Rafter and girder support and location 2

[yusha2005]

I am designing a steel tank of supported cone roof without column which dia is 16 m and height 12m.According to API 650 this tank requires 2 rings of rafters. No. of outer rafters and inner rafters are 28 and 16 respectly.My question is how 2 rings of different types of rafters and girders are supported if no column.Can anyone explain as I am new in this field?

 

[IFRs]

To my knowledge, API 650 does not require two sets of rafters - please tell us the section that does. This size tank could have a self supported roof or a rafter supported roof. If you choose rafter supported, you only need one center column and at most 26 rafters. If you choose a self supported roof, it could be with or without rafters.

 

[yusha2005]

IFRs
Thanks for your feedback.I misunderstood the section 3.10.4.4 which refer spacing of outer ring and inner ring.
Is there any girder required to support rafters without colomn and if required how rafter and girder are arranged-slope to roof plate or horizontally?
If with column,is there any girder required and how girder is arranged(slope to roof plate or horizontally)?

Thanks

 

[IFRs]

Rafters are radial, sloped with the roof. Girders are circumferential. If you use a center column to support the rafters, no girder is required. If you do not use a column, you will need a compression ring in the tank center. If you don't use rafters or girders, design for a "self supporting roof" (cone, dome or umbrella) per API 650.

 

[yusha2005]

Thanks.If girders are circumferential where is the support for girders?Do you mean the compression ring as girder?

Can I use truss at roof instead of rafter at a slope of 3/32(.75 m height at centre)in case no column.

 

[IFRs]

Girders would normally be in a polygon shaped ring with columns at the intersections, used for much larger tanks. You can certainly use a number of trusses in lieu of rafters, but I question why you think you would need them. By the way, API tells you the maximum span between supports at the tank shell.

 

[JStephen]

Certain types of roof structure are normally used because they have proven to be reasonably functional and the cheapest to construct. But a lot of variations are possible, and are permitted by the standard. The details will vary depending on the relative cost of labor versus materials, on the availability of various structural items, on the forming processes available, on coating requirements, hoisting equipment available, etc.

Normally, the general tank configuration is specified by the tank owner or their engineering consultants, and the details are furnished by the tank construction firm. If you are working for the tank owner or their consultant, all you really need to specify is a "fixed roof" and let the actual tank designer work out the details. If you are the tank designer working out the details, I would suggest you have somebody more knowledgeable do the design and also get a lot of input from the people that will be fabricating and erecting the tank. There are a lot of details in API-650 that are easy to overlook if you are not familiar with the standard and with common tank construction methods. And a lot of the details that are not specified by the standard are chosen to make the work easier or quicker or safer. For a tank company, it is advantageous to use similar detailing from tank to tank as well.