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Comment by gibsonf1 | Hacker News Reader

As an architect in San Francisco, I would definitely prefer to be in a modern high rise here over other buildings for safety purposes during an earthquake. I am highly skeptical of this article as I have in depth knowledge of seismic engineering, the building code and the updates after every earthquake around the world to the code and engineering practices.

The leaning building across from my office, for example, has nothing to do with seismic issues but basic design/engineering/construction flaws in not extending the pile footings deep enough to solid ground as others in the area have such as the new Salesforce tower.

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Your comment seems to echo that of another architect quoted in the article.

Ron Klemencic, the chief executive of Magnusson Klemencic Associates, the company that did the structural engineering for Salesforce Tower, says he agrees that water and sewage systems need higher strength requirements, but not high rises. “Buildings falling on top of other buildings — that’s not going to happen,” Mr. Klemencic said.

They never explained why Klemencic's reasoning. Care to explain why "buildings falling on top of other buildings — that’s not going to happen"?

Hollywood treats falling buildings like falling trees [1]. The building basically gets cut off at the base, then stays together as it falls to the side until it hits the ground. I'm no expert, but IRL I think any significant amount of lean would lead to massive structural failure in your average skyscraper. Once the structure starts to fail gravity is going pull the pieces straight down since nothing is pushing them to the side. It's certainly a danger to surrounding buildings (see the WTC) but vastly different from buildings falling over on their sides to crush their neighbors.

1: https://youtu.be/r0tgEYiTNd4 (0:35 and 0:45)

My understanding is that the way buildings collapse in Hollywood is not how they collapse in reality. If the base of the structure begins to fail, it doesn't fall over as a tower, but crumbles into itself.

The Twin Towers took out the blocks surrounding them. Literally they fell on top of other buildings, even though they "collapsed into themselves."

I am very skeptical of anybody, no matter how experienced, that gets on the news and says "that'll never happen," regardless of what the "that" is.

Has he considered a skilled terrorist cell lacing only the east side of Salesforce Tower with explosives?

With the Twin Towers, the issue wasn't so much that they fell on top of other buildings (they didn't - you can see that on footage of them coming down), it's that when they hit the ground, the debris cloud billows out at high velocity and consists of projectiles that can structurally damage neighboring buildings. All that gravitational potential energy of the structure gets converted to kinetic energy as it collapses, and that kinetic energy is basically like a bomb going off at ground zero. When a bomb goes off near a building, the building is gonna have a bad time, even if it doesn't directly hit it.

http://s3.amazonaws.com/nasathermalimages/public/images/4158...

http://s3.amazonaws.com/nasathermalimages/public/images/fema...

http://s3.amazonaws.com/nasathermalimages/public/images/5489...

http://s3.amazonaws.com/nasathermalimages/public/images/core...

http://s3.amazonaws.com/nasathermalimages/public/video/Relic...

http://nate.flach.s3.amazonaws.com/Abolhassan_Astaneh.avi

Like I said, it's like a bomb going off at ground zero. The pictures you link support that: notice how the damage is mostly near street level (from projectiles moving outwards from the crash site) rather than at the top of the building (from projectiles falling on the building).

I agree. https://web.archive.org/web/20021004010033/http://www.chiefe...

http://s3.amazonaws.com/nasathermalimages/public/images/Impa...

Even blowing out only one side of a tall tower, would not cause it to fall like timber. Instead, one of two things will happen. If there is low lateral strength (unlikely), the east side of the floors above will shear and collapse off, straight down into a pile on the east side. If there's good lateral strength, The forces formerly supported by the now blown east structures will largely fall on the adjacent structures around the building, overloading them, causing failure, and rapidly continuing around the supports in a zipper-failure, whereupon the above structures now unsupported again fall straight down with gravity's vector.

Either way there's probably a bigger pile on the east side, but it's not like the top floor of a 1000' building will land 1000' laterally from the base. 100' would be a long way (tho there's be lots of flying debris for a good distance).

Skyscrapers have a tremendous amount of mass and are engineered to resist the force of gravity. That's hard, so they only bother to engineer it so that it resists that force in one direction relative to the building.

