[HN Gopher] The most expensive number in engineering ___________________________________________________________________ The most expensive number in engineering Author : as89 Score : 262 points Date : 2021-05-31 16:34 UTC (6 hours ago) (HTM) web link (surjan.substack.com) (TXT) w3m dump (surjan.substack.com) | bjt2n3904 wrote: | Obligatory Calvin and Hobbes comic: | https://www.gocomics.com/calvinandhobbes/1986/11/26 | dhosek wrote: | Calvin's dad is my role model as a father. My 7-year-old | children believe that in the past the world used to be black | and white. | aetherspawn wrote: | I used to work for a place that built fast cars. We had a mate | that used 5.0 or more for the factor of safety everywhere. | Everything he designed was about 30% heavier than it needed to | be, but we could easily adapt his parts for prototyping because | it never mattered if you drilled a hole through the middle or cut | them in half. They were plenty strong and reliable. | | We called this the "Factor of Lloyd" and we had a few sayings | about it. | GCA10 wrote: | I'm liking the Robert Norton chart about 2/3 of the way down, | showing how safety factors need to be adjusted quite radically | once we think about how reliable or rickety our estimates might | actually be. | | Particular kudos to thinking harder about whether we've truly | tested the actual environment where our product might be used. | | I wish social scientists would do the same in controlled studies | of human behavior -- which are then extrapolated to the ways that | people make real-world decisions. A particularly vexing examples | involves the way that psychology students make decisions in short | experiments involving small amounts of money or other rewards. | (Endless variations on the "marshmallow test," etc.) | | Knowing what a college student will/won't do for a whimsical $5 | reward says almost nothing about how an adult on the brink of | poverty will balance bigger, more difficult decisions. Yet we | apply a 95% confidence level to the college-student experiment | and think we've learned something about the power of all | financial incentives | jvanderbot wrote: | The factor of safety probably doesn't have a specific definition | because it's application and part specific. Its an axiom like the | 5-sigma rule not a property of the system. | iab wrote: | Exactly, it buffers against modeling inaccuracies | BurningFrog wrote: | I'm sure engineers across geography and time all use a Factor of | Safety. | | I'm almost as sure that everyone keeps using the number people | used before they joined the profession. Because if you decide to | lower it, and a disaster happen, you are in very deep shit. | | So once set, the number will tend to stick until forced to change | by something extraordinary. | | Which makes me very curious about how the number varies between | independent domains. Do Japanese, Norwegian and US bridge | builders all use the same number? Do builders of bridges, | skyscrapers, and dams use similar numbers? | | The answer would tell us something about how arbitrary these | numbers are. | victor106 wrote: | Reading this it seems like something like this could be used for | software estimates as well? | | Bake in a factor of safety into your estimates depending on the | type of work, the track record of the team that's doing the work | etc | rossdavidh wrote: | I have seen attempts at it. One is to multiply your estimates | by the number of different pieces you're estimating. So, if you | have estimated for three different pieces, multiply those | estimates by 3 when deciding how much the whole thing will | take. If you have estimated for five different pieces, multiply | the sum by 5, etc. The idea is that the more estimates you have | made, the more likely that at least one of them will "blow up" | and take far longer than expected. | | Generally speaking, though, software is far less advanced than | civil or aeronautical engineering in this kind of thing. | iscrewyou wrote: | Factor of Safety or F.S. for short was something us civil | engineers were taught to never forget. You got grades deducted if | you solved the problem correctly but forgot to include it in the | very last line. | | It makes sure we calculate the loads correctly and use | appropriate materials. You can't fix a bad design. | | The Arkansas bridge that has been in the news lately probably | would have collapsed if it wasn't for the F.S. | https://www.ardot.gov/divisions/public-information/40-ms-riv... | unethical_ban wrote: | Aren't all the major bridges in New York built with ridiculous | safety factors? It's why these century old bridges built for | carriages and small trucks in a city of a million, can deal | with 2021? | | It's fascinating to me, and I feel like over-spec'ing certain | chokepoints in infrastructure makes sense like this. | firebaze wrote: | The funny thing is a safety factor is a factor after all. | Just one needs to be small enough, and the whole construction | (pun intended) may collapse in the worst imaginable manner. | | This happened recently in Italy: | https://www.bbc.com/news/world-europe-57219737. "Engineers" | didn't consider the safety brake essential (i mean, why do | you even need it?), and Murphy took his chance. | boulos wrote: | That was the original article which didn't have the cause, | and I hadn't seen that they'd decided it was disabling the | brakes [1]. A couple days ago it was "not sure which was | first: support cable snap or emergency brake". | | It seems like they've decided that the support cable was | functioning after the main cable broke. | | [1] https://www.nbcnews.com/news/world/blame-italy-cable- | car-dea... | hypertele-Xii wrote: | Then there's the factor that, in the times such a bridge was | built, having it collapse would've been a larger catastrophy | than today. Today, we can quickly fix things and build | another. In the old times, that bridge might've been the only | bridge making trade at all possible and it might've taken | years to stack stones. | | An unnecessarily strong castle takes