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.

Conclusion

This is a fact-set, not an argument — there is no main conclusion offered. The passage tells a historical story: a bridge designed using engineering rules of thumb collapsed in 1907, and the inquiry that followed pushed twentieth-century bridge engineering to depend on far more rigorous mathematical analysis.

Evidence

The Quebec Bridge collapsed during construction. The collapse triggered an inquiry. As a direct result of that inquiry, the engineering "rules of thumb" by which thousands of earlier bridges had been built were abandoned. Twentieth-century bridge engineers shifted to far more rigorous mathematical analysis.

Evaluate

Because this is a Must Be True question, the correct answer must follow directly from what the passage says — not require an extra inferential leap. The passage tells us the rules of thumb were discarded after a bridge built using them collapsed. That is enough to conclude that the rules of thumb (and the math built into them) were not sufficient to fully guarantee bridge safety. Anything stronger — that pre-1907 bridges were unsafe to use, or that more math would have saved this particular bridge — goes beyond the text.

Goal

Find an answer that is directly supported by the facts: pre-1907 mathematical methods were not sufficient to fully ensure bridge safety. Avoid answers that overstate (call old bridges unsafe) or invent causal stories the passage does not tell.

21.

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

Bridges built before about 1907
Too Strong
The passage says the rules of thumb were abandoned after the Quebec Bridge collapsed during construction — but it does not say bridges built with those rules were "unsafe for the public to use." Most bridges built before 1907 presumably stood and carried the public just fine. "Insufficient to completely assure safety" is a much weaker claim than "unsafe to use," and only the weaker claim is supported.
5% picked this
Cooper’s absence from the Quebec
Out of Scope
The passage never says where Cooper was, why the cantilever broke off, or whether his presence would have prevented the collapse. We are told he received word of a deflection and tried to telegraph a freeze order before the cantilever fell. Nothing about an absence-causes-collapse relationship can be inferred from that.
1% picked this
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.
2% picked this
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
Correct
Prior to 1907 the mathematical
Why this is right
This is the modest, well-supported inference. The passage tells us the Quebec Bridge — built using rules of thumb that incorporated some mathematical analysis — collapsed during construction, and that as a direct result of the inquiry, those rules of thumb were abandoned. If the math built into the rules of thumb had been sufficient to completely assure bridge safety under construction, the bridge would not have collapsed and the rules would not have been thrown out. So the math must have been insufficient to fully assure safety.
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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