girder

The date is visible along the upper right side: 8/4/13 reversed (August 4th, 1913) The words "Jones & Laughlin" appear in raised letters on almost every vertical I-beam, indicating the source as Jones and Laughlin Steel Company, Pittsburgh, PA.

D'oh! Thanks for the correction, Wystan!

technogeo: Thanks. We'll add the date to the record next time we update.

The building nearing completion -- a Bain/LOC photo posted on Flickr in March, 2010:

Hi, I'm an admin for a group called a new world, and we'd love to have this added to the group!

The smiles suggest this test team met their goal or schedule deadline. Note the pocket protectors and that there are no smartphones. Hand-carried phones didn't exist, had poor coverage, or were expensive. In higher resolution photos, some of the team nametags may be readable.

After some head scratching, I believe this is probably a structural test for some kind of engine pylon. This would be a tough test to design because the pylon doesn't just support the static load of the engine hanging off the wing. It also must support tens-of-thousands of foot pounds of maximum forward trust and reverse thrust.

Tags are locked on this image also.

I think the "double exposure" tag is wrong. this is probably just a long exposure, and the fellow moved.

Yes, Art Siegel - I just came here to say the same thing.