4% is ~ 5%. 10% of 75 is 7.5. To get the 5% I have to divide it by 2, so 4% of 75 is close to 3.25. I will have to multiply it with some safety coefficient at the end, so the exact value doesn’t matter.
Yes, in elevators usually one cable could hold far more than the full weight, then they add 5 more for the safety.
For rail speed limits this is the exact way they calculate it. For road speed limits they consider braking distance, which grows by the square of your speed, so if you go 120 on 60 road, you will need 4 times the distance to stop. I wrote 1.5 as a safety factor, not 4, With a 1.5 safety factor you can go by 75 though, but I would use a 1.1 safety there, as in my country the speed cameras are set up that way, you can go +10% of the official speed limit, they only send a cheque if you went even quicker than that.
Speed limits are trickier than structural safety margins because of several factors:
In some areas, particularly remote areas, the process isn’t very well defined. Sometimes the speed limit will be set by one guy who just felt like that was fine. Doesn’t even have to be an engineer really.
Standards evolve over time (trending towards lower speed limits) but speed limits only change when a tragedy or major road renovation happens. Where I live there’s sometimes a 40 km/h spread on posted speed limits for similar roads depending on whether they were rebuilt last year or 50 years ago.
Car culture means drivers hold a ton of political power. There are a myriad of traffic devices that cannot be built not because of practical or financial constraints, only because they would “inconvenience drivers”. Lower speed limits are often one of those. People complain so the government backs down despite engineering recommendations.
A driver is always liable if they drive too fast for the conditions, not the traffic engineer. That goes to the previous point, with zero penalty for not sticking to the sensible engineering choice, political pressure easily wins out. Hard to argue against a work order when the person signing off on it cannot be sued for negligence.
The upshot is speed limits in my local experience have a lot more to do with the municipality/region’s political climate than engineering standards and safety factors. Sometimes I feel like I could safely go 2x, sometimes the limit is 90 km/h on a two-way one lane road with 30 m of visibility where 30 km/h feels like I’m pushing it.
That’s because elevators use counter weights usually equal to the weight of the car and half the occupancy load so that it takes less energy to lift it and if it falls for any reason it won’t hit the bottom as long as the counter weights are still attached. The occupancy load is determined by the counter weighting system not the cable load capacity.
Yeah, as I understand it, the elevator will refuse to move instead of collapse, and hopefully you’re not between floors when it happens because it was close and someone shifted their weight or bounced slightly or they might write a sitcom episode about what happens next (and the reality will be far more boring).
It’s not wrong, it’s close enough. And the point it works with more numbers and more type of calculation. Let’s calculate 4% of 1243. That’s the same as 1243% of 4, right, much easier to calculate by simply changing the 2 numbers… While my method is the same, by simply rounding everything.
And in engineering you always multiply/divide your results by a 1.5 or 1.25 safety factor, depending on situation. So you don’t have to calculate exact results, just close enough. E.g. G is always 10m/s2. π is only 3.14, the other digits doesn’t matter.
Huh? It’s not “close enough”, it’s exactly accurate. 4% of 75 is 3 exactly. I don’t know where the rest of what you wrote comes from. This post is about pure arithmetic
Rounding once may be okay but rounding multiple times and that errors add up. Astrophysics?! If im working with wood, i don’t care measuring to 0.1 mm and it might be okay in astrophysics to use 10 for pi, but that doesn’t make guessing your math correct in general.
Maybe we are doing things differently here in germany.
My engineering brain says it’s 3.25.
4% is ~ 5%. 10% of 75 is 7.5. To get the 5% I have to divide it by 2, so 4% of 75 is close to 3.25. I will have to multiply it with some safety coefficient at the end, so the exact value doesn’t matter.
Mine brain just does 0.75 × 4.
Thought process was…
That’s why you can always double the maximum limits engineers give.
60 mph roadway?
I can do 120 on it no problem.
Eight person elevator? Sixteen.
0.08 BAC? 0.16 easy peasy.
Can I also smoke two joints before I smoke two joints, and then smoke two more?
At that point you will have smoked 4% of 100 joints. Hey, math is easy.
How do Europeans even measure weed lol
In grams bro.
That’s trippy man, so do we.
Yes, in elevators usually one cable could hold far more than the full weight, then they add 5 more for the safety.
For rail speed limits this is the exact way they calculate it. For road speed limits they consider braking distance, which grows by the square of your speed, so if you go 120 on 60 road, you will need 4 times the distance to stop. I wrote 1.5 as a safety factor, not 4, With a 1.5 safety factor you can go by 75 though, but I would use a 1.1 safety there, as in my country the speed cameras are set up that way, you can go +10% of the official speed limit, they only send a cheque if you went even quicker than that.
Speed limits are trickier than structural safety margins because of several factors:
The upshot is speed limits in my local experience have a lot more to do with the municipality/region’s political climate than engineering standards and safety factors. Sometimes I feel like I could safely go 2x, sometimes the limit is 90 km/h on a two-way one lane road with 30 m of visibility where 30 km/h feels like I’m pushing it.
That’s because elevators use counter weights usually equal to the weight of the car and half the occupancy load so that it takes less energy to lift it and if it falls for any reason it won’t hit the bottom as long as the counter weights are still attached. The occupancy load is determined by the counter weighting system not the cable load capacity.
Yeah, as I understand it, the elevator will refuse to move instead of collapse, and hopefully you’re not between floors when it happens because it was close and someone shifted their weight or bounced slightly or they might write a sitcom episode about what happens next (and the reality will be far more boring).
They send you a cheque for speeding? Sounds like you should be going at least 1.11x to collect your bonuses 😜
(the word you were meaning to use was “fine” or “ticket”)
Did engineers come up with that last one though?
I’m confused by this statement, the answer is 3. Why do all these extra steps for a wrong answer?
It’s not wrong, it’s close enough. And the point it works with more numbers and more type of calculation. Let’s calculate 4% of 1243. That’s the same as 1243% of 4, right, much easier to calculate by simply changing the 2 numbers… While my method is the same, by simply rounding everything.
And in engineering you always multiply/divide your results by a 1.5 or 1.25 safety factor, depending on situation. So you don’t have to calculate exact results, just close enough. E.g. G is always 10m/s2. π is only 3.14, the other digits doesn’t matter.
Huh? It’s not “close enough”, it’s exactly accurate. 4% of 75 is 3 exactly. I don’t know where the rest of what you wrote comes from. This post is about pure arithmetic
That’s the stupidest shit I have heard today. You should feel ashamed if you really are an engineer
That’s how engineering is. In civil you can round π=5 for a lot of calculations. In astrophysics I’ve seen e=π=10
Rounding once may be okay but rounding multiple times and that errors add up. Astrophysics?! If im working with wood, i don’t care measuring to 0.1 mm and it might be okay in astrophysics to use 10 for pi, but that doesn’t make guessing your math correct in general.
Maybe we are doing things differently here in germany.
That’s exactly why we have safety coefficients.
Safety coefficients are for nerds