Anchor Chair Spacing

[merton]

Does the spacing of anchor chairs on the tank circumference always need to be equal? I have a situation where I would like to increase the distance between two chairs. Even with the change the total distance between all anchor chairs is still less than 10' on the circumference of the tank as specified in API 650.

Thank you in advance.

 

[JStephen]

API may address this somewhere. It comes up with flush-type manways, for example. Anyway, it would generally be acceptable to spread two anchor bolts farther apart to clear an obstruction.

Normally, anchor bolts are designed assuming stress distribution is just Mc/I - P/A, and treating the anchor bolts as an equivalent thin ring. But, you can also take your actual bolt locations, calculate I, find the centroid, and calculate the bolt loads based on that specific load distribution, and show that it problably doesn't change a lot.

The motivation for putting bolts at equal spacing is to give equal loading to all bolts. But another arrangement that would do this is placing them in pairs- alternationg big space and little space.