How to Engineer Buildings That Withstand Earthquakes
scientificamerican.com
scientificamerican.com
1) The period fundamental period of a building is roughly .1 sec/floor. (+-, OOM) Earthquakes tend to have an energy peak in the 1 second range. Houses and skyscrapers have an advantage here, 5-10 story apartment buildings, no. (Houses move with the ground, it hits higher modes of skyscrapers, but apartment buildings resonate)
2) Unreinforced masonry or lightly reinforced concrete sucks in an earthquake. Very low tensile strength, which leads to crumbling under shear loads. Combine this with 5-10 story buildings and you have a death trap. Reinforced concrete helps, but it takes a _lot_ of steel, so much that you have difficulty placing the concrete. It's mostly for containment, so that you have this core of concrete wrapped in a steel jacket that can retain enough strength that the building doesn't collapse. It's still a write off after the quake, but hopefully you haven't killed anyone.
3) You want relatively consistent height vs stiffness and stiffness vs orientation profiles. One weak floor (e.g, double height ground floor with columns) concentrates the motion there, and with that the forces. One of the California quakes (Maybe Northridge) showed that the typical for the time arrangement of 3 walls + one open side for parking on the ground level, + 2 stories above was a really bad idea because that one weak wall doomed the buildings. You can't do that in California anymore.
4) Stiffness attracts load. This is a displacement driven regime, rather than a force driven one. If you make something stronger, it often becomes stiffer, and then attracts more load. This can go in a loop. The solution is to make things _really_ flexible, such as using base isolation.
(source, Masters in Civil, working EIT for a couple years in an earthquake region before switching to internet stuff)
Buildings like this are all over Turkey, parts of Central & South America, etc and just as you mentioned the soft story (usually the ground story) tends to collapse causing the building to pancake. It's still popular because it's easy to put parking or a commercial (large open space) on the ground floor for density.
The fix is moment frames, but the typical cost is north of $100,000 for even a single-family home, largely because of the deep footings required, which means tearing up and rebuilding large parts of the foundation and walls. I really wish there were more economical options available. A lot of engineering and regulatory effort went into developing and permitting cheap but highly effective sheer walls, but when it comes to bridging open spaces the only available option (market or otherwise) seems to be a traditional, bog-standard moment frame developed for large, commercial structures, then scaled down.
Contractors don't even offer laminated timber moment frames, only the more expensive steel frames, despite the former technically being available from suppliers and (presumably) capable of passing muster with the building inspector (at least with sufficient cajoling). The premium for steel beams probably only accounts for a fraction of the ultimate cost, but it's an example of the dearth of options, and at $100,000+ those fractions add up.
My 3-story house has a window which compromises the sheer strength of the ground floor back wall, and of course a garage opening in front. Even for the back wall the only options offered by licensed contractors were a deep, concrete footing-anchored moment frame; or removing the window entirely so the sheer wall is continuous, without requiring any footing or foundation work. The distance in complexity between those two options seems huge. In principle there must be cheap techniques for strengthing headers, etc, to provide more than adequate reinforcement, but if available or known they're just not in the average contractor's repertoire, even for earthquake retrofitters, and probably unlikely to pass muster as legitimate reinforcement.
3-5 unit residential buildings are described as the critical "missing middle" in the housing affordability debate in the U.S. I would assume that magic figure holds true in Turkey and most other locales, meaning large fractions if not the majority of residential units are in buildings of that size. The dearth of standard, economical solutions for this middle class of building size in SF makes me think Turkey will find it difficult to reform their system. As evidenced in both SF and Turkey, people will go without if the cost is too high; it couldn't be any other way, economically speaking.
Garage doors are as difficult as ground floor parking. They often take up the entire building frontage so you need significant reinforcement.
My house has a steel moment frame set back from the door opening half the depth of the garage because it has to support a 1.5 car width span and the primary sheer wall for the second floor directly above. The house pre-renovation had none of that, just toe-nailed pillars like lots of SF garages which would have made that part of the house vulnerable for sure.
The R-2/R-3 neighbors of the city are filled with non retrofitted soft-stories indeed!