you more time and | resources to build. | | A slightly too weak castle means you die, your family dies, | and you lose all wealth and power. | whatshisface wrote: | > _It makes sure we calculate the loads correctly and use | appropriate materials._ | | No, it make sure that nobody dies when you calculate the loads | incorrectly and use inappropriate materials. | rossdavidh wrote: | I think the idea is that, if you calculate the load | incorrectly enough, or use inappropriate enough materials, | the safety factor will not save you. But, if you have done | those things correctly, then the safety factor should be | enough to save you from normal unknowns, unexpecteds, etc. | Gibbon1 wrote: | Yeah four things going on. Design errors | Probabilistic nature of the loads applied. Material | defects Fatigue Deterioration | | All structures have a service life and it's the service | lifetime an experienced engineer is trying to hit. | | For the impeller in a rocket turbo pump the service life is | like 5 minutes. For the impeller in a hydro electric dam | it's 50 years. | | The other thing that one of my professors pointed out was | that 80% of engineers end up designing once off designs. | Where the NRE cost is a lot more than the material costs. | Shaving the safety factor is false economy. | wiredfool wrote: | Fabrication errors. Off by one errors. | wiml wrote: | Also helps when management decides to defer maintenance for a | decade or two and someone drives a truck over it that's just | a _bit_ over the weight limit what could it hurt? | whatshisface wrote: | Perhaps implicit institutional knowledge of large safety | factors is why management feels safe deferring maintenance. | iscrewyou wrote: | Can't tell if this is sarcasm. | | Because of goal of civil engineering is building man made | objects with public safety in mind. | zdragnar wrote: | Another way to look at factor of safety is margin for | error. Implementation variance, material variance, etc can | all go wrong if something is designed to be _exactly_ safe. | | You need to _know_ that something is redundantly safe, and | which parts. | LegitShady wrote: | Material variances are included where the capacity is | calculated. materials with hIgher variance like concrete | (implementation variance) and wood (material variance) | get their capacity lowered more than more consistent | materials like steel. | | The factor of safety is above and beyond material | variance. You calculate the worst case load combinations | for that component, then you check your factor of safety. | Civil engineering is relatively conservative in its | estimations for everyone's safety. | | In my experience serviceability requirements (like | reducing uncomfortable deflections that dont threaten the | safety of the structure) often govern, rather than the | ultimate capacity. | mp3k wrote: | "You can always be thinner" | | https://www.youtube.com/watch?v=7PphbSFZWuU | a9h74j wrote: | (To the OP title: More expensive than disaster?) | | Many comments here relate more to one-off design. There is also | the medium-high-volume manufacturing end. There, a prototype run | might be in the dozens or hundreds of units, more than the entire | manufacturing run in other heavy industries. | | As the OP hints, "safety factor" is not the only term to use. A | _design margin_ (including reduction in margin) can be planned | with one or more motivations: safety, reliability, weight, | volume, reduced BOM costs, unit costs of repair, fleet costs of | repair, logistic and warehouse costs of parts for repair, planned | obsolescence, and so on. | | Probabilistic design, also realized through "Monte Carlo" | analysis, can take into account multiple simultaneous non- | linearity in various models, where symbolic or formula-based | analysis might fail. | | For example (and roughly speaking), if one has millions of miles | of over-the-road data, say of wheel-to-road forces or geometric | road or track profiles, then one might manage to calibrate the | following together: 1) a specific vehicle physical model, | including parts tolerances and probabilistic discrete flaws; 2) | material cycle-fatigue damage properties; and 3) some set of | Weibul-distribution-like parameters as an intermediate in | predicting failure rates and "lifetimes." .. AFAIK the kind of | business analysis one might do could include predicting how many | parts one should overproduce and warehouse (in a one-time batch) | to service in-warranty and post-warranty repairs out to N years. | | At that scale it can also become sociological. "Safety margin" is | a loaded term when it comes to liability and imprecision in | intent. _You reduced the safety margin, as it says right here?!!_ | | Not a bad article, but there could be a whole article on | ramifications of different margin-related wording, high-N | statistics, and explicit accounts of simultaneous goals. | snowwrestler wrote: | Grossly increasing the factor of safety is a subtle way that | science fiction stories connote a feeling of very advanced | technology. | | For example in the JJ Abrams movie Star Trek Into Darkness we see | the Enterprise operating at depth in an ocean, then dramatically | zooming away into space. Then later another ship falls from | orbital height and plows through San Francisco without losing its | hull shape. | | In Star Wars the Millenium Falcon is constantly doing things that | would seem to be outside a normal design for a spacecraft, and it | survives (aside from the radar dish). | | Even as far back as the movie 2001, the monolith is made out of a | material that humans can't dent or cut. Why so strong? It's | basically just an automated radio. | | The idea is: this advanced civilization has such command over | physical technology, that they can effortlessly engineer | unnecessary strength without losing any of their designed | performance. | im3w1l wrote: | I think we need to distinguish between must-have and nice-to- | have safety. To give an example, a car _must not_ sponaneously | disintegrate during normal high-way driving. That 's