Dear members

Can anyone explain what were the background formulae used calculation of actual weak& strong axis bending for vessels on legs (leg check tab). PVElite calculation doesnt show any calculation or reference. I tried calculating the same as per Bednar, but no's are not matching.

If required I can post pv elite file.

Regards
Prudhvi Teja

Hello pruchviteja

We have a small article in our knowledge base called 'LegDesign.pdf'. If you use our eCustomer support system, you can request that article.

Hi Ray

Thanks for the reply and advising about eCustomer support system. I didnt know about it. Now, I have requested for access through my organization.

I will find the article and solve the problem. If, still there are any more questions, then I will post here.

Hello pruchviteja

When you gain access to eCustomer, you can ask me to send the article to you if you have any problems accessing it.

Dear Ray

I have received access for smart support. But, I am not able find the file you have mentioned.

I could find one file related to legs with name Moment_and_ShearForce_at_top_of_the_legs. pdf. But,It is about calculating moment on the leg. I have calculated it and matched it with PV-Elite. I am looking for calculation of actual weak & strong axis bending in the leg check tab.

Can you please guide me.

Every structural member has its strong as well as weak axis; it's about the principal axis of inertia.

Hi Prudhvi I have attached the file to the forum. Please let me know if you received it.

Hi Ray

Yea. I got the file. Thanks.

Dear Mr. Delaforce,

I have some remarks on your paper. They are not changing the results rather are just some remarks in forum.

1. In Figure 1, the legs seem to rotate together with vessel when only M acts on. In fact the model developed (by you, by Bednar, by others) assumes axial load of the legs only.

2. In figure 1, you've drawn the legs distribution and the origin of “alpha” angle is taken from a leg. Developing the calculation, it appears that alpha=0 is linked also to the M axis definition. I mean your M acts in a plane that has a particular relationship with legs distribution. No doubts, when analyze wind or seismic effects this approach is correct because you don’t know the “direction” of such occasional event, hence you must analyze the most unfavorable case- which is exactly what you analyzed.
However, proceeding in this way you have lost the “general case”.
For this reason, I would comment that you made a remark that refers to a degree of generality that is not confirmed.
I refer to the page 3 where one can find “in other software written by our competitors, the force on the leg […] is given by […]; although this is conservative, the missing Cos(alpha) term makes it less accurately […]”.
In fact you started the calculation with index “i” as i=1 and arrived to i=n, case in which the last angle you’ve considered is 2*π*i/n=2*π*n/n=2*π (or “0” if you prefer), and that means that you have also the most loaded leg as 2*M/(n*R) because cos (alpha)=1 for that angle.
What I try to say is that you assumed- conservatively and correctly- a particular relationship between legs distribution and M direction, you developed – correctly- the model starting with alpha=0 (or equivalent, arriving at alpha=2*π), and you’ve lost the degree of generality that you claim that your model has. And I repeat that your approach is correct because we are not interested on the general case (which, BTW has to be modified in case one wants to develop it, because the sum involved has a different development), but to the particular case in which M acts maximizing the effects on legs and the most loaded leg is indeed 2*M/(n*R) for that case.

3. When you've analyzed the horizontal shear force effect, you made an assumption. “Now, for a conservative approach, we make the following assumptions:
– the legs are pin jointed at the base (bottom of the legs)
– the top pf the legs cannot rotate or twist”
Making such assumption seems to be in contradiction with the ending paragraph of your work. “The legs are considered to act as guided cantilevers […]”.
My remark is that you don’t need the assumptions “conservatively” made at page 4, because the “stiffness” can be evaluated for a general case- by Castigliano theory, for example- and the result of force distribution to legs is identical with yours.

My Best Regards.

Hello mariog

Most of the folk on the forum are not aware of the article I wrote. As a suggestion, it would be better that you submit an article from which we could all benefit. You could make the angles more generalalised. After reviewing your paper, we would be happy to include it in our set of knowledge base articles. In that way we could all benefit from you expertise.

And would it help to write an article up with the same results as yours?
Otherwise why do you think that most of the folk on the forum are not aware of the article you wrote but they shall be happy to see my article?

Just to be clear: my remarks are just about your paper. You might want to improve it in some details and explanations (or not), it's after you- of course.

I would add that I think the reason of a forum (particularly this forum) is to share ideas and knowledge, at least this is my perception. And of course my remarks are addressed to those that are aware of the article you wrote.

Hello mariog

I just thought that you could give a more detailed insight on the principles of leg design. Should you wish to write such an article, my article could be withdrawn and replaced with yours. That could be the improvement you suggest.

Intergraph are always interested in customers getting involved in any educational work. We regard ourselves as part of the engineering community.