Quote:
Originally Posted by Peter Ward
For circular sections the moment of inertia is proportinal to the diameter to the 4th power. If my arithemetic is correct you have reduced the stiffness of the pier by about 6000x by using 10mm bolt(s) instead of the full diameter of the pier (150mm and say 4mm wall).....hence my comment.
I use a 10” steel pier with 1/2”walls. doesn’t move much
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Quote:
Originally Posted by Peter Ward
Indeed, ideally you want low mass and high moment of inertia, so any perturbation will have a low amplitude (eg immune to small breeze) and decay quickly. That said even an EQ6 probably deserves better than a 8-10mm diameter bolt...the attached diagram from my old engineering text don’t lie.
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The formulae are obviously correct, but they are only considering a circular section in isolation, not a group of 3 or 4 where they are some distance from the neutral axes. A single bolt will have fraction of the stiffness of a larger CHS section as you say, but a bolt group can have similar stiffness with sufficient spacing and/or diameter.
I do agree that pier heads don't need to be 100% level, although it does make polar alignment easier the more level they are, less iterative.
I do also agree that it seems a bit silly to put a "soft storey" if not needed when what we're after is minimising risk of deflections. However the calcs do show that stiffness can be maintained by carefully selection of bolt diameter and spacing.