Partial Bulkhead Scantling Calculation ISO 12215

I don´t think so, if an element, whatever it is, is considered a stiffener, it must be calculated according to the continuous beam theory. In any case, in my opinion, according to what the ISO says, it must be calculated with the design pressure corresponding to a stiffener and the rest of the formulas corresponding to stiffeners.
The ISO says many things but it is difficult to find them. It is not easy to get clear answers in the ISO and, therefore, I can only talk about how I interpret it.

 

Ok, so you have these fibreglass longitudinals - but it would appear to me that you will still have relatively weak areas where the depth of the transverse frame is reduced.
Ad Hoc proposed running longitudinals in way of these areas - do you think that the single (each side) longitudinals shown in your sketch will be sufficient?

 

They are probably more than enough, or it could even be the case that they can be removed. Nothing can be said without knowing the TOTAL structure of the hull. If the steps of the sole are long enough they could act as "natural stiffeners" so nothing more would be needed. As the boat will certainly have spray rails on the outside of the bottom, they would act, if necessary, as longitudinal reinforcements of the hull. (If the spray rails do not exist, they can be placed). In this way, the interior structure would be much simpler, therefore cheaper, and probably lighter.
Scantling a boat is much more than reading a few texts. Practical experience, like everything in life, is very important.

 

ToMek-
Since you are designing a partial bulkhead it is classified as a stiffener “section 9.3.4” whose length is defined by the point from deadrise to the first primary longitudinal as illustrated by AH.

The depth of the stiffener (at its lowest point) is 7*Db (mm) in which Db is in meters. The proportion h/tw (slenderness ratio) should be =<10 using table 20, flat bar, plywood.

If you want to check for core shear AREA just to be sure use equation 48 and 49. The design edgewise shear stress for plywood is given on Table 18. Since this is not a constant height stiffener, the shear strength will be greater in the keel with a greater height. Use lightening holes if you want.

Lastly, you must satisfy equation 51 (design shear force) and 52 (bending moment). Because you are using plywood and it is made of dissimilar material, the bending moment formula is different.

Note the terms s and lu the frame spacing and length of transverses. Go back and check if you have satisfied the arrangement to satisfy the Aspect ratio arrangement 9.13. Is your arrangement square or rectangular? If s=lu, you have a square panel. If s> lu, you have a rectangular panel. If s<lu, you need to subdivide.

Edited to conform to ISO terminology.

 

That of course depends upon the span, but it still does not resolve the issue of the weak joints.

As drawn, there is no contribution by the sole region vertical parts, to act as effective longitudinal stiffeners, because there is no shear path to the hull.
Any stiffness it offers, as drawn, will be minor compared to an effective longitudinal that is a primary structure. Its scantling, as a sole, will be an order of magnitude below what is required.
Just simple pax loading per sq.m is rarely more than 5kN/m^2....one does not get such low loads on a hull bottom; one doesn't need to even crunching any numbers or look at rules to know this.

To suggest one can rely on a spray rail, as contribution, suggests that the whole structure is "on the limit" of pass-fail, and is therefore extremely unwise to do so. Any contribution of a spray rail, on its own, is so minor, compared to the hull girder, it is just a 'numbers' game to present as a "pass". This is just playing with rules and numbers and is not fit-for-purpose structural design.

 

This is, simply, not knowing the normal practice of scantling small boats and it is a nonsense without any foundation no matter how much "very technical" words are used.
ISO 12215-5 accepts the use of spray rails as longitudinal reinforcements as long as, of course, they have the necessary mechanical properties and that their angle is less than 130º. Their properties are calculated as if they were a normal reinforcement of the bottom and the normal practice is to add one or two more layers of fiber to reinforce it, although it is not mandatory, as long as they comply with the minimums required by the standard. The ISO calls these types of reinforcements "natural reinforcements" and there are various types in all boats.

 

Won't 2 longitudinals per side supported by bulkheads be sufficient? Looking at your sketch, it seems feasible.

 

As noted previously, regardless what the "rules" allow or suggest:

That's the essence of structural design, being fit-for-purpose....
Anyone can play with numbers or rules...

 

Anything is feasible, it really depends upon the spans and the loads expected. Then how he can integrate this into the arrangement.
Then of course the law of diminishing returns. The long.ts may become so 'large' (because of spans and pressure it must support) that such long.ts become impractical, size wise.

So, without more meat on the bone... as always, it depends.

 

Yeah. Agree. Seems there is going to be more tweaking based on calculations and interpretation of the rule.

 

Indeed.
Since the spans may simply end up being between WTBs, and in which case that is a large amount of area to support structurally.
Hence, in the absence of more data, it just depends...all one can do, is offer guidance on how structural design is achieved, to yeild a fit-for-purpose arrangement, rather than satisfying a rule, which does not.

 

The defining terms for ToMek would be how far apart his WT bulkheads/frames are spaced and the max height the 1st primary longi would be. Third would be the designed height of the stiffeners on the short side.

 

ToMek-

Sometime ago I think I showed you how to subdivide the longtitudinal section of a boat to ISO standard Par 9.1.2 with the “s” being the span or center to center distance of the transverses.

The subdivisions need not be all WT bulkheads but can be any major transverse structure like frames or web frames which will support the longitudinals.

In your design, it appears that you have longitudinals in the form of webs of the floor or deck and it is supported at the ends by transverses (bulkheads or any primary transverse structure).

If you look at figure 6 of the illustration, the s becomes l (length) and the width of the panel is b. If you have a large panel, your scantling will be heavy. Subdivide it.

Looking at the midsection with one primary longi on one side because the design calls for it, you need to add another longi to further divide the point from the center of the boat to the edge of the chine. Make it two. This makes for a very high aspect ratio panel whose AR would probably be in the range of 3.5+. 2 to 3 is ideal. If calculations show you will need to increase moment of Inertia (combination of primary longi height and plate) because of the long l. Further divide the l by a stiffener. Partial bulkhead in your case. Now the panel will have a low AR in the range of 1.5 to 2 causing a reduction of Mb. Then you can calculate plate or foam sandwich as the l and b is finally defined.