API-12B tank roof support?

[11echo]

Got a question about roof supporting on API-12B bolted tanks. What is the limit (diameter wise) when external rafters are no longer a good engineering method of supporting a tank roof and a center pole would be required? …THX!

 

[Bonswa]

From what i gathered in section 4.7, 4.8, 4.9, & 4.10....

It seems that no matter what, you will need SOME type of center support. On your low and medium capacity tanks, you can get away with simple structural support, basically a channel-iron support column. On your high capacity tanks, you will need a pole type center column. Either way, it seems you will need a center support regardless of rafters.

The only mention of rafters states that they are required on your low tanks of 1000 bbls capacity or greater. basically, your short stubby tanks above 1000 bbls, will need rafters.

It sounds strange that a small 100 bbl tank will need a center column, but that's clearly what 4.7 says. I'm sure it only applies to bolted tanks, because i KNOW welded of higher capacity that dont require a center column.


I've posted the sections for others:

4.7 Low Capacity Tanks—Supports
Tanks of 100-, 200-, 300-, 250-, high 500-, 750-, low 500-, high 1,000-, and 1,500-bbl capacity shall be furnished with structural-type center support, with the upper end fastened inside the dome with three bolts in each leg and the lower end fastened to a bottom attachment base designed to permit height adjustment.

4.8 Medium Capacity Tanks—Supports
Tanks of low 1,000-, 2,000-, and 3,000-bbl capacity shall be furnished with a structural- or pole-type center support, including a rafter support ring. The distance from the tank center to the point of attachment of the rafter to the rafter support ring clip shall be 38 1/32 in.

4.9 High Capacity Tanks—Supports
Tanks of 5,000-bbl and 10,000-bbl capacity shall be furnished with pole-type center support. The distance from the tank center to the point of attachment of the rafter to the rafter support ring clip shall be 28 in. for the 5,000 bbl tank and 60 in. for the 10,000-bbl tank.

4.10 Rafters
Tanks of low 1,000-, 2,000-, 3,000-, 5,000-, and 10,000-bbl capacity shall be furnished with rafters attached to each radial deck joint. Rafters shall conform to the allowable design stresses as given in 4.6. Each rafter shall be attached to the center support ring and by suitable brackets to the top chime area. The depth of rafters, measured from the underside of the deck at each end shall be 5 1/2 in. for low 1,000-, 2,000-, 3,000-, and 5,000-bbl tanks, and 6 3/4 in. for 10,000-bbl tanks. Rafters shall be punched or drilled to accommodate supporting clips for bolt retainers. Except as provided above, rafters shall be furnished in conformance to design details as agreed upon between the purchaser and manufacturer