Mullions / Pillars Calculation

Discussion in 'Boat Design' started by ToMeK, Sep 20, 2021.

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1. Joined: Nov 2006
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ToMeKYoung naval architect

Dear Boatdesigners,

For quite some time I am struggling in finding method to do the mullion or pillar calculation.

I am working on the 50 ft GRP planing boat project with flybridge. After designing internal structure, hull and deck, it is time to do the flybridge calculation. In preliminary design idea was to support GRP flybridge with two ALU frames (100 x 100 x 3 rectangular cross section) that are supported on deck; and two vertical GRP mullions that are in front part of cabin (near windshield).

I am familiar with analytic (hand) calculation of ALU mullions, but I have never done it with GRP (problem is EI of the GRP mullion cross section). Does anyone have any experience and an advice on how to approach the problem?

Many thanks,

Kind regards,

Tom

2. Joined: Oct 2008
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Hi Tom,

As a compression post like anything, it is proportional to the EI.
As such the Young's Modulus is the key as well as the allowable deflection. GRP being some 3 times less than ally.

In GRP it shall be more difficult to define the E than an isotropic material when laminated into a tube/pillar. As that requires a high degree of quality control, and then you really need to do a coupon test to establish the E in the axis of compression.
Unless you can buy a ready made GRP pillar/tube that suits what you want, and with a cert to guarantee its strength (via quality control), I would advise against using GRP as a pillar.

If as a mullion, that is different, since you can treated it in the same manner as any structural section frame/mullion etc, but again, noting the EI, and the allowable defection (that's the key).
Just as if it were any isotropic material.

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ToMeKYoung naval architect

I apologise, I formed the question not very precisely because of translation. I am asking about a calculation of structural members that are supporting flybridge. That would be pillars and columns.

@Ad Hoc - Thank you for your answer. Just to check if I understood correctly, you would recommend using Aluminium Alloy (like 6061) to form supporting structure for fly?

I haven't found any guidelines or example of well designed flybridge (or any deck above main) on GRP boats. So my idea is to define strong grid of transversal and longitudinal GRP beams that would be supported by columns, and add some additional longitudinal panel stiffeners if needed to support deck panels. Grid would be defined as stringers according to ISO and pressure would be take as a mix of flybridge load case and ISO pressure for weather deck.

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TANSLSenior Member

When calculating the design pressure for the fly bridge according to ISO, the value can be very small, so you should not forget to also consider some local loads, such as the accumulation of people on a panel. This weight, to which a factor must be applied that takes into account the vertical acceleration in that area (a factor that the same ISO provides), may be greater than the value given by the ISO.

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In which case, my replies remains. I would not recommend using a pillar made from GRP.
Use an aluminium one.

The beams and Long.ts in GRP... not a problem., But the pillar supporting said load, see above.

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rxcompositeSenior Member

Tomek
Now I am confused. Do you want to know how to design pillars/mullions to support the weight of the deck or do you want to design a deck grid to handle deck loads?

They are two different calculations.

Your first question was how to design mullions/pillar in GRP.

While it is difficult and complex, there is a methodology and I was preparing something to guide you into it. I think I will wait.

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rxcompositeSenior Member

It is bad practice to combine aluminum and FRP.

I know you can do it (pure FRP) Tansl.

Last edited: Sep 23, 2021
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TANSLSenior Member

Strongly agree, it is a practice to be avoided. I am not able to imagine any circumstance that can recommend it, although it may exist, of course.

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rxcompositeSenior Member

FRP has a lower modulus than aluminum and thus tends to be bulkier to have the same EI. But being bulkier, there is no need to worry about Eulers or slenderness ratio.

The greatest problem would be in the connection. Bolt it to FRP sheet thru a flange? Problematic. What is there is an FRP plate? How do we bond it to the aluminum mullions?
The only time it works is when you use aluminum/stainless tubes and canvass awnings typical of flybridge.

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rxcompositeSenior Member

Tansl- In the old version of the ISO, there is sufficient information on the grillage structure using the plate tabulations, plate with stiffeners tabulations.

Is there anything on the latest version where FRP can be designed as a pillar/mullion?

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TANSLSenior Member

There is nothing in ISO 122125-5: 2019 that talks about the requirements that pillars must meet, as there was not in the previous version. But we always have Euler to help us with them. What the ISO does indicate is how to obtain the mechanical properties of any laminate, even if it is used for pillars.

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ToMeKYoung naval architect

Dear rxcomposite,

I ment composite mullions or columns/pillar calculation. Vertical compression structural members that support flybridge deck and connect it to main deck.
Can you please explain why would you avoid it joining alu with composite? Particularly in case of column?

If you have any literature or guidelines how to do it that you can share with me, I would be very grateful!

For flybridge deck structure I would make small "load case" that would give me worst scenario for loads to apply for calculation and then would calculate it as normal weather deck (weather deck loads + additional weight of passangers, equipment, water ... )

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ToMeKYoung naval architect

Exacly, working on it at the moment

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ToMeKYoung naval architect

This is how I would join metal pillar with GRP deck structure. In between even some glue can be put.

Attached Files:

• IMG_20210923_160444_2.jpg
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15. Joined: Jan 2005
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rxcompositeSenior Member

To answer your first question, It is about the EI as AH has mentioned. The I is fairly straightforward bh3/12 but finding the E in composite is quite complicated. The E varies with the type of fabric used, the ratio of glass content, the type of resin used, and the type of load whether it is in compression or tension. This is complicated by the fact that sometimes you have to mix and match different fabric types.

You need a large database or you can derive the E using the formulas provided by LR or ISO. Attached is the common properties of typically used fabrics laminated in polyester resin with a typical glass content that can be achieved using hand lamination.

To find the Effective E, the basic formula is (Sum of individual E x Sum of individual thickness)/Total thickness. Since the elements can be either in compression or tension (and Shear), and different fabric types will be used, it is best to define the effective E per element.

IF you are familiar with ISO or LR composite analysis, you will find the tabulation rings a bell. Attached is a snippet of my Excel tabulation. Look closely and it shows it uses the basic formula.

Page 1, stiffeners is a tophat stiffener with plate typical of what you will use in a deck layout. Note that I ommited the plate calculation as you was asking about how to find E.

Page 2, beam is a layup of what you will use for a naked crossbeam. No plates.

These are all designed to take care of bending loads. Pillars and mullions a different topic later.

Attached Files:

• Stiffener to Beam.xlsx
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