During construction of the Quebec Bridge

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsMust be True

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Stimulus

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. Before he could telegraph to freeze the project, the whole cantilever arm broke off and plunged, along Quebec Bridge. Twentieth-century bridge engineers would thereafter depend on far more rigorous applications of mathematical analysis.

What this question is testing

Must be True

Conclusion

There is no real argument here — just a historical story. A bridge fell in 1907 because the rough engineering shortcuts of the time were not good enough, and afterward engineers switched to far more careful math.

Evidence

Three concrete facts: (1) the Quebec Bridge collapsed during construction, (2) as a direct result of the inquiry, the engineering rules of thumb that had been used on thousands of earlier bridges were abandoned, and (3) twentieth-century engineers moved to far more rigorous mathematical analysis.

Evaluate

For a Must Be True question, you want the answer that just walks straight off the page. Here, what the passage really tells us is: the math behind the old rules of thumb was not enough to fully guarantee bridge safety — if it had been, the rules would not have been thrown out. That is a modest, defensible inference.

Be careful of bigger claims the passage flirts with but does not actually make: that old bridges were dangerous to use, that this specific bridge would have been saved by more math, or that nineteenth-century engineers chose rules of thumb because they had no choice. Tempting, but each requires extra information.

Goal

Pick the modest answer: pre-1907 engineering math was not sufficient to fully assure bridge safety.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question

21.

Which one of the following statements can be properly inferred from

Answer choices

Too Strong5% picked this
Bridges built before about 1907 were built without thorough mathematical analysis and, therefore, were unsafe for
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%
Out of Scope1% picked this
Cooper’s absence from the Quebec Bridge construction site resulted in the breaking off
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%
Out of Scope2% picked this
Nineteenth-century bridge engineers relied on their rules of thumb because analytical methods were inadequate to
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%
Too Strong10% picked this
Only a more rigorous application of mathematical analysis to the design of the Quebec Bridge could
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%
Correct81% picked this
Prior to 1907 the mathematical analysis incorporated in engineering rules of thumb was insufficient to completely assure the
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%

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