ISO 12215-5 - variable height stiffener calculation

Since ISO do not give any requirements for variable height stiffeners, a certain degree of interpretation is needed here, as well as a specific and higher Factor of Security. All the comments already given are very wise, and their effects on your stiffener composition should be compared with the following method.

A detailed analysis of this case would allow us to "split" the omega in two parts, resolving an end-bay problem -Fixed-Supported beam-. The curves in red are copied from the ISO case given in the standard -(b) case -, and is placed on the diagram to show the approximated bending moment.


The nominal composition is to be found by checking ISO requirement at the fixed-end, for the idealized beam in green. Also checking the requirements of table A.12 ( buckling check), but using the maximum height of the actual stiffener, here 500 (mm).

 

Thanks Alan. Seems like we are getting there.

Interesting graph. Seems like the Point of Contraflexure (the point where Mb=0). Will check if it agrees with my data.

 

I guess we all missed the boat. The OP is basically asking how to design a crossbeam, whose load model is a cantelever design with diminishing load. Diminishing load because the pressure diminishes as it moves towards the waterline.

We are all accustomed to Fixed end UDL.

"So why make things simple when it can be made more difficult". The solution is Area Moment Method. Although the area remain constant, the pressure does. The cross section area thus vary also. This is a topic beyond ISO.

 

Indeed, ISO assumes the panels with their four recessed sides and the stiffeners with both recessed sides. Everything else is not covered by the ISO. But, in my opinion, whatever the OP says, his beam cannot be considered as a cantilever because neither of its ends is free to move or rotate. In any case, one could assume an elastic link at the far end of the C.L. but I think that this is complicating things too much and neither does the ISO (nor the C.S.) contemplate it.

 

Tansl- He is adamant about it "This is transversal structural element (floor/frame); longitudinal structural elements are not continuous through transversal element" and won't answer anymore.

The ends does not have to rotate. This just a method of analysis for a beam/mast/crossbeam/kite/airplane wing only longer. It is still a cante.

The gross load is supported by a fluid and its pressure is governed by an area. If this is a barge, flat bottom, it will be a constant chord (uniformly loaded). If there is a deadrise/dihedral it will be a tapered wing (diminishing load).

 

Simplify using shear and moment diagram.

 

Rx, ok.

 

Many times in ISO meeting, the (non academic) term of "degree of fixity" has been spat out, so as to have something like an evasive term to express how much some panels/stiffeners configurations just sit between the fixed and the supported cases.

 

Yes. I noticed that. That word "fully fixed/Encastre" that seems to be commonly used in Structural analysis like primary, secondary. tertiary.

ISO uses only one constant for bending moment (1/12), right?

That is the reason I plotted the various constants used in plates, primary, secondary, tertiary in my post #17.

I noticed also that bending moment is always positive which means the result of the analysis is only on one point along the length.

 

Well, it will possibly be one more inaccuracy of the ISO that does not allow us to know if it considers the ends recessed or just a little recessed or simply supported. In any case, cantilever is a term that does not allow one to speak of a very recessed or slightly recessed end, much less simply supported. Not even a ball joint at one of its ends would allow it to be classified as a cantilever.
Having said all this, I think that the properties of the floor (or frame, whatever it is going to be put) should be studied at the same point where the design pressure is measured and, on the other hand, I think that the OP you should consider designing your structure differently. For example, it doesn't make much sense to put such a high floor under a transverse bulkhead.
I do not want to evade the responsibility of giving a concrete answer to the initial question. What I think is that before calculating the scantlings it is necessary to conceive a structure that is as correct as possible.

(1/12, isn't that the factor that appears in the moment of embedment of a beam with its two ends embedded?)

 

1/12 is Indeed the factor that appears in the expression of the bending moment at the end of a beam, fixed at both end, under constant load. Like 1/8 appears in the bending moment at the fixed end of a fixed-supported beam under constant load.
I see no mistakes in the diagram given in the ISO, although these formulas should be applied with care, making valid hypothetis of the actual configuration.
As long as a good justification is given along with the calculations, I see no reasons why one would be let to avoid the reference to the ISO formulas. They are quite basic, and quite handy, to my opinion.

 

Tansl- It is a first principle analysis but forget about it because we are running around in circles. You know as well as I do that is an impractical analysis for that out of proportion variable height structure. That is a bulkhead, plain and simple and ISO tells us how to design a bulkhead. (Also a cantelever)

Now if this is a transverse frame built out of tophat profile, ISO calculate the pressures needed for the bottom, sides, and deck. It is a simple "stiffener with plate" analysis of the bottom, side, and deck. Bracketed of course because this is a primary structure

 

Thank you. I did not say there was a mistake.

 

@rxcomposite . We're 100% inline.