# Method for calculating the scantling of Tee-Section?

Discussion in 'Boat Design' started by Jo Ho, May 23, 2021.

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### Jo HoNew Member

Does anyone know a method for calculating the scantling of Tee-Section?
I have calculated the design pressure and the section modulus of the girder.
What is the next step?

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### Ad HocNaval Architect

Welcome to the forum Jo Jo

Are you a student?

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### Jo HoNew Member

my pleasure,
yes I am a student.

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### Mr EfficiencySenior Member

No text books ?

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### Ad HocNaval Architect

Have you studied structural design and second moment of inertia calculations of shapes?

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### TANSLSenior Member

You have probably used a certain rulebook to calculate the right pressure. Well, that same regulation will have a formula to define the minimum admissible value of the elastic section modulus of the girder. And don't forget to include the stiffener's attached plate when calculating that modulus.

Last edited: May 24, 2021
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### gonzoSenior Member

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### jehardimanSenior Member

And just for info, if you are not using a set of rules, a typical accepted value for effective plating flange width is 20 times the plating thickness.

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### CDBarrySenior Member

Whatever code you are using will specify the effective width. (Note that effective breadth is different than width. The latter is for buckling.) Most class society codes are available for download free, notably ABS at www.eagle.org.

Meanwhile buy a copy of Blodgett, "Design of Welded Structures" directly from the JF Lincoln Foundation (not Amazon). The Lincoln Foundation subsidizes their publications so they are very affordable. You may also want to look at some of their other publications. It used to be they offered a discount for multiple orders, so you may want to put together a joint order with your classmates.

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### TANSLSenior Member

I do not know that rule nor does it seem correct to me. You must go to a reinforcement with a web of a thickness similar bigger but not too much that of the associated plate so as not to deform the latter welding both elements. From there, what you have to look for is that the reinforcement + attached plate has the right module. But, in addition, you must take care that the proportions of the reinforcement are such that local buckling of the reinforcement does not occur. For this purpose, the various standards give maximum values for the width of the flange/web thickness ratio and/or similar.

Last edited: Feb 2, 2022
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### gonzoSenior Member

Thickness and width are two very different things.

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### TANSLSenior Member

Wise words.

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### jehardimanSenior Member

It isn't absolutely correct, but is a good rule of thumb for preliminary design we always used in the office. Otherwise the plating gets too flexible and you lose geometry and are forced to add additional stiffening. For many years the US Navy used 30t, then along came MIL-HDBK-264 which uses the formula 2*SQRT(E/Fy)*t which gives a range of 60t for MS to 35t for HY-100. However there are two problems with this: first it doesn't consider buckling geometry, openings, or attachments, secondly it didn't account for different materials in the plating and the stiffener. This lead to a whole other set criteria set down in Design Data Sheet 100 and other specs. Finally, if you go into the rules (ABS, GL, etc.) and actually work out the spacing between frames/stringers/longs/etc. and then compare it to required shell or strength deck plating thickness those stiffener support you are never going to get anywhere near 60t; same thing for rolled W shapes, the flanges never get that wide compared to their thickness.
So, yeah, it's not correct, but nothing else is either, just the belief that we are doing it correctly.

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### Ad HocNaval Architect

We have used 50t for 99% of our structural designs. These are for 'simple' Tee and OBP/FB stiffeners.
If the stiffener is more like an RHS/channel... then it becomes a little different...as it starts to depend upon the width of the base of the RHS/channel too - which adds more complexity and variability to be a 'rule of thumb'...

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