3 ms·

Roughly, There are a few things that are generally applicable for earthquake performance. 1) The period fundamental period of a building is roughly .1 sec/floo

Roughly, There are a few things that are generally applicable for earthquake performance. 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)

xenadu02 4y ago
3 is called "soft story" and is something San Francisco now requires be fixed or the building is red-tagged - no grandfathering of existing structures is allowed. 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.
- wahern 4y ago
  The deadline for remediating multi-family soft stories dwellings in San Francisco was 2020. But this only included buildings with 5+ units, or buildings with a commercial space on the ground floor. There are still many residential buildings, including multi-family buildings (< 5 units), with these 3- or 2-wall soft stories, and there's no mandate in place going forward, AFAIK. 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.
rkagerer 4y ago
If I understand you correctly elasticity is the way to go. What sort of building materials and techniques achieve that? (If I want a structure that can still be used after a quake) eg. Does a floating slab help, can you place the whole thing on big shock absorbers, etc?

xenadu02 4y ago

3 is called "soft story" and is something San Francisco now requires be fixed or the building is red-tagged - no grandfathering of existing structures is allowed. 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.

wahern 4y ago
The deadline for remediating multi-family soft stories dwellings in San Francisco was 2020. But this only included buildings with 5+ units, or buildings with a commercial space on the ground floor. There are still many residential buildings, including multi-family buildings (< 5 units), with these 3- or 2-wall soft stories, and there's no mandate in place going forward, AFAIK. 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.

rkagerer 4y ago

If I understand you correctly elasticity is the way to go. What sort of building materials and techniques achieve that? (If I want a structure that can still be used after a quake) eg. Does a floating slab help, can you place the whole thing on big shock absorbers, etc?