As soon as you might start to turn the building onto its side it falls apart like a sand castle. Were this not to happen the building would have to be engineered such that it could withstand being turned on its side like that without collapsing. But they don't design them as such.

Wouldn't there be bigger concerns, terrorist attacks really don't happen that often.

It'd be like constantly worrying about protecting yourself from lightning.

>It'd be like constantly worrying about protecting yourself from lightning.

A lightning rod (US, AUS) or lightning conductor (UK) is a metal rod mounted on a structure and intended to protect the structure from a lightning strike. If lightning hits the structure, it will preferentially strike the rod and be conducted to ground through a wire, instead of passing through the structure, where it could start a fire or cause electrocution. Lightning rods are also called finials, air terminals or strike termination devices.

In a lightning protection system, a lightning rod is a single component of the system. The lightning rod requires a connection to earth to perform its protective function. Lightning rods come in many different forms, including hollow, solid, pointed, rounded, flat strips or even bristle brush-like. The main attribute common to all lightning rods is that they are all made of conductive materials, such as copper and aluminum. Copper and its alloys are the most common materials used in lightning protection.[1]

https://en.wikipedia.org/wiki/Lightning_rod

You're right, but over the course of a building's lifespan, the "chance" is greater, right? So the chance of a terrorist attack against salesforce tower this year is less than the chance it might happen sometime in the next 30 years. I'm not good at statistics, though.

Anyway, this is why we have a lot of earthquake preparedness, though the chance is 5% in the next 30 years.

It's not a "constant worry," so much as a plan. There's non-negligible chance of terrorist attack, so they architect in such a way to prevent it being catastrophic. There's a non-negligible chance of your office catching fire, so they put green exit signs at the door. That sort of thing.

> though the chance is 5% in the next 30 years.

Not even close. USGS estimates over the next 30 years are: 72% probability of a M6.7 or higher 51% probability of a M7.0 or higher 20% probability of a M7.5 or higher

Source: https://pubs.usgs.gov/fs/2016/3020/fs20163020.pdf

And?? California building codes have improved alot since the 1980s or so. I remember the 1989 LA earthquake as I drove through the aftermath to visit my grandma about 2 weeks after, I live on the Central coast.

All in all not since 1933 has CA lost more than 100 lives in an earthquake: https://en.m.wikipedia.org/wiki/List_of_earthquakes_in_Calif...

That tells me our building codes are pretty damn good.

I work at ucsb and the catilina islands recently had a 5.2 about 2 weeks ago and the building shook abit. But all in all unless we suffer an 8.0+ I'm not terribly worried (knock on wood). To put that another way, realistically for any moderate sized earthquake CA is generally well prepared in terms of architecture, unless the big one hits ... In which case just kiss your butt goodbye.

I do wish, even for the big one, that CA invested in an early warning system like Japan has ... Imagine a 5-50 seconds warning of an earthquake, that would save lives, not stricter building codes: https://youtu.be/OXXZouxPT7U

Here is a better, live example of the early warning: https://youtu.be/n-FMpNBfna8

An early warning system might be nice, but stricter building codes are what is going to save lives. Japan has those too.

It's also worth pointing out that there has arguably not been a large quake close to a major city center in California since 1906 (for some values of "large" and "close"). The 1994 Northridge quake, which did some $15B in damage, was probably the best recent preview we have. Loma Prieta certainly did serious damage, despite being (as I recall) some 75 miles from San Francisco.

The real test is when the Calaveras Fault that runs up the east side of the SF Bay breaks. The last time was 1868, when there wasn't much there; estimated magnitude was 6.3 to 6.7. A quake of that size on that fault now — and it's getting to be likely, in the next two or three decades — is going to make a hell of a mess. We'll find out then how well a lot of things hold up.

Earthquakes are interesting, I was mostly talking about terrorist attacks, which are basically rounding errors when compared to other causes of death.

I don't think it would cumulatively add up to much, but would love to see the math.

I don't agree with Klemencic on this - given a big enough earthquake with large enough building sway (imagine a tower being a vertical beam with the base fixed and the top swaying back and forth) the structure can fail such that the momentum of the building during maximum sway will either break the building and/or cause the building to impact other swaying buildings.

