Annular bending of stiffened ring

[DerChad]

I have a mind bender for everyone. We have an annular plate at the base of a thin shelled tubular structure. The base is usually unstiffened. When this fails, we try one stiffener per bolt and change the method of calculation to PL/6 (partially fixed) between stiffeners and then size the stiffener for compression (ungrouted base). Often, when one stiffener is used, the calculation shows that the plate has a higher capacity in annular bending than it does as a stiffened plate (the stiffeners are too far apart). When this occurs, we then use a two stiffener per bolt solution. Usually, the stiffened plate calculation governs and me move on down the road.

I've been presented with a case study which shows that the annular bending capacity is much higher than the stiffened plate capacity and the geometry is locked in. The only reason I can come up with for the answers given are that the stiffeners are too far apart to work effectively and the annular bending will control the design. I don't buy that the stiffeners are doing nothing...merely going along for the ride. Does anyone know of a way to calculate a combination of annular bending and stiffened plate behavior working in conjunction, without the use of FEA. I was unable to find anything in Rourk and the problem is very complex for statics.

 

[civilperson]

The distance from the bolt to the vessel must be less than the distance from the bolt to the stiffener for the annular case to be stiffer than the gusseted case. Analyze as a three side fixed plate with a load at the bolt center.