Posted Sep 6, 2018, 1:21 AM
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1st Ward
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Join Date: Sep 2006
Location: The Big Onion
Posts: 2,570
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Quote:
Originally Posted by Clarkkent2420
Believe it or not, even in San Francisco (and especially in Chicago) the wind load design criterion for tall buildings are the controlling factor for the structural design - not seismic. In Chicago, the result is very prescriptive wind design considerations, and no specific seismic considerations.
Interestingly, seismic design criteria allow for some % failure on the outer bands of the probability distribution for earthquake events. Meaning, in San Francisco, the code allows that some portion of buildings will fall down in an earthquake. Contrast that with wind load design - where you have to design to a 0% failure rate, plus safety factor.
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Interesting. So we've got as stringent or more stringent safety codes than earthquake zones. Good to know! 
Quote:
Originally Posted by LouisVanDerWright
Yeah I've also read that the New Madrid fault isn't very dangerous at longer distances. It's got something to do with deeper earthquakes like that proliferating shockwaves differently through the soil. Also I've heard that the soil here is basically a bowl full of jelly, if you shake it then it bounces back and forth, but it doesn't really tear or shake for long after you stop because it absorbs the energy. Our soil is not rocky where you have very jarring concussive seismic waves and it's not sandy where it liquefies when vibrated. It's dense, mucky, and thick. That also contributes to the dampering of earthquakes proliferating through great plains soil. Parts of the South Loop or south side might have some settlement issues because it's much sandier and the water table is high near the lake.
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It's funny, I've heard the opposite. The geology in the eastern US is older than that of the western US, and the older bedrock lacks the elasticity to absorb seismic shock waves well, allowing them to carry for much further distances. In the 1811-1812 New Madrid quakes (which most seismologists assume were about 7.5-8.0 on the Richter scale, making them the strongest ever quakes in the contiguous United States) caused much of the east coast to feel the effects of the quake. Church bells rang in places as far away as Toronto, Charleston and Boston.
Some text from Wikipedia:
Quote:
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There are estimates that these stable continental region earthquakes were felt strongly over roughly 130,000 square kilometers (50,000 sq mi), and moderately across nearly 3 million square kilometers (1 million square miles). The 1906 San Francisco earthquake, by comparison, was felt moderately over roughly 16,000 km2 (6,200 sq mi).
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Source: https://en.wikipedia.org/wiki/1811%E...id_earthquakes
Quote:
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In a report filed in November 2008, the U.S. Federal Emergency Management Agency warned that a serious earthquake in the New Madrid Seismic Zone could result in "the highest economic losses due to a natural disaster in the United States," further predicting "widespread and catastrophic" damage across Alabama, Arkansas, Illinois, Indiana, Kansas, Kentucky, Mississippi, Missouri, Oklahoma, Texas, and particularly Tennessee, where a 7.7 magnitude quake would cause damage to tens of thousands of structures affecting water distribution, transportation systems, and other vital infrastructure.[21] The earthquake is expected to also result in many thousands of fatalities, with more than 4,000 of the fatalities expected in Memphis alone.
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Source: https://en.wikipedia.org/wiki/New_Madrid_Seismic_Zone
Obviously the hope is we never see such a quake in our lifetimes, and if we do that Chicago is far enough away and our buildings engineered to be able to withstand any seismic waves coming from Missouri.
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"Eventually, I think Chicago will be the most beautiful great city left in the world." -Frank Lloyd Wright
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