what | safety factor covers. If you floor it during heavy rain, and | start skidding, crash into a tree, then that's kind of on you. | Nevertheless the car will try to save you with abs breaks, | crumple zones and airbags and what not. | | So the future you speak of is already kind of a reality with | cars. But maybe expressing it as a pure factor is the wrong way | to think about it. | djoldman wrote: | I wonder if the strength of the monolith isn't targeting | durability on the order of eons? | 05 wrote: | > For example in the JJ Abrams movie Star Trek Into Darkness we | see the Enterprise operating at depth in an ocean, then | dramatically zooming away into space. | | Subverted in Futurama: | https://www.youtube.com/watch?v=7GDthiBGMz8 | cardiffspaceman wrote: | What about a craft like the space shuttle? You have it | powered through the atmosphere by a set of powerful rocket | engines. The forces on its hull while it is accelerated | through the Atmosphere by rocket engines or gravity would | seem to be analyzable as atmospheres of pressure. So the | Enterprise and Bessie both need to be designed for more | atmospheres than 1.0, due to the fact that both ships | encounter gaseous environments while under acceleration. | vladTheInhaler wrote: | What your're talking about is called the max q | condition[1]. It's definitely a significant design | consideration, but I believe that the loading would very | different when the rocket is plowing through the air in a | particular direction, compared to an "equivalent" | hydrostatic stress applied uniformly over the surface, so | even though the structure might be fine with the first, it | wouldn't survive the second. For instance, think about | corrugated or honeycomb materials - they often have a | "strong" orientation and a "weak" orientation. | | [1] https://en.wikipedia.org/wiki/Max_q | hef19898 wrote: | Back in the old Star Wars RPG from Westend Games, hull | integrity was amplified by ballistic shields that were always | on. Reason being, that hull had to withstand debris hits at the | rediculous speeds in space. Given the speed and energy, I would | assume kinetic impact would matter much. Pressure is different | so, as even the hardest space ship only has to hold 2 bar, give | or take, of pressure _in_. | tzs wrote: | > Even as far back as the movie 2001, the monolith is made out | of a material that humans can't dent or cut. Why so strong? | It's basically just an automated radio. | | It's an automated radio that is supposed to operate unattended | for millions of years. You want a strong case for that. | UncleMeat wrote: | I don't think this is science fiction doing this to connote the | future in many of these cases. | | The cars in the "fast" franchise also survive ludicrous damage. | The bodies of action heroes survive falls from ridiculous | heights and blows and stab wounds that would kill somebody with | ease. | | This is more a property of action movies (and adventure movies, | to a lesser degree). | proggy wrote: | The loose industry term for this is "plot armor" [1]. There | is no explicitly stated reason for the hero(es) being nigh | invincible (be it person, spaceship, car, etc.). The only | reason why the character survives is that it has a reason to | continue existing for sake of the plot. | | So yes, I suppose you can rationalize in your head that most | ships are made out of super strong materials in science | fiction, but unless that's clearly laid out, you may just be | rationalizing writer's convenience. | | [1] https://tvtropes.org/pmwiki/pmwiki.php/Main/PlotArmor | glitchc wrote: | Seems disingenuous to lump a science fiction movie where the | focus is future technologies to an action movie with | exaggerated physics and lack of real damage. In action | movies, bad guys die with one bullet while the hero finds his | way to safety (and survives!) despite having 10 or more | lethal bullet wounds. | | For the record, Star Wars is not science fiction nor has it | ever been portrayed as such. It's very much an action | adventure set in space. | UncleMeat wrote: | Three movies were referenced in the comment I responded to: | Return of the Jedi, Star Trek: Into Darkness, and 2001: A | Space Odyssey. Two of those movies are definitely not | focused on future technologies. | | Many people call Star Wars "science fantasy", but I'm | extremely confident that this is used less frequently than | "science fiction" to describe it and I am absolutely | confident that I'd be able to find marketing copy by | LucasFilm or distributors describing it as science fiction, | even if that aggravates people who are really into more | cerebral sci-fi. | ben_w wrote: | > For the record, Star Wars is not science fiction nor has | it ever been portrayed as such. | | FWIW this is literally the first time I've encountered | someone claiming that it _isn't_ science fiction. | | ( _Soft_ SciFi, to be sure, but still: https://tvtropes.org | /pmwiki/pmwiki.php/Main/MohsScaleOfScien...) | gourlaysama wrote: | > FWIW this is literally the first time I've encountered | someone claiming that it isn't science fiction. | | Well, there's Science Fantasy [1]: Jedi and the Force are | very much Wizards and Magic. | | I've heard it called Space Fantasy, too. | | [1]: https://tvtropes.org/pmwiki/pmwiki.php/Main/ScienceF | antasy | snowwrestler wrote: | A bunch of classic "golden era" science fiction novels | feature characters with unexplained mental powers, like | Asimov's Foundation series, Dune, Niven's Known Space | series, etc. | | Those seem like obvious fantasy now, but from about the | 1950s through the 1970s, a lot of serious people believed | that there were undiscovered powers of the human mind | that science was on the verge of discovering or | confirming. Mental powers are therefore a common | anachronism of sci fi from that era. | | Most science fiction stories are going to feature some | elements that are essentially unexplained and therefore | act like magic in the story. I think most folks would | consider 2001 to be science fiction but the powers