API 650 - Minimum Number of Rafter and other roof design

[takiyasamsama]

Sorry for such title and I appreciate if someone could guide me through this question

I'm still learning to design a API storage tank and one thing messing in my head is roof design. Basically in API 650 only mention the rafter length and not number of rafter, so how do we design on how many rafter is required? Is there any codes and standard mandatory to be followed for storage tank?

I've been searching some tank calculation and some of them said "Minimum number of rafter shall be multiple of 5" others stated 8, 16, 32, and etc.

Thanks in advance

 

[IFRs]

Not sure why you can't find rafter spacing in API 650. Per API 650, rafters used to be on maximum 2 x Pi spacing in feet ( tank diameter divided by 2, round up ). Now see 5.10.4.4 for rafter spacing. The maximum is now 84". Often it is a multiple of the number of girders, so the layout is consistent.

 

[JStephen]

The minimum number of rafters comes from the maximum spacing allowed.
It may be advantageous to use smaller spacing for erection purposes, specifically, to use rafter spacing smaller than the plate width being used. That would depend on erection procedures and past experience.
On a smaller tank, it may be advantageous to make the number of rafters a multiple of 4 or 8 if there are a lot of roof nozzles.
On a larger tank with multiple columns, the number of rafters will be a multiple of the number of outer columns, however many that is.