Design pressures in ISO 12215-5:2019

One of the factors that modify the design pressure is the "pressure reduction factor kAR", which in turn depends on another factor kR. For bottom side and deck stiffeners kR = 1 - 2 * lu/10^4.
When the unsupported length of a stiffener is equal to 5000 mm, this factor is reduced to zero, so the design pressure is zero. So how can a stiffener of this length or greater be calculated?
Furthermore, the larger the beam, the lower this coefficient and therefore the lower the pressure acting on the stiffener.
Thank you for your opinions.

 

As I understand it, the design pressure is based on a local "peak", which is migrating along the panel. This peak is "smeared out" over the actual panel area to get the design pressure. Consequently, the design pressure will vary with the size of the panel. Now, you may have a panel that is longer than the distance between frames, if you use "floating" longitudinal support. Could it be that you use a panel length that differs from frame distance?

I made a simple spreadsheet for calculation of single skin bottoms and support structure (long and transverse), and I don't see the problem you get, the required bending resistance comes out ok. But there might well be something weird coming up with certain combinations (a bit late now, but I'll check my calcs again and see if something pops up).

 

Thanks for your response @baeckmo. I think I haven't been able to explain myself. The problem that I have tried to pose is not how to calculate a structure but how to calculate the design pressure, according ISO 12215-5:2019, I should apply to it, when it comes to a reinforcement of 5 or more meters of free span.

 

If the value of kR reduces to a low value, then kAR has a minimum value it cannot go below and is dictated by 8.4, i.e where the value of kAR becomes less than 0.25, one must use the means for pressure calculation noted in 8.4.

 

Aha, my misinterpretation; Ad Hoc nailed it. Note that there are analog threshold values kicking in for lower (and upper) speed limits.

 

Thanks @Ad Hoc, you always help with precise advice. But, honestly, I don't quite understand what you mean. What I'm asking is what to do in the formula in the picture below, when "lu" is equal to 5 m or greater.

 

I don't understand this either. Sorry, but thank you for your good intentions.

 

The statement regarding a minimum value for kAR appears in a tabula further down in the document. It is also used in the "Guidelines for commercial craft" submitted by the Finnish authority, and which is based on the ISO 12215 to a large extent. In the paragraph defining the kAR it says that 0<kAR<1, which is overruled by the minimum value, currently 0,25 , stated later, which might be the root for confusion.

This way of stating a main rule, which can later be adjusted on a detail level, is not uncommon in legal texts. It allows detail adjustment without "disturbing" the overall rule.

Edit: Btw, I see that you are marketing software for scantling calculations (with reference to the ISO 12215), how have you treated the "borderline cases" there?

 

@baeckmo, what appears in Table 9 of the latest version of ISO 12215-5 is that kAR will not be taken <0 nor >1. It does not indicate anything else. That is, it could be equal to zero (if kR is negative) as I explained in my first post, with which the design pressure would be zero.
In reality, this is a very strange case of a 5 m span reinforcement, and up to this point I had never even considered it.
In fact, I sell this software, and I have corrected it so that the value of kR is never less than 0.25. I do not know if that is correct or not, but it would be more incorrect to have a negative value for kR.
Thanks for your comments.

 

kr is used to calculate kAR, which is "not to be taken <0 nor >1" (table 9. ISO 12215-5:2019 harmonised version). So, even is kr <0, kAR is limited and cannot be negative.

Design pressures are then calculated as the maximum between :
- Pressure having kAR in factor
- Minimum pressure that does not include kAR as a factor.

So Design pressure cannot become null, nor negative.

 

Thanks for your cllarification Alan .
Let me continue with my doubts. kAR cannot be negative, okay, but it can be zero. In that case, how should the "Minimum pressure that does not include kAR as a factor" be calculated?
What I do, in case of kAR=0, is to set a design pressure for the stiffener equal to that of the corresponding panel. I already know that the design pressure of stiffeners is lower than that of the panels, but I have to go somewhere.
Thanks again.

 

Check the applicable formulas (exemple for bottom structure, due to copyright, I cannot reproduce all the standard ) given in
Table 12 —Design pressures for motor craft (kN/m2) / kAR = 0, so PBMD (Bottom Design Pressure) = PBM MIN. PBM MIN does not depends on kAR, so here is your Design pressure independent of kAR


or Table 13 — Design pressures for sailing craft (kN/m2), / kAR = 0, so PBS (Bottom Design Pressure for Sailing Yacht) = PBS MIN. PBS MIN does not depends on kAR, so here is your Design pressure independent of kAR


In these tables, you will also find expressions for Design Pressures in different zones. For cockpit, side or superstructure, the tables gives the minimum pressure to be used, if kAR goes to null. (3.5kPA or 5.0 kPa, depending on the zone)

 

@Alan, you are absolutely right. Everything is (on this occasion) perfectly studied and explained in the standard. It was a silly mistake, really, because I have always done that calculation correctly, but this time I was wrong and I didn't know how to see what I had in front of me all along.
Thanks Alan and thanks to those who have tried to help.

 

It happens to me every time I dive into this standard ....