> The fix is moment frames, but the typical cost is north of $100,000 for even a single-family home, largely because of the deep footings required, which means tearing up and rebuilding large parts of the foundation and walls. I really wish there were more economical options available
They do have solutions (Simpsons makes steel strongwalls that are a lot cheaper than moment frames to avoid you having to have a shearwall on the entire front wall), and you'll find structural engineers that will do plans without moment frames that'll definitely get accepted by the city. You'll also find the other half of structural engineers only willing to do moment frames that'll cost you 100-200k$.
Now in northern Syria, you could argue the cost would have been too large. The alternative there, in the midst of war, was probably in many cases "build incorrectly, or don't build at all and freeze in a tent".
eg. Does a floating slab help, can you place the whole thing on big shock absorbers, etc?
There were probably some such cases, anyway this is as well interesting:
From https://www.nbcnews.com/news/world/erzin-turkey-earthquake-b... :
The city that didn't collapse: How Erzin became a haven from Turkey's earthquake - Residents and officials say Erzin suffered no deaths and saw no buildings collapse, and they credit a long-standing policy not to allow construction that violated the country’s codes.
From https://en.wikipedia.org/wiki/Erzin,_Turkey :
Erzin was the sole district of Hatay Province to have none of its buildings collapse in the aftermath of the 2023 Turkey–Syria earthquake, despite being closer to the epicenter than other cities such as Iskenderun and Antakya which suffered greater damage. The mayor, Ökkeş Elmasoğlu claimed it was due to strict construction of housing; the district mostly consists of single houses, local authorities prohibited unsafe and substandard construction to a much greater degree than nearby areas, and apartment blocks do not have many floors. Even older structures such as houses from 60 years ago survived the earthquake.[4] Additionally, the city had fared relatively well during previous earthquakes. However, engineers and scientists (such as Omer Emre) attributed the town escaping unscathed to geological factors, such as Erzin's relatively higher sea level compared to surrounding towns, and it being built upon harder ground, consisting of bedrock and coarser grains than sand, compared to softer, water-laden sediments like that of cities to the south.
However, I feel we'd be remiss to not investigate the geology of the different places. Are they similar, different, one alluvial one on bedrock? Seeing the iconic olive grove have a 30 meter collapse, I'm not sure what kind of building would survive that upheaval, unless the pilons went deeper than that into bedrock.
However, engineers and scientists (such as Omer Emre) attributed the town escaping unscathed to geological factors, such as Erzin's relatively higher sea level compared to surrounding towns, and it being built upon harder ground, consisting of bedrock and coarser grains than sand, compared to softer, water-laden sediments like that of cities to the south.
This is nice: https://youtu.be/7eO23oeXJlw
shear walls, braces, and moment frame connections are a lot more common than base isolation and dampeners, especially for smaller buildings.
More common, but much less interesting to look at :)
Watch this with English subtitles (an ad by government): https://www.youtube.com/watch?v=LJGueK-Ocgg . State exists to solve the problems of its citizens :(
For example "How do Earthquake Affect Reinforced Concrete Buildings? [1] describes the "strength hierarchy" for an RC frame building to remain safe during earthquake shaking, columns (which receive forces from beams) should be stronger than beams, and foundations stronger than columns.
Aside from the flexibility of wooden vertical load bearing posts, the typical weather board / external wooden slat construction which is widely used in New Zealand allows for the narrow boards to slide independently under the lateral shaking forces. The external walls can deform into trapezoid shapes more effectively - hence a higher threshold for catastrophic damage and better future usability.
Brickwork chimneys, internal plasterwork and the subsurface conditions are another story but a least the dwelling is still functional :/
I know you didn't write this, but lots of people think that the flexibility works because it "absorbs" the energy of the earthquake. It does not. What it does is spread a short sharp move into longer but lower amplitude move. The energy imparted is the same.
If everything is linear elastic, you can take the principle components/eigenvectors and get fundamental modes for a first approximation. Once you hit plastic deformation, things go non-linear and get more interesting.
So, Wood:
1) Less stiff, so KX means less force due the ground motion. 2) Less mass, so the MA term is less.
However, wood also tends to be used in smaller structures, so your on the left side of the resonance peak, were first mode structure motion is greater than the forcing function (but not into the resonance peak)
Steel tends to be used for really tall buildings, where the earthquake doesn't excite the fundamental modes, so the ground basically wiggles the bottom of the skyscraper, but the top is nearly unaffected. (the right side of the resonance peak)