The Pantheon is awesome, but one shouldn't think, "They don't make 'em like they used to."
Almost all structures built 2,000 years ago do not exist today. The ones that do exist are all marvelous feats of engineering because if they weren't, they wouldn't have stuck around for 2,000 years. I guess it's an example of http://en.wikipedia.org/wiki/Survivorship_bias
OK, back to gawking at the Pantheon. It's absolutely stunning that people could build such a dome using 5,000 tons of concrete, no steel, and no power tools.
For this to be the case you would have to assume that the main reason that a 2,000 year old building is not around is because it fell down due to structural flaws. I think in Rome and Athens there is a lot of evidence that people knocked their houses down to build something fancier, or they were destroyed during war or civil unrest.
So it's not a simple case of survivorship bias, though undoubtedly that is one factor.
No, you wouldn't have to assume that. All the alternative factors you mention can tear down a sound building, but none of them can help an unsound one survive 2000 years.
A building that lasts that long is robust almost by definition. So, as long as any buildings do survive, we have fertile soil for survivorship bias.
I don't think you are applying the statistical notion of survivorship bias completely accurately.
The original comment questioned the accuracy of the statement "they don't build them like they used to", which I took to refer to building standards 2000 years ago.
Consider the following two scenarios:
A. 2000 years ago, all buildings were shoddily engineered except the Pantheon.
B. 2000 years ago, 90% of buildings were well engineered, of which the Pantheon is one example. The rest were deliberately torn down for one reason or other.
Survivorship bias does not allow you in itself to distinguish between A and B, which is the interesting question. It just allows you to disprove C:
C. Since the Patheon still stands, all buildings 2000 years ago must have been brilliantly engineered.
My initial reaction was that 'millions' must be an exaggeration here. Surely, I thought, there were far fewer than a million cars sold that early. But I would have been wrong: "From 1908 to 1927 Ford produced more than 15 million Model T automobiles." http://www.modelt.ca/faq.html#2
i spent some time looking at the local newspaper archives, it is astounding in 1900~1910, all photos go from being devoid of cars to being exclusively cars
I'm always amazed by the Egyptians. They cut enormous obelisques(sp?) out of solid rock, rolled them onto enormous barges, floated them down the Nile, then erected them (pun intended) with nothing more than rope, sweat, logs and dirt. Pretty awesome stuff really.
Just goes to show you how powerful bread is. Egypt had great harvests on a regular basis, something few other places could accomplish, so they were able to spend more time developing knowledge. "Western Civilization" was born in Egypt, don't let anyone tell you different.
I'm always amazed by the Egyptians. They cut enormous obelisques(sp?) out of solid rock, rolled them onto enormous barges, floated them down the Nile, then erected them (pun intended) with nothing more than rope, sweat, logs and dirt. Pretty awesome stuff really.
A three mile long ramp made of dirt. I said they used dirt. Dirt.
I like to ask my students how they think the Egyptians did stuff like this. They usually guess something like "a wooden thing with rope and they pulled it up." It's hard to comprehend how much effort Egyptians put into their projects. Building an enormous ramp and then tearing down an enormous ramp? That's a lot of shoveling.
Hm, not sure. When I posted, I think I may have had you and bloomshed mixed up. He was expressing some skepticism about a ramp being used to build pyramids. Sorry about that!
I didn't mean to sound like I was being skeptical about ramps. Ramps make the most sense, especially the enormous three mile ones that lead right to the top level of the pyramid being built. I love big ramps made of dirt!
Egyptians also used the "build an earthen ramp to get the heavy stuff up, then clear the dirt away after you're done" strategy to build their ridiculously awesome temples. Any History Channel fan knows that.
The lack of power tools may have exactly been why the building is as sturdy as it is. It took the builders several years to build the whole building. Most of the concrete they used hardened out before they built on the next section.
If they would have had powertools, construction would have progressed a lot faster, the concrete would not have hardened as well as it should and the building would have cracked and collapsed a lot more easily.
Plenty of shoddily constructed buildings were also made without power tools.
There might be an element of truth to your argument, but I suspect the more important factor is the influence of the party motivating the construction.
A Roman businessman building a hi-rise tenement building would probably not bother enforcing the use of quality engineering and materials. They also didn't use power tools, but if I'm not mistaken, these hi-rise apartment blocks were known to collapse.
But if you're building something like the Pantheon, in Rome, for the emperor Hadrian, you're going to have access to the best materials, and are not going to want to risk your neck by slacking.
The fact that a building survives for 2000 years isn't necessarily a 'marvelous feat of engineering'.
It might just be that the architect accidentally designed something strong enough. For example, many Gothic cathedrals collapsed during construction, and the ones still standing often are way stronger than needed.
No, they are not. Or, rather, they are only if they follow a(n inverted) chain line (i.e. exp(x)+exp(-x)), but the lower parts of the half spheres favored in architecture are steeper than that, so there is massive tensile stress there. This is, by the way, the genius in Wren's consruction of St. Pauls in London: he hid a chain line dome that carries most of the load between the visible outer and inner half spheres.
> This is, by the way, the genius in Wren's consruction of St. Pauls in London: he hid a chain line dome that carries most of the load between the visible outer and inner half spheres.
I think that's the wrong way around. The inner and outer dome follow catenary curves (they only need to support their own weight), and between them is a cone shape, supporting the heavy ornament on the roof. A 'chain line dome' is not a good way of supporting a heavy load on the roof (just imagine how the shape of the chain changes as you hang a weight in the middle of it).
I was planning to mention arch/keystone before, but decided against it. One of the neat things about a DOME is, you can get the keystone effect without, y'know, a keystone. That ring of stone(s?) around the oculus fills the same compressive role, and lights the place up a bit in the bargain!
There is one issue that's NOT clearly answered just looking at the building or its design (not to me, anyway): an arch/dome pushes outward on its supports, not just downward, and requires some inward force/resistance to counter it. This is what the flying buttresses on many medieval cathedrals are for. I don't really see such a structure on the Pantheon, but I notice the walls are a bit thicker at the base. Whether that's enough to counter the expansive force, I can't say.
Clearly, something has kept it up there for two millennia.
The clever part is that the dome is incredibly light.
It thins toward the top, where it is taking less load, and uses empty jars in the concrete to lighten parts.
The relatively thick heavy walls are enough to hold it.
More importantly, I don't think rebar has anything to do with durability (except insofar as being able to survive rare earthquakes). Rebar isn't impressive because it makes buildings last longer than without it, it's impressive because it allows building to be built which couldn't exist period without it.
Rebar generally reduces the life of concrete - almost all concrete failure is due to water getting into the rebar, creating rust which expands and cracks the concrete.
A lot of the ruins of Greece were damaged by Victorian engineers securing the stones with iron bars to strengthen them
I read* that most of the temples of Rome actually did survive for centuries after the western empire fell, until some Eastern emperor decided to remove all the steel brackets to melt down. The buildings then only stayed up until the next earthquake.
* in a tour guide, so not exactly an impeccable source.
I didn't downvote you, but I guess I just don't get it. Here's the guidelines, seems like this one pretty clearly stands in the on-topic realm of things.
"On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.
Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon. Videos of pratfalls or disasters, or cute animal pictures. If they'd cover it on TV news, it's probably off-topic."
Almost all structures built 2,000 years ago do not exist today. The ones that do exist are all marvelous feats of engineering because if they weren't, they wouldn't have stuck around for 2,000 years. I guess it's an example of http://en.wikipedia.org/wiki/Survivorship_bias
OK, back to gawking at the Pantheon. It's absolutely stunning that people could build such a dome using 5,000 tons of concrete, no steel, and no power tools.