Scalloped Stiffening Rib

[MDDSI]

Picture attached.

I am posting here because I am at wits end on how to handle this situation. My company often deals with large flat surfaces that are subject to typically low pressures (approximately 1 psi). In the past, a stiffening rib has been added to the surface to help rigidity - usually a flat bar welded directly to the vessel skin. Recently, a new design has been suggested, one where a large section of the flat bar is removed or "scalloped". One argument for this design is that it utilizes less steel and reduces the welds needed. If this "beam" were to be attached to a plate, the beam stress would then be shifted from a uniformly distributed load to a series of point loads, so in theory, this unit should work just fine. However, something doesn't smell right to me. I cant help but feel like there is more that needs to be analyzed with this design. I have seen some guidance for putting holes in the web of I beams, and castellated beams, but this is slightly different as the cut outs are not midspan on the web. I would argue that by welding the rectangular section to the plate it makes a sort of T section, where the scallops are right against the T sections flange.

Overall, this design bothers me, though I have not been able to fully justify why, and perhaps my concerns are unnecessary. Using beam theory to work this out seems to be the logical path forward, but again, I think there is more to it. Do any of you have a recommended resource for attempting to analyze this member or is this so out of the ordinary that there really is not a reliable method? Please let me know if I can further explain my question.

Thanks

 

[jayrod12]

how thick of a flat bar are we talking here? I'd like to think the scalloping procedure likely costs more than the material saved.

 

[MDDSI]

Jayrod,

The flat bar thickness varies depending on where we would apply it. This is more of predesign research phase. 1/2" thick typical. I forgot to mention one other big reason this design is desired. This is going to be in an environment where the prospective customer would require full and complete welding. Even if we put a flat bar in place, we would need to weld continuously on it. Sanitary environments and all that. So the scallop would allow them to clean under the flat bar, whereas simply skip welding a flat bar would not. So the main benefit for the scallop is that it allows much much less construction time.

 

[jayrod12]

And this has been ran by the client? Another part of me feels that my manufacturing clients would say "No way"

 

[MDDSI]

So far so good as far as they are concerned. Of course, the task now would fall on us to make sure its not going to fall apart, which is where I am starting to pump the breaks. Glad to hear that someone else sees some issues with this design. Nevertheless, I am going to have to justify why if and when I shoot this down, and so far I just dont like the way it feels, you know?

 

[MDDSI]

It's one of those things that works really good for everyone other than engineering.

 

[jayrod12]

Ya I've been through these before. Someone thinks it's a good idea, until it's actually implemented. Then they realize, shit these little holes are impossible to clean, and then they pay double what they would have to get someone to come seam weld it closed anyway.

From a structural perspective, I don't see a huge issue with the intent. I feel the logistics are a different animal altogether and people are having trouble seeing the forest for the trees. The additional money up front for the unit will probably be offset by the tedious cleaning required rather quickly.

 

[BUGGAR]

I used to design work boats and we would typically cut the bottom hull plate rib stiffeners like this to prevent entrapment of water in the bilge. No special design rules. For practical reasons, we kept these ribs thick which reduced their tendency to buckle as a column between the welds under compression when the rib is loaded in beam bending.