ISO 12215. Aluminum double bottom structure scantlings

Hello everyone,
I'd like to read your opinions about how to calculate thickness of the floors and girders of an aluminum boat (see attached picture), according to ISO 12215.
Thanks in advance for your ideas.

 

Floors taken in normal way.

Longitudinal stiffeners with openings - take only section of part adjacent to the bottom.

Double bottom plating to be calculated as tank.

 

Thanks Alik for your answer. Let me make the following comments :
Floors taken in normal way : I do not see the ISO specific reference to the calculation of floor plates
Longitudinal stiffeners with openings - take only section of part adjacent to the bottom : Like a flat bar welded to the bottom?
Double bottom plating to be calculated as tank : do you mean as tank top?. I do not see how ISO recommends this calculation. Could it be done as a deck?

 

1. There are diagrams in ISO for floors.
2. Yes.
3. If double bottom should hold pressure (say, from liquid cargo or raking damage) it should be taken as tank top. If not, it can be taken as deck 5kPa. Though You can take even less, say 2-4kPa for internal deck.

 

1.- In ISO 12215-5, Figure 11, there are schemes that apply to certain types of floor plates but, in my opinion, are not applicable to the case that I indicate here. My question is whether we should calculate as a stiffener with loads (even load distribution) due to external pressure on the bottom or should be calculated for the case where the double bottom is filled with water, ie, as a bulkhead (triangular load distribution), waterthight or not according cases or even as a tank bulkhead.
3.- OK

 

1.Formally should calculate both, but usually only the first approach as it presents worst case.

 

Thanks a lot, Alik.
If someone wishes to express some other interpretation, please feel free to do so.

 

Another interpretation would be: You can calculate this as stiffening system without main stiffeners, i.e. grillage. There are corrections for equivalent span given in ISO12215-6.

 

Yes, I had not thought of that possibility and probably this interpretation is more consistent with the structure as it is conceived.
I guess it would have to consider two grillajes, one of the bottom and one for the "beams and girders" of the deck. Each reinforcement height equal to half of the plate's height of the floor ?.

 

They can be be taken half, BUT usually if loads are in opposite directions we take it full height. We do so for example for small catamarans on wet deck - taking slamming loads and deck loads. And in presence of plates on both sides can be taken as I-beam.

 

I calculate the deck with the wet deck as a single structure, subjected to slamming, loads on deck and torques due to the relative movements between the hulls.
In the case of the double bottom of this boat, if you used the simile of an I-beam, the norm would say that the values of h/tw or d/tf (Table 20) are greater than allowed. What do you think?

 

In my opinion, the main point of 'table 20' is to prevent buckling of stiffener. Taking the bottom scantling as grillage, we should consider the buckling of the vertical panel replacing the stiffener. The effective plate to be not caculated in this situation.

Just for discussion. Thanks.

 

According to your image:



you have a bottom frame (floor) that is deep with a lightening hole, of sorts, in the web. You treat the frame no matter whether it has a lightening hole in or not. What matters is the stiffness, the modulus, of the frame to satisfy the minimum rule compliance. So take a section through the frame like so:



at the location which has the least depth. Calculate the modulus and if it passes, hey presto.

The fact the web has a lightening hole in means you only need to double check
i) the required shear area is still satisfied
ii) that the web wont buckle under load, which means either a) TBs or b) rider bars around the lightening hole.

That’s it.

Table 20 just refers to maximum depth v thickness of an supported, or support web.

That's correct.

 

Thanks for your constructive interpretation.
And there is some further question for the web frame.

1. For TYPE A arrangement, the beam 5-6-7 to be calculated as a whole H-beam bearing the bottom load.(the span lu equals the chine beam BC) I think this is what the rule requires. At the same time we noticed that the keel is strong enough to act as a primary stiffener. Should we consider the beam 5-6-7 separately as 56-and 5-7? And the point 6 as a support point?

2. For TYPE B arrangement, the beam 1-2 and 2-3 at least to hold the bottom load individually.(the span of each beam lu equals almost the half chine beam BC ) In this way, should we consider the beam 1-2-3 as whole one to bearing the full bottom load(beam 1-4-3 only take the deck pressure)? If it is true,the question same as question 1 comes out. And the pillar beams 2-4 reinforce the keel more from aft to fwd. If not, I assume that the beam 1-2-4-3 totally acts as a whole beam. Generally it is too strong to design or build. Isn't it？

Thanks.

 

For this you need to understand the definition of "span".
The span of a beam/frame is from one support point to the other support point. However when there is shape in the hull, when does the support become influenced by natural supports, such as chines or turn of bilge?

The answer to this, in ISO, is shown in 9.1.5, fig.10:-



The angle shown, alpha, is the natural break, or support that you are referring to, and noted in the rules:



So if the subtended angle is less than 150 degrees, it may be considered as a natural break. Or put another way, like this:



The angle of deadrise now becomes the "alpha" angle shown in fig.10.

Thus, if the deadrise angle is greater than (180 - 150)/2 = 15 degrees the keel becomes the location of a natural support, because the total subtended angle (inside) is less than 150 degrees.

And this is exactly the same for Type B. I used these 2 extreme examples to show the same is applied. The only difference is how you wish to arrange the structure.... normal deep frames as Type A, or very deep frames as Type B which may be used to perhaps be the foundation/support for a sole, for example. The method of calculating the requirement becomes the same.

The advantage with Type B, is that once the deadrise angle becomes less than 15 degrees, these frames, by inspection, are stiff enough not chnage. Whereas Type A, the frame depth, rider bar or both, need to change to increase the stiffness to satisfy the greater span being imposed by the lesser deadrise angle.

This is the same in most Classification Societies too, such as shown here from ABS :