of the | monolith are at least as crazy and unexplained as what | the Jedi can do. | dragontamer wrote: | And Vulcan are space elves, Klingon are space orcs and | Borg are space undead. | | All of this stuff has inspiration from classical fantasy. | That doesn't change whether or not something is SciFi or | not. | | There is an intersection of SciFi and Fantasy for sure, | and the line gets blurred. Artificial human stories can | be cyborgs, androids, clones, golems, or chimera, or even | explicit fantasy races like minotaurs. | | If it's an android, then it's SciFi. But if it was a clay | golem made with magic (but otherwise the same story) it's | fantasy. | | See golem stories for instance. They explore a lot of sci | Fi themes but are basically a fantasy theme. | https://en.m.wikipedia.org/wiki/Golem | ben_w wrote: | Certainly many tropes are shared, but that is true beyond | the scifi-fantasy intersection. (Although: I would say | Klingons are the avatar of Russian-ness _in the eyes of | America at the time any given episode gets written_. | Beardy humans in TOS, impoverished but hard as nails in | TNG, anarchic and self-destructive in ENT, sneaky and | dangerous warmongers in DIS). | | I regard the superhero genre as a modern version of the | old divine pantheons -- heroes and villains, supposedly | far beyond human, yet oddly well-balanced against each | other (Hawkeye should not be in the same battleground as | Thor in the same way and for the same reason that Choi | Mi-sun should not be in a battle featuring an attack | helicopter). | | Sufficiently advanced technology being indistinguishable | from magic and all that. | glitchc wrote: | Lucas himself has stated that although science fiction | inspired him to create these stories, Star Wars is indeed | not science fiction but rather science fantasy: | | https://scifi.stackexchange.com/questions/46481/did- | george-l... | | Because all the stories are " a long, long time ago in a | galaxy far, far away," it's not even clear if Luke and | the gang are humans or some other species. Of course it's | easy to think of them as humans, and most people do, but | it's not necessary. This ambiguity is also by design. | robbrown451 wrote: | True but Lucas doesn't have final say on classifying his | movies. People who make things have an incentive to claim | that their work is somehow special. | | I'd just say Star Wars is science fiction with lots of | fantasy elements, as well as lots of influence from | different genres (Earth-based war films, adventure | serials, etc.) | | I see the fact that humans in Star Wars seem to be the | "same species" as Homo sapiens to be similar to the fact | that movies that take place in ancient Greece have the | characters speak modern English. We are expected to | understand that humans in Star Wars evolved there, not in | Africa on Earth. It would be more realistic to have the | main characters look very different from us, but it | probably wouldn't make for as enjoyable a movie. | [deleted] | TeMPOraL wrote: | Things JJ Abrams touches tend to border on the absurd, but the | principle is true in pre-Abrams Star Trek too. | | O'Brien sheds a little light on one of the aspects of this: | https://www.youtube.com/watch?v=UaPkSU8DNfY. | GILORA: Starfleet code requires a second backup? O'BRIEN: | In case the first backup fails. GILORA: What are the | chances that both a primary system and its backup would fail at | the same time? O'BRIEN: It's very unlikely, but in a | crunch I wouldn't like to be caught without a second backup. | | From a different franchise, I keep thinking about the Ancients | of StarGate universe, known for their technology, which could | remain fully operational for _millions of years_. That 's the | true over-engineering. | | (But then I'm thinking, a species that mastered "stasis | technology", commonly present in many sci-fi franchises, should | eventually be able to make artifacts that can survive | indefinitely, at least when not in operation.) | codeflo wrote: | About the Abrams movie: Too bad the same factor of safety | wasn't applied to the buildings, where that should have been | even cheaper. | | Your point is good though. Just like we're now starting to put | wi-fi chips into absolutely anything just in case, why wouldn't | an advanced civilization simply use their super strength | nanoparticles for everything? Why go out of your way to use | worse materials? | ben_w wrote: | In fairness, a building-sized battleship was crashed into the | buildings in _Into Darkness_. What would happen if someone | catapulted the (CVN-80) USS Enterprise through a bunch of | skyscrapers? | WalterBright wrote: | We use super-strength thousand year plastic for one time | uses. It's a big problem. | manquer wrote: | It is only a problem for single planet species. Advanced | multi planet or multi system civilizations may worry less | about that than we do. | | Also if the bio diversity dropped, and people are used to | living in closed ecosystems to limit damages from such | harmful chemicals, over long enough period everyone will | forget what was there before , further making bad things | will not impact their quality of life anyway. | codeflo wrote: | The article suggests using a probabilistic failure model instead | of a large a safety factor, and explains how a safety factor | established in the 1930s affected the cost of the Space Shuttle. | But spacecraft might be a special case, where any additional | weight is so expensive, and you also expect the models to be | especially accurate and manufacturing to be extremely precise. | | For more everyday civil engineering, I think the safety factor | "covers up" a lot of systemic inaccuracies everywhere in the | system, from modeling to design to manufacturing to unintended | uses. Some of those you might account for in a probabilistic | model. But it's very difficult to probabilistically model errors | in the model itself, as the financial industry found out the hard | way. | | When driving over a bridge built the way suggested here, how | comfortable can