Logical Reasoning

PT4 · S4 · Q21 During construction of the Quebec Bridge

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During construction of the Quebec Bridge in 1907, the bridge’s designer, Theodore Cooper, received word that the suspended span being built out from the bridge’s cantilever was deflecting downward by a fraction of an inch.

Facts

causal (as a direct result)

Inquiry about the >> changing eng >> 20th cent eng use cantilever crash rules of thumb more rigorous math

Evidence

On Must Be True, the #1 thing we read for is conditional relationships, and then secondarily for causality / pivots / math / comparisons.

We are given a long causal chain here:

1. we used "rules of thumb" to build Quebec Bridge 2. which led to cantilever deflecting down a smidge 3. which led to cantilever crashing into the river 4. which led to an inquiry 5. which led to ditching the "rules of thumb" 6. which led to far more rigorous math analysis for the rest of 20th century bridge engineering.

Goal

It's hard to know where the answer will go, but it will probably reward our understanding of this causal chain. We should just be careful about suggestive / speculative causality. This is Must Be True, so our burden of proof is extremely high.

21.

Which one of the following statements can be properly inferred from the passage?

  1. Bridges built before about 1907

    Too Strong

    We know that bridges built after 1907 had far more rigorous math analysis, but that does mean the prior standard was "not thorough"? We know that this one bridge built in 1907 had an accident during the construction process, but do we know that any bridge prior to 1907 was ever "unsafe to use"? No. And this answer is saying literally all of them were unsafe.

    5% picked this

  2. Cooper’s absence from the Quebec

    Out of Scope

    This is speculating a causal backstory for why the cantilever broke off. It could just have been because of the faulty math making it deflect downward a smidge. That might have happened whether or not Cooper was on the scene at the time. We can't prove that his absence had any causal effect.

    2% picked this

  3. Nineteenth-century bridge engineers relied on

    Out of Scope

    The passage never tells us why nineteenth-century engineers used rules of thumb. They might have done so because the math was inadequate, but they also might have done so because the math existed but was too cumbersome, or because tradition was strong, or for any other reason. The passage simply does not say.

    3% picked this

  4. Only a more rigorous application

    Too Strong

    The word "only" is the trap. The passage tells us more rigorous math became the norm after the inquiry, but it never claims that more rigorous math was the only thing that could have prevented the Quebec Bridge collapse. Better construction supervision, a faster telegraph, different materials, or many other interventions might also have prevented it. The passage rules none of these out.

    10% picked this

  5. Correct

    Prior to 1907 the mathematical

    Why this is right

    This is a super weak claim, because saying "X was insufficient to completely assure Y", just means that at least once, we had X and we weren't guaranteed Y. Well, with the Quebec Bridge, we used the rules of thumb math analysis, and the Quebec Bridge was not entirely safe during its construction (in fact 84 people died). So, yes, we can prove that the rules of thumb math did not 100% guarantee safe construction.

    Skill tested: Must be True · how this choice captures the argument's function is the move to repeat next time.

    81% picked this

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