Watching news reports from various parts of Turkey. I was struck by the number of tall, thin minarets still standing amidst the generalized destruction. Was there an ancient grasp of architectural principles that allowed this, or just luck?
I’m not an expert, but I did find this article.
To hold the stairs and the sides together in such a way that they won’t collapse in an earthquake or severe windstorm, molten iron was poured into pre-cut hollows within the stones. When the molten iron cooled, it served to clamp the pieces together. The system worked well so much so that engineers today are again considering using it, given the collapse of many minarets in modern times.
The Wikipedia article mentions the use of molten iron as well.
I am a lazy weasel. Never even checked Wikipedia. Thanks for taking up the slack! It’s a fascinating idea. Somebody was clever…
There’s also the survivorship bias. Those old minarets that are still standing are the ones that survived every previous earthquake.
Japan has similar issues with the tall pagodas that are often found at temples/shrines:
Tall narrow structures have to be able to survive the wind and other uneven loads and bearing issues. In doing so that makes them better able to survive an earthquake too. As @Horatius said, there’s survivorship bias as well, and I suspect many poorly designed towers never made it to the earthquake test before toppling for other reasons.
Tall spindly structures tend to bend and sway in a way that dampens the motion of an earthquake. Think of it like a tree in the wind. Shorter more boxy structures on the other hand just have to ride it out and tend to get broken apart because they’re too stiff to absorb the forces. Granted, the frequency of the earthquake’s shaking matters too (long and slow vs short and fast), as the resonant frequency is amplified in different building heights.
Isn’t fire the bigger issue with old Japanese buildings? I seem to remember the majority of temples we visited in Japan said that they had been rebuilt in the 19th or 20th century (although that is definitely biased by the fire-bombing of Tokyo).
Fire certainly is an additional hazard for wood structures, and you’re right, I do recall seeing quite a few signs on temple/shrine structures that mention a rebuild taking place after a major fire. OTOH, I don’t recall seeing many temple/shrine signs mentioning a major rebuild after a quake. In a land that’s frequently hit by destructive earthquakes, that ought to be interpreted as a sign that there is merit to their earthquake-resistant designs.
I seem to recall that wood structures are generally more earthquake resistant, because wood is inherently more flexible than stone, concrete or steel. As discussed above, it’s well-known that structures that can flex under shaking stay standing better than more rigid structures.
PBS’ Nova had an episode about the building of The Forbidden City and how it survives earthquakes:
It was a while ago that I saw it but ISTM that the columns holding the roof were not attached to the floor - so they could slide around a bit when the shaking starts.
Also many Japanese buildings are regularly rebuilt just as a matter of course, which changes the calculus a bit when examining their longevity. There’s many Ships of Theseus that are ostensibly centuries old.
According to an article I read, Japanese construction involves two or more layers of cradles on top of the posts holding crossbeams, not solid pegged construction. The beams are laid in the cradles, not pegged solid - so in an earthquake, the beam-to-pillar connection is flexible and sways, rather than breaking . The cradles, (U) shape in each direction, mean that the beam has to move quite a lot before it comes loose.
Much Greek column construction used a similar trick to the minarets - butterfly-shaped hollows were carved where two column drums joined, and lead was poured into that. (The days before cheap and easy iron). The net effect was that when civil chaos happened, the locals would chip out the valuable lead - you can see these chipped holes on many old ruins. IIRC a similar butterfly connector technique, but using wood, was also used in some stonework in ancient times. Wet wood was inserted and shrunk to create a strong connection.
Well, especially a minaret has to be made to sway in the wind… if it was just bricks it would topple in no time !. And suffer lightning damage … and so on… they had iron back a few thousand years now…
Looking at the failed buildings at the latest Asia Minor earthquake… they suspect that a many of the failed structures that pancaked might have only pancaked because of failure at the ground level.
Certainly a row apartment blocks of a similar build appear to have toppled sideways , rolled over… broken ground floor walls hanging off the rest of the building high in the air… The foundation broke apart.
They were meant to apply the earthquake proofing all the way down and including the foundation. But that was in conflict with their desire to have commercial, or parking space at the ground floor. Its also cheaper … eg they didn’t cut into the bedrock to get the foundation in there …
There are several videos of the aftershock, of apartment buildings failing. Basically, the ground floor concrete support crumbles - often, explosively - and the building either crumbles like the WTC downward or, if the failure is mainly on one side at first, it topples sideways on the wider side.
Some buildings toppled but the upper floors are somewhat intact, suggesting that people higher up may have survived. Presumably this reflects buildings that were earthquake-proof for less magnitude quakes. The structure most in touch with the ground failed.
I read an account from a survivor in Syria who grabbed one of his children and ran up to the roof, since they were higher up in the building.
So I assume that with minarets, the base levels are seriously overengineered as many stone construction buildings were built. Plus they’d recognize the risk of making the higher part too heavy. I do wonder how many architects way back when had the same question, and looked at the surviving minarets and asked “Why did they survive? What’s different?”