Comment by abduhl | Hacker News Reader

Comment by abduhl | Hacker News Reader

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For someone who is self-admittedly not knowledgeable about the testing requirements, you seem very certain of your conclusions about this Toronto falling glass problem. Testing of components is usually by the manufacturer and it is their responsibility to provide a product that meets the requirements of the design. This is not a problem from the design side and is very difficult to prevent without the engineer being onerous with his requirements to a point that no engineer is really willing to go to.

I'm not familiar with the specifics of your rebar example so I can't comment. Your example with the train makes no sense - the design loading for railway is codified in the design manual (AREMA in the USA) and includes dynamic forces. Braking forces are applied longitudinally to the track so unless you were in a curve there is no downward force. I find it hard to believe that your boss agreed with a fictitious force and then just multiplied everything by 2 to get around it.

Your opinion on your colleagues is concerning to me and is probably more indicative of your lack of experience than the other staff's incompetence. Your experience reads like someone suffering from 200th hour syndrome, I wouldn't be surprised that if you stuck with it another 3 years you would have realized your initial impressions were way off base. At worst, it sounds like you may have been working at a firm that did commodity work and didn't attract top tier talent. If you are as good as you seem to think you are then you should have jumped ship when you got "fed up".

I don't intend for this post to sound dismissive but it will probably come off that way.

As an aside, knowledge of advanced math is not necessary for structural engineering in my opinion, nor is it common.

"Braking forces are applied longitudinally to the track so unless you were in a curve there is no downward force." Why is that? Is it because the cars behind are pulling on it and keeping the usual forward weight transfer from happening?

Think of a motorcycle doing a "stoppie" i.e. read wheel is in the air under braking, all weight is on front wheel.

This is hard to describe without being pedantic and without being able to draw but I will attempt.

A motorcycle performing a stoppie experiences rotation because the inertial force couples with the braking force to create a moment about the front axle of the bike (this isn't technically correct language but you get the gist). While this idea holds true for the train, we have to take into account the differences in mass and contact between the two systems. Train cars typically ride on more than 2 axles and this provides stability from front or rear tipping. Train cars are also typically very heavy meaning that the braking force is not sufficient to 1) move the center of forces of the system ahead of the front axles and 2) tip the car. Increases in load because of this are, therefore, not sufficient to double the load on the front axle as you would see with a stoppie.

In general I agree with the idea that is put forth; however, it is important to note that what we are discussing is the BRAKING force. The inertial forces that result in differential axle loads is not a braking force (certainly, it is a result of braking in this case but this force also exists when a train begins pulling). These loads are DYNAMIC loads and are already considered in another part of the analysis. Dynamic loads also include consideration for bumps, etc. Because of this, the code is explicit that braking forces are applied only longitudinal to the track so that the forces are not counted twice.

Thanks for responding, and I'm very happy that there are people like you out there; but trust me when I say that despite my poor recollection from the time I practiced, my fundamental point is not wrong. Most engineers I've worked with in Canada are not to be trusted with advanced design. If you disagree I'd like to really talk to you about it because I felt like I was surrounded by people that had no idea what was going on and I would really like to be proven otherwise.

I'm not competent to explain why Tacoma Narrows failed, but it obviously wasn't up to design load. The existence of large sheets of glass falling off buildings and endangering people from multiple different installations strongly indicates that someone botched something.

What is 200th hour syndrome?

The 200th hour syndrome refers to when a pilot has reached their 200th hour in the air and their confidence in their abilities is enough that they begin to get careless:

"Enough experience to be confident, enough to screw up real good." is how a nice TV show put it.

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