we be that certain stresses aren't correlated in | ways that the engineers didn't anticipate? Or that a certain | distribution isn't actually as well approximated by a Gaussian as | it was assumed to be? Intuitively, it's a lot harder to be wildly | wrong with the factor of safety approach. | | To put this another way, a more complex way to reason about | safety necessarily has more moving parts, and is thus more likely | to be wrong. So in effect, adopting more complicated safety | models introduces a safety risk all on its own. I think that | needs to be considered as well. | wiredfool wrote: | I'll go out on a pretty small limb and say that the vast | majority of Civil Engineering failings are not a matter of an | incorrect safety factor, but are things that are explicitly not | part of it. | | 1) Blunders. (Many places. You do the math wrong, or approve | the wrong shop drawing, and no factor of safety is going to | save you). (See the Hyatt Regency Walkway Failure) 2) | Inadequate Geotech Info. (Basically every dam failure ever) 3) | Genuinely new behavior. (Tacoma Narrows) 4) Contractors. (I-90 | Bridge Sinking) 5) Deferred Maintenance. (Fatigue on bridges, | Minneapolis) | kortex wrote: | 6. Corner-cutting. I consider that distinct from blunders or | bad contractors. See: the Pal-Kal construction method. | | I consider that not part of the safety factor because IIRC it | was basically fudged or outright ignored, often times with | building inspectors paid off. | | https://en.m.wikipedia.org/wiki/Versailles_wedding_hall_disa. | .. | ghaff wrote: | Agreed. Although I'd probably also argue that a safety factor | probably papers over those kind of problems in many many | cases. | londons_explore wrote: | But the question is, if you used a probabilistic approach, | and tried to model, even very roughly, those things | (probability contractor bodges the job in a way inspection | doesn't notice: 10%), then would you end up with a safer | bridge for the same money spent? | ghaff wrote: | It feels like you're picking numbers out of the air (or | basing them on historical experience) either way. Unless | you actually have historical data on certain types of | problems--but then it seems like you're pretty much back | to a safety/fudge factor. | londons_explore wrote: | But a large number of guestimated fudge factors all added | up will approach the true value as long as there is no | bias. | | The same does not apply for factors of safety - a 1.5 FoS | is always between 0 and 50% too much. | wiredfool wrote: | That can be an unbounded black swan event. There are | distributions where there is no mean value. | | The difference between a square section and welded | channels. The difference between 53F and 27F. The | difference between putting the waterproofing on before or | after the post tensioning anchors. Leaving watertight | doors open during a storm. | lamontcg wrote: | Or more simply: planes that get hit with massive unexpected | turbulence shouldn't just drop out of the sky and kill everyone | on board. | | And to paraphrase the great philosopher Donald Rumsfeld, it is | all about the unknown-unknowns. | WalterBright wrote: | Aircraft have a smallish safety factor, because of the weight. | They make up for it with much more careful design, manufacture, | and maintenance along with frequent inspections. | abduhl wrote: | Most modern bridges you drive over are designed the | probabilistic way that is suggested in the article. Bridge | design followed vertical construction as material science and | manufacturing got better for steel and concrete. The | probabilistic approaches haven't been adopted much in | engineering fields that deal with too many unknowns. I'm | actually incredibly surprised that there was no mention of the | technical terms for these approaches: Allowable Stress Design, | Load and Resistance Factor Design, and Yeah That Looks Right | Design. LRFD is highly probability and materials testing based. | ASD is a hybrid approach of old factors of safety and some | probabilistic theory. YTLRD is based on the long and storied | history of guys who have been doing it this way since before | you were born, no matter when that was. | wiredfool wrote: | And they're all used, to some extent. If your Wizzy design | based on the latest everything doesn't pass the grumpy old | partner's YTLRD review, you're going to redo it till you do. | | (In my case, that was Bill. He was one of those guys who knew | where to put the $50k mark) | wiredfool wrote: | """A non-empirical alternative to the factor of safety has been | around since the 1940s, but still doesn't have widespread | adoption. I think the image below describes the concept, called | probabilistic design, best. | | """ | | This is _exactly_ LRFD (Load Factor Resistance Design) which has | been in the Civil Engineering building codes since the mid 80's, | and became common in use the 90's when I was an Engineer (in | training). | | (It's the difference between the older green book and the newer | (at the time) silver steel design handbook) | | It was absolutely drilled into us in school though, that Safety | Factors and LRFD factors covered material and other uncertainty, | they did not cover blunders. | greesil wrote: | Probabalistic failure analysis is certainly something engineers | do for determining system risk, else how do you determine how | many redundant components to include? I seems like having a | higher safety factor just means having a lower probability of | failure, and these two concepts are very compatible. | wiredfool wrote: | The probabilistic failure analysis (as practiced in LRFD) is | essentially a pencil sharpening exercise where the margins can | be reduced a bit. For example, some loads are better known than | others (e.g., dead load vs live load), some materials have | better QC or a more uniform quality than others (think concrete | vs steel). | | The end result is generally in the ballpark of the old factor | of safety, but might be up to 10% less in some cases. | steve76 wrote: | ??? | | You build people