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# Joint Efficiency for a lap welded roof

## Joint Efficiency for a lap welded roof

(OP)
I am currently reviewing the design of an API 650 tank built in accordance with App F.
D = 86.5'
Design pressure = 0.5psi

Is it the intent of App F 7.3 that the roof plate on a lap welded should be designed with a joint efficiency of 0.35 as per API 620?

### RE: Joint Efficiency for a lap welded roof

If lap-welded from the top-side only, that would be correct.  The roof could also be double lap-welded or butt-welded if required.  Also, the type of weld can vary across the roof.

### RE: Joint Efficiency for a lap welded roof

(OP)
As I feared...

I had designed the tank to have a double lap weld for the outer 3rd of the tank to roof to increase the joint efficiency to 0.70.  I just found out they didn't do the back weld on the roof and now the tank has been coated.  Sigh...

### RE: Joint Efficiency for a lap welded roof

(OP)
Does anyone know why API 620 is so much more conservative on their joint design then AISC ASD?

For 38W material a single lap weld would have an allowable stress of:

API 620
S_allowable = 16,500psi x 0.35 = 5,775 psi

AISC (table J2.5)
S-allowable = 60,000psi x 0.3 x 0.707 = 12,726 psi

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