## #21 - During construction of the Quebec Bridge in 1907, the

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Complete Question Explanation

Must Be True. The correct answer choice is (E)

The basic thrust of this stimulus is that it took a major bridge accident to force bridge designers to stop using rules of thumb and to start using modern engineering principles.

Answer choice (A) starts off correctly – bridges before 1907 were built by rules-of-thumb and not with thorough mathematical analysis. But, then it draws the unjustified conclusion that all such bridges were (and presumably still are) unsafe for public use. But, we only know that one such bridge was unsafe – it could be many other bridges were perfectly safe even though they were designed with rules of thumb. In fact, many bridges in Europe are several hundreds of years old and are still in use.

Answer choice (B) assumes that if Cooper had been at the bridge site, the cantilever would not have broken off. While this is possible, it is by no means certain. As a result, this choice is wrong.

Answer choice (C) speculates why bridge engineers used rules-of-thumb. But, the stimulus gives no reason for their doing so – it could have been that rules-of-thumb were just easier than sound engineering.

Answer choice (D) is close: it appears that the Quebec Bridge collapsed because it was not mathematically engineered. But, there might have been other methods of preventing the collapse, such as using better rules of thumb. The tip-off in this choice is the word ‘only' – when we see this, the first question ought to be "is it really the only way"?

Answer choice (E): This is the correct answer choice, as it draws the right conclusion. We already know that the rules of thumb were insufficient to assure the safety of bridges, otherwise the Quebec Bridge would not have gone down. Therefore, to the extent that the rules embodied any mathematical or engineering principles, those principles must have also been insufficient.
PB410

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Hi,

I am struggling with understanding why E would be a better selection than D. From your explanation you point out D relies on the narrow commitment of there "only" being one way, but E threw me off with its use of "completely" assuring the safety of bridges under construction. I found the language to be equivalent in force. Can someone help explain how I might think through eliminating D?
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Thanks for your question!

With respect to answer choice (E), we know that the rules of thumb were insufficient to completely assure the safety of bridges under construction because at least one particular bridge built with the "rule of thumb" measurement technique -- the Quebec Bridge -- collapsed! We don't need to know about other bridges since the collapse of one bridge tells us, beyond a doubt, that this measurement technique did not ensure complete safety.

Answer choice (D) can be eliminated because it essentially presents the opposite problem. Using the rigorous mathematical model could have been one of the ways to prevent the Quebec Bridge from collapsing, but we don't have enough information to know that it was the only way to prevent the bridge's collapse. There could be dozens of other ways to secure bridges other than the mathematical method mentioned in the passage -- extra supports, more costly construction methods, balloons, floating docks, and who knows what else!

I'm being a little facetious in my examples, but my point is that it's very easy to prove that something is not completely safe (e.g. if it fails one time) but it's quite hard to prove that using a particular method is the only way to make something safe. The imagination is really the limit on exhaustive ways to make something safe. Here, we only know that the mathematical formula is one way to make the bridges safe, not the only way.

I hope this makes sense! Good luck studying!
martinbeslu

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It seems like answer C is making an assumption that isn't proven in the stimulus. The stimulus states that engineers depended on more rigorous applications of mathematical analysis after the Quebec Bridge collapsed but it doesn't say anywhere in the stimulus that the use of rules of thumb was the cause of the collapse. The cantilever arm could have broke because the metal used in the construction was not properly heat treated or because of improper construction techniques by the workers building the bridge or any of a thousand other reasons. We don't know that the rules of thumb that were used were insufficient just because engineers started using more rigorous applications of mathematical analysis after this happened.