movers, and wonder about safety factor??? | | WTF!!! | | There's also service factor. Protip: Multiply them both please. | akhilpotla wrote: | I've actually used this concept at an old job. When I was given a | new project the business people always wanted it done at a | particular date, but it was always an unrealistic timeframe. I'd | then spend sometime thinking about how long I thought it would | take me, but I would always add 2 weeks or 25% to the estimated | time, which ever was larger, just to deal with the human element. | | This could include changing requirements, poor communication, | illness, being blocked by other changes, etc. | | I learned that you can get away with giving people extended | deadlines as long as you hit them. | CharlesW wrote: | Is there a software equivalent of safety factor? How do you/would | you calculate it? | bretpiatt wrote: | I'm not a civil or aerospace engineer so this could be built into | the safety factor models already. Reading the post had me | wondering: | | If safety factor adds mass and additional mass requires | additional force to accelerate is a lower safety factor safer | since you'll lower the amount of force required thus increasing | the structural safety? | | Calculating safety factor for a given scenario feels like a | complex multivariable equation. Is that the case? | bandrade wrote: | Yes, all (essentially all) engineering design ends up being | multivariable. For even something as simple as a cantilevered | beam supporting a load, if you can change the shape, material, | material treatment, length, width/height, all of which affects | cost. Usually due to limits of manufacturing and availability | of standard parts, the exploration space can be greatly | reduced. | Galxeagle wrote: | Also not an engineer but watching a real-world example of that | thought process was fascinating during NASA and SpaceX's design | process for Dragon Capsule, that contained a requirement that | the capsule needed to have a statistical probability of loss- | of-crew less than 1:270 flights, which is the alternative | design measure in TFA. | | One challenge was NASA's modelling of in-orbit micrometeorite | strikes was complex, and there were concerns that extra | complexity to provide redundancy and armor would make an | overall less-safe vehicle. | | "Blindly striving to achieve a statistical loss of crew number | may drive you to design a system that is less safe" -Bill | Gerstenmaier, NASA associate administrator for human | exploration and operation [0] | | [0] https://spacenews.com/commercial-crew-vehicles-may-fall- | shor... | amelius wrote: | Clickbait title, not what I expect from an engineer. | iab wrote: | This article is definitely written from the perspective of a | novice without real-world experience. Empiricism is not a dirty | word! | edem wrote: | `null`? | ghaff wrote: | Apropos of nothing really but I always loved this story because | it tells of building something so that _all_ of its components | were perfectly matched in longevity: | | http://holyjoe.org/poetry/holmes1.htm | | The Deacon's Masterpiece | | or, the Wonderful "One-hoss Shay": | | A Logical Story | | by Oliver Wendell Holmes (1809-1894) | | Have you heard of the wonderful one-hoss shay, That was built in | such a logical way It ran a hundred years to a day, And then, of | a sudden, it -- ah, but stay, I'll tell you what happened without | delay, Scaring the parson into fits, Frightening people out of | their wits, -- Have you ever heard of that, I say? | | Seventeen hundred and fifty-five. Georgius Secundus was then | alive, -- Snuffy old drone from the German hive. That was the | year when Lisbon-town Saw the earth open and gulp her down, And | Braddock's army was done so brown, Left without a scalp to its | crown. It was on the terrible Earthquake-day That the Deacon | finished the one-hoss shay. | | Now in building of chaises, I tell you what, There is always | somewhere a weakest spot, -- In hub, tire, felloe, in spring or | thill, In panel, or crossbar, or floor, or sill, In screw, bolt, | thoroughbrace, -- lurking still, Find it somewhere you must and | will, -- Above or below, or within or without, -- And that's the | reason, beyond a doubt, A chaise breaks down, but doesn't wear | out. | | But the Deacon swore (as Deacons do, With an "I dew vum," or an | "I tell yeou") He would build one shay to beat the taown 'N' the | keounty 'n' all the kentry raoun'; It should be so built that it | couldn' break daown: "Fur," said the Deacon, "'tis mighty plain | Thut the weakes' place mus' stan' the strain; 'N' the way t' fix | it, uz I maintain, Is only jest T' make that place uz strong uz | the rest." | | So the Deacon inquired of the village folk Where he could find | the strongest oak, That couldn't be split nor bent nor broke, -- | That was for spokes and floor and sills; He sent for lancewood to | make the thills; The crossbars were ash, from the straightest | trees, The panels of white-wood, that cuts like cheese, But lasts | like iron for things like these; The hubs of logs from the | "Settler's ellum," -- Last of its timber, -- they couldn't sell | 'em, Never an axe had seen their chips, And the wedges flew from | between their lips, Their blunt ends frizzled like celery-tips; | Step and prop-iron, bolt and screw, Spring, tire, axle, and | linchpin too, Steel of the finest, bright and blue; Thoroughbrace | bison-skin, thick and wide; Boot, top, dasher, from tough old | hide Found in the pit when the tanner died. That was the way he | "put her through." "There!" said the Deacon, "naow she'll dew!" | | Do! I tell you, I rather guess She was a wonder, and nothing | less! Colts grew horses, beards turned gray, Deacon and deaconess | dropped away, Children and grandchildren -- where were they? But | there stood the stout old one-hoss shay As fresh as on Lisbon- | earthquake-day! | | EIGHTEEN HUNDRED; -- it came and found The Deacon's masterpiece | strong and sound. Eighteen hundred increased by ten; -- "Hahnsum | kerridge" they called it then. Eighteen hundred and twenty