of rolls in a machine not unlike a tube mill, which would gradually flatten and widen the part in a continuous process.

I suppose it's possible that the original extrusion, with zigzag web still folded, might have been intended as the major part of an air to air heat exchanger, and was repurposed into joists. ... which is kind of a stretch.

OR

There may be a completely different explanation; I have not seen a part quite like that before, and I have been wrong on occasion.

The sections are a _lot_ lighter than any Aluminum Association shape I have ever seen, even in a catalog, and don't look like they would withstand the usual 'choker sling around a bunch' sort of handling normally used in the metal supply chain. I'd be interested to learn whatever you can find out about them.

Mike Halloran
Pembroke Pines, FL, USA

Thanks for the reply. You apparently know the extruding process very well. The slots on each flange with the semi-round cross-section look familiar to me. I assume the purpose would be to connect to other extrusions or other parts. I attached a close up photo FWIW.

• http://files.engineering.com/getfile.aspx?folder=068c8aec-5ffe-40e7-b4ec-e6