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Rand raises a good point; if you do sign the drawings, what are you claiming?  You can't say that the tank meets applicable codes and standards, because to the best of your knowledge it does not.  

They say that these things have worked fine since 1947, but are you sure that the other tanks don't vary in any critical ways from this one?  All too often I have observed that some critical difference will exist in such cases between what I am asked to do and what has been done in the past.  It is always irritating to spend time trying to figure out how someone has built something time and time again without trouble only to later discover that the case you are looking at is the tallest or the heaviest or in the highest seismic area that they have ever tried, but it didn't seem like a big difference to them.  

You also have to consider that if something goes wrong, even through someone else's negligence, you will have a hard time defending yourself.  Even if an identical case has worked for years, does it have the factors of safety that are legislated through applicable codes and standards?  Something that works but has a marginal factor of safety will be more prone to failure through accidental overload, mistakes in construction, or other issues that a more robust factor of safety might have rendered moot.  

Lastly, without knowing the specifics it is hard to say, but I am always hesitant to design anything assuming the most favorable load case will necessarily exist.  Can the tank be drained?  If the tank is emptied for inspection and maintenance personnel to climb into it, that seems like a bad time for a failure.  Depending on the situation, that could be one of the most critical life-safety cases.  

Of course, you could always try to consult a tank expert to see if there are some applicable analysis methods that you have missed that would demonstrate code complaince.  Otherwise, signing seems like a bad idea.  Good luck!

Paul Crocker, PE, SE

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