came; | -- Running as usual; much the same. Thirty and forty at last | arrive, And then come fifty, and FIFTY-FIVE. | | Little of all we value here Wakes on the morn of its hundreth | year Without both feeling and looking queer. In fact, there's | nothing that keeps its youth, So far as I know, but a tree and | truth. (This is a moral that runs at large; Take it. -- You're | welcome. -- No extra charge.) | | FIRST OF NOVEMBER, -- the Earthquake-day, -- There are traces of | age in the one-hoss shay, A general flavor of mild decay, But | nothing local, as one may say. There couldn't be, -- for the | Deacon's art Had made it so like in every part That there wasn't | a chance for one to start. For the wheels were just as strong as | the thills, And the floor was just as strong as the sills, And | the panels just as strong as the floor, And the whipple-tree | neither less nor more, And the back crossbar as strong as the | fore, And spring and axle and hub encore. And yet, as a whole, it | is past a doubt In another hour it will be worn out! | | First of November, 'Fifty-five! This morning the parson takes a | drive. Now, small boys, get out of the way! Here comes the | wonderful one-hoss shay, Drawn by a rat-tailed, ewe-necked bay. | "Huddup!" said the parson. -- Off went they. The parson was | working his Sunday's text, -- Had got to fifthly, and stopped | perplexed At what the -- Moses -- was coming next. All at once | the horse stood still, Close by the meet'n'-house on the hill. | First a shiver, and then a thrill, Then something decidedly like | a spill, -- And the parson was sitting upon a rock, At half past | nine by the meet'n-house clock, -- Just the hour of the | Earthquake shock! What do you think the parson found, When he got | up and stared around? The poor old chaise in a heap or mound, As | if it had been to the mill and ground! You see, of course, if | you're not a dunce, How it went to pieces all at once, -- All at | once, and nothing first, -- Just as bubbles do when they burst. | | End of the wonderful one-hoss shay. Logic is logic. That's all I | say. | Terretta wrote: | I like the "most expensive number" hook, and enjoyed the read. | | However, I'd hazard the most unnecessarily costly variable in | engineering (over time, in aggregate; as well as on most any | given substantial project) is the number of days later a project | starts than it could have if it had just gone ahead and started. | charcircuit wrote: | Are we sure this is the most expensive? I would guess the number | which represents the bit pattern of a Windows 10 iso cost | Microsoft more than $1.5 billion to find. I am sure you can find | other examples of numbers that were expensive to find. | marmaduke wrote: | Is your username a play on charcuterie? | charcircuit wrote: | No, it isn't | supernova87a wrote: | If you had the luxury of throwing away bridges or planes or space | shuttles to test every possible circumstance, then I guess | eventually the safety factor could conceivably come down to 1.0, | right? You would've satisfied yourself that nothing in the real | world was not in your simulations? | whatshisface wrote: | A safety factor of 1.5x does not guarantee that nature will not | throw 1.6x the expected force at you. That's why the author of | the article calls it a "libation," because it isn't related to | anyone's knowledge about the uncertainties in the situation at | hand. | preinheimer wrote: | I don't think so. | | - You'd also need to let them stand for 100 years or something | to get a better view of all possible weather events. Oh wait, | weather events are becoming more extreme. - Materials of | production are imperfect. We're well past poorly made cast | iron, but maybe something wasn't quite perfect when that bolt | was cast. - Improper usage or external emergencies may still | impact usage. | ajuc wrote: | > If you had the luxury of throwing away bridges or planes or | space shuttles to test every possible circumstance, then I | guess eventually the safety factor could conceivably come down | to 1.0, right? You would've satisfied yourself that nothing in | the real world was not in your simulations? | | Isn't that the main advantage Space X has over NASA? | ghaff wrote: | It's not really Space X vs. NASA. NASA doesn't build rockets. | ULA is probably the relevant comparison. NASA's also not | really into exploding rockets so saying Space X is about | shaving safety factors is pretty simplistic. | gerdesj wrote: | "Safety factors started being formalized in the mid-1800s for | bridge building, where factors as high as 6 were used to cover | for the massive inconsistencies in the quality of early cast | iron." | | I'm not a real Civil Engineer but I was a graduate one from 1991 | - I'm now a IT bod. Anyway, Civ Eng uses established factors of | safety or safety multipliers or safety factors or whatever. | Structural steel uses 1.2 I recall - so you work out your worst | case (in 100 years - look up tables) bending moment and mult by | 1.2. Civ Eng is one thing and despite our bridges still failing | after 2000 years of really solid knowledge. Tacoma Narrows (who | knew the bloody things fly and shake) or London Millenium bridge | - lol - shake, shake, shake the room - BOOM. | | The thing about safety factors is that they need to be derived | conclusively. In Civ Eng - wood is a bit wayward so the safety | factor for it is quite large compared to steel. | | I have no idea what you do for space thingies (yes I do) but I | would expect _my_ first 50 experiments to blow up - I need to | explore the extremities. | | If I ran a Space Agency I would say something like: "Soz, we are | going to make some cracking firework displays first and then we | will know what to avoid." | TeMPOraL wrote: | > _If I ran a Space Agency I would say something like: "Soz, we | are going to make some cracking firework displays first and | then we will know what to avoid."