API 650 Supported Cone Roof

[olmedo]

I am trying to find API 650 requirements regarding arrangement of center columns, rafters and girders for given tank diameters. If API is silent on this, can somebody point a website where I can find one.

 

[TankDude]

API 650 provides very general guidance for a structural roof design. It gives you maximum allowable rafter spacing (2*PI) as well as some slenderness ratio guidelines for columns. As for arrangement it provides nothing that I am aware of. This is more of a designer and/or tank contractor specification. The basic rules need to follow AISD methods, but a lot of this comes from experience within the industry regarding economy, fabrication, and ease of installation.

 

[richay]

There is a chapter on "supported cone roof design" in the text by Brownell & Young; Process Equipment Design. I believe this text has been recently republished, so you should be able to find it.

Richard Ay
COADE, Inc.

 

[JStephen]

"Steel Plate Engineering Data- Volume 1- Steel Tanks for Liquid Storage" from AISA/SPFA includes some information on roof design. This and perhaps the Brownell book are based on older AISC or API allowable stresses, but the design methods should be about the same.

Typically, a tank will have a single center column up to maybe 60-80' diameter, with multiple columns in circular layouts for sizes much over this, with details depending on fabricator and designer. What exactly are you needing?

 

[olmedo]

For my current project, I need to know if I can do away with a center column because our operation people thinks that it might provide problems in maintenance. Our tank diameter is 15 meters and tank content is domestic water. I need to show reference standards to convince end user to accept our design.

 

[JStephen]

API 650 does not specifically require a rafter-type roof. It has some general requirements for that style of roof, but also for umbrella and self-supporting cone roofs, as well as open-topped.

The center column (and rafters themselves) do involve some extra maintenance liability, but also lower the initial cost. On a tank of that size, an umbrella would work pretty well.

You can use a columnless arrangement with rafters, or use trusses, but these don't really improve the maintenance aspect. Another option would be an aluminum dome roof.

 

[TankDude]

I would agree that an umbrella or geodesic dome would work well in this instance. You certainly cannot expect to simply remove the center column and keep the same structural arrangment that you currently have. Even a columnless structural roof would likely require heavier rafters at a greater pitch. Depending upon your location an aluminum geodome would work well (be cautious if you are on the coast however). And umbrella of that size could be easily erected and set in place.

 

[sme75]

You design parameters will dictate whether or not a center column is necessary, but if you do not have significant internal or external design pressures there is no reason that a center column would be required. Assuming that this is a new tank, a diameter of 15 meters can easily be spanned with a stiffened cone roof or a dome roof and there are many tanks of this size in service without a center support. Talk with a tank manufacturer and tell them what you want. Your proof for the end user would be the recommendation of the manufacturer and having them provide stamed design drawings and/or calculations

 

[olmedo]

Thanks to all, your comments are well taken and are certainly helpful.