Answer A seems like just as good of an answer choice (or better) if we are going to assume that the bridge collapsed because it was built without thorough mathematical analysis. If the use of rules of thumb instead of more though analysis was the cause of the bridge collapsing then this was an unsafe way to build bridges. We don't need to know whether any other bridges have collapsed to know that they were unsafe given the fact that they were not built with sound engineering that has lead to at least one bridge collapsing and the same thing could happen to any of the other bridges that were built using the same rules of thumb.
Jonathan Evans
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Martinbeslu,

Could you please clarify your question? The credited response to this question is (E). Thank you very much!
martinbeslu

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It seems like answer E is making an assumption that isn't proven in the stimulus. The stimulus states that engineers depended on more rigorous applications of mathematical analysis after the Quebec Bridge collapsed but it doesn't say anywhere in the stimulus that the use of rules of thumb was the cause of the collapse. The cantilever arm could have broke because the metal used in the construction was not properly heat treated or because of improper construction techniques by the workers building the bridge or any of a thousand other reasons. We don't know that the rules of thumb that were used were insufficient just because engineers started using more rigorous applications of mathematical analysis after this happened.

Answer A seems like just as good of an answer choice (or better) if we are going to assume that the bridge collapsed because it was built without thorough mathematical analysis. If the use of rules of thumb instead of more though analysis was the cause of the bridge collapsing then this was an unsafe way to build bridges. We don't need to know whether any other bridges have collapsed to know that they were unsafe given the fact that they were not built with sound engineering that has lead to at least one bridge collapsing and the same thing could happen to any of the other bridges that were built using the same rules of thumb.
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The stimulus tells us that the old rules of thumb went down with the bridge, martinbeslu, which is another way of saying that they were no longer considered adequate. That suggests pretty strongly that those rules were used in the construction of that bridge (as opposed to those engineers taking shortcuts and not following the rules), because otherwise there would be no reason to change the rules (which in fact happened as a direct result of the inquiry that followed - again, supporting the idea that the old rules had been followed).

Regardless of exactly why the bridge collapsed, it did collapse, and that means the old rules of thumb (which we must assume were used in its construction, for the reasons I've described) were not enough, by themselves, to ensure the safety of the bridges. Why? Because that one bridge, at least, wasn't safe - just ask the 7 dozen workers who went down with it!

Then there is our old friend the Uniqueness Principle, which tells us that only one answer can be the credited response. If answer A is true, then it must also be true that answer E is true, mustn't it? When one answer forces another to also be true, the one doing the forcing (A, in this case) cannot be the credited response, because then E would be just as good and would also have to be credited. That alone is enough to kill answer A.

But wait, there's more! Answer A is the one that assumes too much! Just because the old rules weren't good enough doesn't mean that no bridges were built to higher standards. What if some bridges were built to go way beyond what the old rules required? Maybe those bridges met or even exceeded not only the old rules, but the new ones as well? They might be completely safe, right? Also, just because the old rules weren't good enough doesn't mean that there was absolutely no thorough mathematical analysis. There could have been lots of thorough analysis, but nevertheless a mistake about what the math should be or what the tolerances were to allow some variance from that analysis (the "rules of thumb"). We just cannot know about EVERY bridge built before 1907, but we can know something about the rules before then. That makes E the best answer, and A belongs at the bottom of the St. Lawrence River with the debris from the Quebec Bridge.

Take another look and let us know if that makes sense. See you soon!
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AlmaRS
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For the answer choices that I was struggling with in this question were D and E. When I read over your responses to each answer choice for answer choice A you mentioned that not all bridges that were built were unsafe, before 1907, because some are still being used. Wouldn’t it be similar to answer choice E in this case where not all mathematical analysis incorporated in engineering rule of thumb was insufficient, since some bridges that used that rule of thumb, like you said for answer A are still in use like in Europe ? Or am I overthink the answer.
Emily Haney-Caron
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Hi AlmaRS,

Great question. Here, answer A is saying all bridges built before 1907 were unsafe. Answer E is saying at least some bridges built before 1907 were unsafe. "Completely" in E throws some people off, but all that phrase means is we can't be 100% sure of the safety of every single bridge built pre-1907 (which is definitely true, since one of those bridges fell and killed people!). A is much broader than E. I hope that helps!