THICKNESS OF RAFTERS IN API 650 TANKS 1

[jor1492]

Hello

I am new in the design of storage tanks and I have somes question about it.

In the API 650, item - 5.10.2.4 "Structural Member Thickness: All internal and external structural members shall have a mininumm nominal thickness(new) of 4.3 mm (0.17 in.), and a minimum corroded thickness of 2.4 mm (0.094 in.)"

1) are the rafters included in the structural members ?

2) Do the rafters must have that minimum thickness?

3) What kind of beams are usually used as rafter?

 

[fegenbush]

1. Yes.

2. Yes. Interestingly enough, the thinnest web of a W Shape in the AISC Steel Construction Manual has a thickness of 0.17 in. (W6x8.5). Also, the thinnest web of an S Shape is 0.17 in. (S3x5.7).

3. W Shape and Channel are the most common rafter shapes that I have dealt with, although it is possible to use T Shapes, Angles, or some other shape if you are so inclined. Only a two of the Channels have a web thickness thinner than the minimum (C3x3.5 and C4x4.5).

 

[jor1492]

Thanks fegenbush, by the good answers

I have anothers questions,

Is mandatory the use of corrosion thickness for the rafters or is that a design criteria?

Could I use S shape underside W shape ?

 

[fegenbush]

Corrosion allowance is specified by the purchaser. Whether the specified corrosion allowance applies to the roof structure and in what way it applies is also specified by the purchaser. They may dictate that the corrosion allowance is for the total per web, total per flange, per exposed surface, or any combination thereof.

Yes, any combination of shapes that satisfies AISC 360 will meet the requirements of API 650.

 

[JStephen]

Usually, the choice between S shapes, W shapes, or channels is economic. Some sections will cost more per pound than others, and may be available in a restricted choice of lengths (IE, to get pieces 28' long, you may have to buy 40' lengths).

 

[jor1492]

Thank you fegenbush and JStephen. I am going to put in practice your advices