Simulation showing an actual building failure from the NZ Christchurch earthquake [1]. It collapsed in the Feb 2011 Christchurch earthquake, magnitude 6.2[2].

Once the building's supports fail, it drops rather than topples.

[1] https://www.youtube.com/watch?v=TCJq_2-q34k [2] https://en.wikipedia.org/wiki/2011_Christchurch_earthquake

That's a squat cube-like 5 story building. You're probably right, but that video alone doesn't convince me a skyscraper would necessarily fail in the same way.

It wouldn't. The are designed to withstand hurricanes. It takes extensive preweakening to bring one (or three) down.

People are more likely to be killed by flying glass in the Financial District than skyscrapers collapsing. A good approximation of how far glass can be thrown downtown is roughly 2x building height due to the winds coming off the bay.

Is that supposed to make it better?

I have friends who unfortunately got to witness what happens when plate glass comes down. They can barely talk about it. It also means that the injured and trapped folks in the high rises are going to be without aid in buildings with dead mechanicals for awhile.

I think he's saying, in an earthquake, stay inside.

A lot of people think running into the street is what you wanna do. It is, in like, Mountain View maybe. Realistically you want to just jump under the nearest thing you can and hope for the best. If you're in the Financial District in the middle of the street... well your friends apparently have seen what that'll look like.

Right, that was the point I was trying to make, perhaps I didn't state it well. Seek shelter immediately and try to avoid flying glass. I would also avoid UMBs (Unreinforced Masonary Buildings) these buildings WILL collapse. They aren't as common in FiDi but are all over Soma, Chinatown, and the Tenderloin.

Yeah. Lots of stuff comes from above if you are on the street. Going inside a building is a good bet.

"35 deaths and many injuries from falling masonry during the 2011 Christchurch earthquake. Buildings with dangerous parapets, facades and verandahs" [1]. That was 20% of total deaths.

Certainly near a skyscraper, you want to go inside, since if it pancakes, being outside won't help anyway - debris will hurtle horizontally.

[1] https://www.stuff.co.nz/business/71738867/property-council-s...

> like, Mountain View maybe

During the Loma Prieta Earthquake a bunch of my coworkers fled the building and took shelter in the parking lot under the power lines.

> The leaning building across from my office, for example, has nothing to do with seismic issues

Nowhere in the article did anybody make such a claim. It was to illustrate the general problems of building in that area even without an earth quake, which leads me to the other point, what IMO is the main issue, and I think is pretty independent of how much effort is spent on any one building: The potential for soil liquefaction. They included several maps showing that many of the tallest buildings are built pretty much exactly over ground that might suffer that fate in a big earth quake. How would any individual design help? Compared to the alternative of not building the tallest structures right over those spots. The maps also showed plenty of more solid ground, but that is where there are mostly lower buildings.

I would like to hear your opinion on that issue.

As for your doubts on the entire article, it seems to me they asked quite a few specialists? Are they all mistaken?

Well, I certainly would like you to be right, I used to live in SF and I still love the entire greater Bay Area.

Liquefaction mitigation is possible (ground replacement cells, compaction grouting, etc). It costs a lot and the only place that we (my company) have done true liquefaction mitigation is on large state funded jobs. The preferred alternative for the Bay Area is to sink foundations through liquefiable soils, discounting any strength from these soils, and rely on empirical studies that have found that liquefaction does not necessary result in loss of confinement which would induce buckling failures in the piles.

I share the GP's doubts on the article. The practicing specialists they talked to certainly don't think there's a gamble: they actively pursue and design sky scrapers in the City. I think their words are being misused in a way that makes them seem like they agree with the article's premise.

A modern high rise- but only as long as you are confident there were not basic design/engineering/construction flaws?

Which, for those of us who are not structural engineers and cannot review the work to see that it is indeed well done...

If the earthquake were large enough, could the Millenium Tower completely fall over? Or is it only destined to keep leaning?

