Deck Unsupported Span

Hi All,
Currently, I am designing a deck for a client, and he has given me a plan just to verify if the frame system can pass ABS requirements.

I have uploaded a picture describing two sets of systems. in the "A" picture is what I understand to be definition of "unsupported span", if all girders and web transverse pass the requirements this is how their unsupported spans would be defined. Alternatively I can also cut the transverse into 4 sections but I have to calculate the 3 girders with their unsupported span covering the distance between two bhds.

In "B" picture is what I am curious about. Is there a way where I can calculate a system of framing like this?

 

You could reduce the distance between supports by placing intermediate supports, for example pillars. But I don't think your client will like that. On the other hand, if the beams are too high and reduce the internal free height, you could also use box-type beams, similar to the one in the attached image.

 

It is called a grillage.
Standard frame analysis, if you have done the theory for type of analysis...?

 

Yes told him the same. But obviously, he didn't like it. Glad that I could clear that up.

 

Hi Adhoc, I don't think I have done that. To this point, I have only done checking on section modulus of members with plating attached and a longitudinal hull girder strength.
Could you point me in a right direction where I can learn it?

Right now I am also doing an analysis of the stern of this client's boat which have huge openings on its floors. So any additional checks I can do before I give a report is welcome.

 

Well, it sounds like perhaps you have not done basic beam theory yet, that is methods of analysing simply supported and built-in beam, for stress and deflections etc....is that correct?

 

I see. Thank you. I am aware of how to calculate these. However I am very limited with my application. For example, I don't know how to account if the plate has a huge hole in it. The rules are also very clear of not cutting webs more than 50% of the height.
Preferably, I don't really want to make an FEA of this model. Is it possible to do that by hand/excel? Or not practical?

 

You need to be fully conversant with how to analyse using beam theory before moving onto grillages. Since it is a prerequisite understanding of beam theory that allows you to analyse these structures known as 'grillages'.

If you know how to calculate the second moment of area of any shape, then you just draw up the shape and calculate as you would any member, with or without a hole in it.

 

I see. But I would argue that a hole so large would invalidate whatever shape it has in the first place. It must be weaker structurally than a member that has a more appropriate sized hole. But as you pointed out, I have incomplete knowledge on this so I might be wrong.

 

Time will tell, once you do the sums

 

Hi Ad hoc,
Thank you for your ample reply just so I understand what you mean more clearly.
What you mean is regardless of how large the lightening hole would be. I would still calculate for example the frame in this instance with Web H and I do not need to take the Hole H in consideration.
So a 500 mm web would still be calculated as a 500mm web even if there is a 300 diameter hole in the middle?

I apologize if my questions are too simple. Thank you for your patience.

 

No, that´s not correct.
Even if you don't know beams theory, common sense tells you that both solutions cannot be equivalent. You can consult, for example, any Classification Society Regulations. In them, solutions are given to concrete constructive solutions, which can be applied without knowing beams theory.
If you want concrete answers and effective solutions, send me a private message.

 

As noted above, that is not correct.

So, to understand what is going on create a simple I-beam, a rider bar and a web plate.

Make the riders say 300 x 10mm, and make the web, 500mm by 6mm, as an example.
What modulus and 2nd MoI values do you get?

 

Hi Ad Hoc, Thank you for your reply. I am getting 45 265 cm4 from my calculations.

Thank you for the help TANSL. I am reading up on it. To me I just want to know quantitatively how to solve this. Applying the Excel Sheet that I have already made. .

 

Ok, that';s correct, and don't forget the modulus value too!

So, now, in the same example, work out the Z and I with a central hole of 100mm diameter and 200mm and then 300mm diameter.
And what do you notice?