Connection shear

[shacked]

I don't know where else to post this so here goes.

I am designing a folding beam for a trimaran boat and I need to determine the forces in the connection. Basically the beam is a 3" SQ x 1/8" thk Aluminum tube that is centered on the main hull. The main hull is 2ft in width, therefore the beam cantilevers out on each side 18". At the end of that beam there will be a laminated wood beam made out of Ipe and DF Clear. The top & bottom laminations will be made out of Ipe and the center will be DF Clear.

I realize that I am basically trying to take the moment out at this location which is not a good design in structures, but this is a small boat.

If you refer to the attached pdf you can see the connection design and additional info.

I need to determine the shear at both of the bolts on each side of the pivot. I realize that if there were only 1 bolt on each side of the pivot, then the shear would be the moment produced by the point load acting on the end of the beam divided by distance btw'n the 2 bolts, but in this case there are 2 bolts per side.

I realize that this would be easily solved using finite element analysis software, but I do not have that available.

Any help would be appreciated.

 

[Lion06]

Take the shear and moment acting on the group and do an elastic bolt group analysis.

 

[shacked]

Thanks SEIT. Ya, I reviewed that section in the steel book, but I think that that method is a little conservative for what I need.

I also reviewed the instantaneous center of rotation method but that method seems to be more suited for use when homogenous materials are used.

In my case there is shear btw'n aluminum shear plates and an aluminum beam, as well as aluminum plates and a laminated beam.

I guess I could always back calculate allowable moment based on the allowable shear for a bolt in double shear with the main member as wood and the side members as aluminum plates.

 

[JAE]

Is the wood extension beam only connected to the aluminum tube with the two bolts? If so, then your moment arm for the shear in the bolts is only the space between the two closely spaced bolts? As the wood extension is "bent" the only thing resisting this bending is the two bolts through the wood, each only apparently inches apart.

The end of the wood beam, as it rotates, might bear a bit on the end of the aluminum tube, but then the connection gets very complicated.

Not sure if I'm perceiving the connection correctly or not...can you elaborate a bit?

 

[shacked]

JAE, the laminated wood beam is connected to the Al tube via a 1/4" thk Al plate on each side of the wood beam, which pivots about a 1" pin located through the Al beam.

I see your point that the 2 bolts in the wood beam will be resisting the moment created by the point load. Maybe if I were to make an aluminum bent plate/shear pivot plate which is shaped like a square sleave that the wood beam can fit into, then bolt to the Al beam when the outrigger is folded down.

Thanks

 

[JAE]

That might be better - that way you create a moment arm via the sleeve and not the bolts alone. The bolts would still do some work (holding the wood on to the sleeve) but the moment would be via simple bearing on the insides of the sleeve on each end/each side.

The trick then would be to create a sleeve long enough to limit the bearing on the wood of the sleeve. To some extent, you'd have a "bow-tie" shear diagram along the embedded portion of wood inside the sleeve. The ends of that shear diagram (at the end of the wood and at the mouth of the sleeve) would have the greatest shear. Take that shear divided by the wood width and you have some semblence of a bearing pressure.

You would have these limit states to check:
1. Wood bearing on the inside of the sleeve.
2. Wood shear (maximum at the mouth of the sleeve).
3. Wood bending (maximum at the mouth of the sleeve).

I'd keep the bolts away from the mouth of the sleeve just to leave yourself a full section of wood to deal with all of the above.

 

[shacked]

JAE, thanks. Yes, I have already thought about the shear & bearing of the wood on the sleave. I don't think that will be a problem since the design reaction at the end of the outrigger/beam is 350lb.