High rises in general are designed to sway in an earthquake - that sway absorbs the horizontal forces being applied - similar to a tree. If an earthquake gets sufficiently large, say larger than any known measured earthquake to date, that sway differential would increase enough that buildings would either hit each other, or the building would sway beyond the ability of the structure to recover and fall over. A standard approach for tall buildings is to incorporate either a dynamic weight high in the tower to counterbalance the sway so it doesn't go critical, or incorporate dynamic elements in the foundation system. In general, if a modern high rise is falling over, all buildings without the benefit of this level of modern engineering have already fallen over.

> A standard approach for tall buildings is to incorporate either a dynamic weight high in the tower to counterbalance the sway so it doesn't go critical, or incorporate dynamic elements in the foundation system.

As a structural engineer on the west coast, I can say that neither of these are standard. Tuned mass dampers (TMDs) are commonly used for reducing wind-induced vibration, but not for seismic applications. Nor have I ever designed, seen, or heard of 'dynamic elements in the foundation'

The reason you can't rely on TMDs is because they are tuned to a structure's elastic period of vibration. That is to say, the materials are still in their elastic range. When a ductile structure is subjected to a sufficiently large earthquake, elements of the seismic force resisting system (SFRS) will yield. Hence the period will elongate. Hence the damper will be detuned and may not provide any benefit at all.

A link to the next generation TMD's to handle earthquakes: tuned mass blocks, orthogonal poles, and torsional pendulums (TMDPP) : https://www.hindawi.com/journals/sv/2017/5834760/

1) I'm not saying there aren't novel solutions - there's a whole category of devices that use active control. I'm saying its not common place.

2) From what I understand, China has a completely different design philosophy. They design structures to remain elastic, under certain earthquakes, which would then allow for TMDs.

I think he is referring to base isolation which, I agree, is non-standard.

I meant standard for the really big buildings under the highest stress. For lesser challenges, agreed, non-standard.

They don't base isolate tall buildings, they isolate short ones. The whole point of base isolation is period elongation. In a tall building, you're already flexible, and your column loads are tremendous. It makes no sense.

Would you be able to provide an explanation of what a dynamic element in the foundation is, for example?

In mechanical fields, dynamic means "moving". The weight literally moves, so it's dynamic. Another YouTube video that explains it a bit better: https://www.youtube.com/watch?v=f1U4SAgy60c

See how a dampened dynamic build vs a regular dynamic build react to movement (and you can imagine the static build): https://youtube.com/watch?v=xp2pGxFzrzI

>Apologies for the complaint, but could someone please let me know why I'm being downvoted for this question?

It's likely because you're implying "dynamic" is a "buzzword" in a discussion about structural engineering and seismic movements (which is, by definition, a dynamic system).

But I'm not, I was saying that due to my background, I can't help but think dynamic is a buzzword. Therefor I was asking someone who was clearly knowledgeable on the subject to set me straight.

The "why"s (your prejudices) are irrelevant.

Also I don't really think there's a background you can have where you don't understand, or cannot lookup, the difference between dynamic and static if you really cared to understand. Seems more like self-important rhetoric than anything.

I don't think linking it to a buzzword adds anything to the conversation. The question didn't need a justification.

All right. I've removed it. Such is life

What is your background?

This gets at my concern though - it is not that we don’t know how to build skyscrapers safely. It is that there is now positive evidence that those construction and engineering techniques are not being applied properly, at least in some cases.

For me, the question is not about whether high quality projects are going to be okay. The question is: on what basis do I believe there has been sufficient oversight to guarantee the soundness of new construction?

I can remember talking to SPUR members in 2001 about how problematic it was to extend downtown development into SOMA, etc, due to liquefaction and uncertainty. When the building craze hit, it felt like it brushed these concerns aside, rather than answer them. Now we have leaning buildings that don’t meet basic construction requirements. It doesn’t inspire a lot of faith.

Are design parameters such as pile depth part of the public record? Is there a way to lookup the design of a building?

Most cities keep a set of permit drawings, but generally you need to have a proximate relationship to the building to get access to them

The leaning building across from my office, for example, has nothing to do with seismic issues but basic design/engineering/construction flaws

You say that, and then you claim that seismic engineering is free from such flaws? That seems a bit like hubris.

Seismic is different than basic construction. That building tilt would have happened anywhere vs. in an earthquake prone zone like SF.

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