_ | | This is the modus operandi of SpaceX - they just keep tweaking | their rockets and launching experimental tweaks as much as they | possibly can without risking bankrupcy, as failures teach them | more than successes. | chrisgp wrote: | In finance and gambling, the kelly criterion is used to evaluate | maximum bet sizing while keeping risk-of-ruin near 0. Using it | correctly requires understanding your own expectation and | variance to a high degree of confidence. Everyone in these | industries uses kelly to figure out the maximum size they can bet | based on these careful expectation and variance calculations, | then just divides by 2. | [deleted] | boulos wrote: | It's kind of mentioned in the article, but to be more explicit: | reducing safety factors has asymmetric risk vs reward. Reducing | the factors "just" lowers cost or improves performance. But if | your field is padding by 50%, then you need to tradeoff an "up to | 50%" cost reduction (or similar) versus "had a catastrophic | failure". | | So, reducing the padding from 5x to 1.5x was already most of the | benefit. If you were at 1.2, there are probably better ways to | shave costs than reducing your unexpected force multipliers. It's | definitely attractive to lower cost / increase speed / whatever | if you _truly_ think it's "free", but the benefits are | diminishing. | j-pb wrote: | Ok let's wing the entire thing from cardboard then! | | * "poorly representative material test data available" that's 5+ | | * "extremely challenging environment" 5+ again | | * "models are crude aproximation" is another 5+ | | So we should be able to get a cardboard spaceshuttle, if we only | use a safety factor of 125+! Moar cardboard! Great job team! | laurent92 wrote: | You're joking but in aerospace they use factors of not 10x or | 2x but 10%. On a 30m high rocket. Just 10%. Here's a tour with | the CEO of ELA, as a bonus: https://youtu.be/OdPoVi_h0r0 | amackera wrote: | Sounds like a few software projects I've been involved in over | the years D: | londons_explore wrote: | The _real_ number engineers should be considering is not the | factor of safety, but the probability of failure. | | The probably of failure should be calculated considering material | defects, forces larger than predicted, simulation errors, and all | other causes the factor of safety is designed to protect against. | | Then the engineering process can allocate those probabilities in | the most efficient way. | | For example, in a rocket it might make sense to make the engine | bells stronger (decreasing probably of failure) while making the | fuel tanks weaker (increasing probability of failure). The | overall probability of failure remains the same, but perhaps the | craft ends up lighter/cheaper/better than it would be if all | components just built in a fixed factor of safety. | afterburner wrote: | Unfortunately, NASA was really bad at estimating the | probability of failure. Feynman famously dissed their lack of | mathematical rigour in this regard. | | I'm guessing most engineers' grasp of proper statistics math is | worse than their understanding of factors of safety. | santiagobasulto wrote: | For context, the FS of elevator cables is ~10 (depending of the | country). | | EDIT: What's usually limiting in elevators (and that's why they | say "max 4 people") are the breaks. | [deleted] | formerly_proven wrote: | > EDIT: What's usually limiting in elevators (and that's why | they say "max 4 people") are the breaks. | | Even here there's a safety factor (self-limiting packing | density of people in Western countries... "Uh... I'm gonna take | the next one"). | vortico wrote: | You could just square your quantity before applying the 1.5 | factor. Instead of "Our shuttle is safe up to 150% of the | required speed!" design for "safe up to 150% of the required | kinetic energy (1/2 m v^2)". Then you only need to design up to | sqrt(1.5) = 123% of the required speed. | | (My point is that the scaling of the importance of quantities is | arbitrary so a single safety factor doesn't make sense to be | applied to every quantity.) | sobriquet9 wrote: | The definition is | | > breaking force divided by the expected force | | Note _force_ being used here, not energy or speed. | jollybean wrote: | Not just 'stress safety' but all the other things. | | A NASA project I was related to we logged every single bolt that | went on the device, where it came from, the batch number, and had | to keep all the old software around in the event we had to | reconstruct something. | | The amount of overhead was pretty amazing. | | Most of that is for safety. | FredPret wrote: | Overhead that saves lives isn't really overhead | throwaway0a5e wrote: | This analysis is great. It makes for a great blog post, | university lecture or similiar. But unfortunately you can't have | these kinds of discussions in an engineering meeting, or other | shared context because anything that could be perceived as | arguing for less safety will attract opposition because there's | tons of people who want the cheap virtue points and ass-covering | that goes with being the guy who's always in favor of more | safety. | rossdavidh wrote: | I think it tends to come up more in fields, such as | aeronautical engineering, where there is a safety tradeoff. If | you make the plane heavier, it may be safer from material | failure, but now there may be less margin for error by the | pilot because the plane does not respond the same. You have | traded one kind of risk against another. I remember being | present when a friend who was a civil engineer heard that they | used a safety factor of only 1.5 in aeronautical engineering, | and she was kind of shocked it was so low; when you don't have | to fly the thing, you can afford to make the factor | significantly higher. | gostsamo wrote: | If you use such an argument in a real design meeting, you might | be asked to leave. Either you have data to support changing the | parameters of the assignment, or you keep your silence. | Accusing everyone else in virtue signaling is at hominem attack | that brings nothing to the table. ___________________________________________________________________ (page generated 2021-05-31 23:00 UTC)