Washington Monument design safety factor

Tom uses the Washington Monument's aluminum-tip lightning rod as evidence of aluminum's once-precious status in the 1880s. Not a structural-design story in this telling.

Any questions about any of that? I took a little bit longer as I usually do. I was going to talk about aluminum, and maybe I can just quickly do some of the history of aluminum. Aluminum was once more valuable than gold. In 1850, a baby rattle from the royal family in France was made out of aluminum. It's light — babies aren't that strong. What's on top of the Washington Monument, built in the 1880s? A piece of aluminum. It's a lightning rod, intended to attract lightning, and it's aluminum.

Brief mention via Petroski — crushing safety factor of 9, designed in the 1870s. Used to give a worked example of period-typical safety factors for masonry.

You have to remember they were using all kinds of material — steel that didn't have much toughness, cast iron that was very brittle, just inherently brittle. Petroski talks about the Washington Monument in crushing having a safety factor of 9. The Washington Monument was designed in the 1870s.

Aluminum tip of the Washington Monument used to illustrate that aluminum was almost half the value of gold at the time of dedication.

In 1850 the French were interested particularly in aluminum. This is a Napoleonic baby rattle made out of aluminum, because it was more valuable than gold. So Napoleonic babies got to rattle their aluminum chains. The top of the Washington Monument — it should have been 1886 because it was supposed to be the Centennial from 1776 — the top of the Washington Monument is this little piece of aluminum at the tip. It was actually not at that time more valuable than gold, but almost half the value of gold. There's the guy, Charles Martin Hall in 1885, about the time he discovered the way to electrolytically refine aluminum. Here's his sister, who also helped him, in the spirit of gender equality.

Contrast case to the Egyptian obelisks — the Washington Monument is brick assembly, not monolithic stone. Used to highlight that Egyptian obelisks really are single pieces.

[Tom hands a granite fracture-toughness specimen around the class.] When I first took over as an assistant professor here, in the little office around the corner, 8-137, I shared it with a graduate student who was working on the fracture toughness of granite. This is a fracture toughness specimen — you can pass it around. No one had really looked at fracture toughness of granite. Why was he looking at it? Professor Backofen, who taught mechanical properties of materials in course 3, was interested because if you actually did some calculations estimating the toughness of granite and other stones, there was no way to raise the Egyptian obelisk. One of the engineering marvels is the Washington Monument, which they actually built going straight up. But it had always been assumed that the Egyptians carved the stone, because their obelisks were one monolithic piece of stone. The Washington Monument is a bunch of bricks. If you did the calculation you would predict that the bending load would break the obelisk in two when you started to lift it up.