These pictures of the Eiffel Tower under construction are fairly famous, and they deserve to be. They show the extremely rapid and accurate construction of a huge expression of modern (for the time) technology. They also, more or less accidentally, show exactly how the tower works.
Gustav Eiffel spent a lot of time analyzing wind pressure on various structures and was one of the first engineers to think about how a metal-frame structure could best resist wind’s lateral push. The Tower’s shape is a reflection of the wind moment at any given elevation, and is therefore, more or less, parabolic. More importantly, the arches at the base are non-structural decoration meant to link the bare metal to the surrounding Parisian architecture. The important parts of the structure, other than the four main curved legs that merge at the top, are the horizontal struts that connect the legs just above the arches and just below the merge point. They are necessary to keep the legs acting as parts of a single structure and not as four separate structures.
In the first picture of the sequence (for all, click to enlarge), October 8, 1887, you can see the four legs rising from their foundations, each braced by some temporary shoring with the reverse slope to the leg slope. Starting in the third picture, December 12, 1887, you can see a big central shoring tower used to hold the first platform level (the lower strut) as it was built. By March 15, 1888, the platform was well underway; by June 14, it was complete and the legs had been extended past it. By July 10, work on the upper strut had begun and the central shoring tower is gone. By October 14, the upper strut is complete and the merger of the legs above is under construction, and the leg shoring is gone.
In short, the five big pieces of shoring – the shoring at each leg base and the tower below the first platform – were removed as soon as they were no longer needed. What determined when that was? When the partially built tower was capable of resisting gravity and wind on its own. The central tower didn’t help with wind, so was superfluous as soon as the first platform/lower strut was complete. As soon as both struts were complete, the tower was properly joined as a single frame, and the leg bracing was superfluous. The sequence of permanent and temporary construction shows the flow of structural forces, which is, in a very geeky manner, rather cool.
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