BHD stiffeners

Hi,

Could some one tell what the difference is if:

1- Bulkhead stiffeners are attched directly to stringers
2- Bulkhead stiffeners are attched to stringers thru brackets

The following photos are for aluminum structures

 

you don't have to make such judgement against the designer of this boat....WHY?

Because this boat is RIB type selected by American Coast Guard as an alternative of the boats of the same size they have, and now many countries are requesting this RIB...Iraq I thnk one of them.

follow this link: http://www.responseboatproject.net/

Anyhow, I agree with you, deep girders are much easier for designing and building " I guess" , but are they adequate..? if so, why not all designers/builders follow your way... Hope you have the answer?

Still no one answered my question about the difference between attaching BHD stiffeners to the stringers directly or through brackets?

 

Whoosh

My appologies to you for "My friend", in my culture it gives good impression. (I removed it)

My post is a simple technical question, and my intention is to get a simple answer too, nothing else.

I already mentioed that I agree with you about girders even I'm not an expert but I already noticed before posting this thread that some designers use stringers and some use girders & I post photos for bothe cases

http://www.boatdesign.net/forum/showthread.php?t=25825

but you gave a partial answer; still the question is:

why many designers still using stringers NOT girders? you mentioned the benifits of using girders instead of stringers, could you tell the benifits of using stringers, is it because of the light weight, space, ..?

My second question; if attaching BHD stiffeners directly to stringers would cause cracks in high speed crafts, why did the Coast Guard accept that design; Do classes accept this direct attachement?

Forgive me my many questions but I'm intrested in learning about the structures of high speed craft , the practical side.


cheers

 

your definitions/explanations are too vague, can you elaborate otherwise i will draw conjecture rather than facts.

 

Whoosh has deleted all his posts, I don't know why, I ask him as a builder to explain his point view.

Anyhow, when we have to use girders instead of stringers?

 

Well, the reason why it is a bit vague is because the method of constructing a WTB and its associated stiffening is invariance with your Question.

There are basically 2 ways to stiffen a WTB and 2 ways to construct a WTB.

2 ways to stiffen a WTB
a) Corrugated
b) With stiffeners

2 ways to construct the WTB
a) attach the WTB plate directly to the shell plate
b) attach the WTB to a frame, ie butt into the rider.

I would never recommend attaching the WTB plate directly to the shell. I know many yards/fabricators do this, but the WTB is, or should be, the last structural member that is welded up. If it is not the distortion and shrinkage that the boat experiences when being welded up will cause the plate of the WTB to buckle, sometimes badly which would require correcting.

So, what does this have to do with the main hull stringers?

These are either intercostal between WTBs or they are continuous.

If they are intercostal, each hull stringer will be required to have a bracket to the frame, both sides, inline with each other.

If they are continuous, each hull stringer will have a collar.

Ok, now take a typical example. (ignoring corrugated since this is self stiffening and doesn't address your Q).

A normal "T" frame, web and flange, is attached to the shell plate. If the hull stringers are intercostal between WTBs, then say the stringers are 100mm deep. Make the web of the frame say 300mm deep. This allows for a 200mm size bracket. (100+200=300). The toe of the bracket will be welded to the underside of the flange.

If intercostal, you can have a frame of say 200mm web. This still allows room for the welding torch to go up and around the collar without clashing with the flange.

So in both of these cases, the hull stringers finish or pass through the frame.

The WTB Plate

So, now the WTB plate (same if corrugated) is attached to the flange of the frame.

The stiffeners on the WTB plate will be either
simply supported, or, built-in.

If simply supported (you must of course ensure the modulus satisfies the min rule requirement), then there is no connection. The stiffeners stop short of the flange, of the frame. This method allows for the fabrication of the WTB and stiffeners on the floor, not insitu, much easier.

If built-in, (again check against min rule requirements), you can terminate them 2 ways.
a) If the depth of the stringer is say 50mm, make the flange of the "T" frame 55~60mm. The WTB plate will sit about 5mm inside the flange, offset. Then the stiffener will butt directly to the flange to be welded. This is neat and a good structural joint for transfer of shear loads from the WTB.
or
if a large WTB
stop the stiffeners short of the flange, and bracket the WTB stiffeners to the flange. So if flange of "T" frame is say 100mm, stiffeners say 50mm, a bracket will be 100mm roughly, to weld to the flange and then lapped to the web of the stiffener.
This is much more complicated and adds work etc..so not recommended unless necessary.

So, going back to comments about girders etc...no idea what these are for, a longitudinal girder is out of plane and has nothing to do with the strength of the WTB stiffeners and it method of construction. If someone is simply adding long.t girders to the hull etc, they clearly do not understand structural design and are just adding unnecessary weight.