Consider this question: Question

I solve this eventual question and my answer is 7.29667:


Among the question choices are 7 x 7 ft and 7.5 x 7.5 ft. Which one should I choose? I guess if a safety margin is intended, the larger choice i.e. 7.5 x 7.5 ft should be selected. Am I right?

There are situations like this in which two question choices are close to my final answer and I'm not quite sure which one to pick. Is considering safety margin a good criterion to pick the choice? Thanks for your help!

  • $\begingroup$ An aside: I like to think I'm quite well-read, but I've never before encountered the name "unit weight" used for the quantity that ISO 80000-1 would call "weight density". Is that common in some circles? $\endgroup$ May 13, 2021 at 9:44
  • $\begingroup$ @DanielHatton Why would you expect civil engineers in the USA to care about ISO? "We don't need no stinkin' international stamdards!" :) $\endgroup$
    – alephzero
    May 13, 2021 at 11:54
  • $\begingroup$ @alephzero I have read the occasional American document: like I said, I've never encountered that nomenclature. $\endgroup$ May 13, 2021 at 12:02
  • 2
    $\begingroup$ It's civil. Round up! $\endgroup$ May 13, 2021 at 13:08

1 Answer 1


You are worrying about the numbers and forgetting what the numbers mean. This is for the PE exam, so this is a very important topic to make clear in your mind.

You solved the problem. You came up with an answer for what is required. You now have two options:

  1. Choose a small footing that your own solution just proved is too small for the requirements.
  2. Choose a large footing that your own solution just proved is larger than is required. This will be conservative, but it is the only option that meets the requirements.

If you look at the two options above, there is only one solution. This is a question for the PE exam. One of the primary purposes of professional licensure is to promote the safety of the public. When in doubt, always choose the safer option.


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