Timber double shear - internal ply

[Pawel1234]

Hi All,

A very quick question on double shear of timber members connected by bolts.

I know that for timber members acting in double shear the inner member should be twice as thick as the outer members. Would you say that it matters whether the inner member is composed of 2 No. timber plies or it needs to be a homogenous member - i.e. whether the 2 No. cases shown in the figure below are equivalent?

 

[KootK]

I'd certainly deem them equivalent if the two inner plies were laminated together suitably with some kind of fasteners.

 

[r13]

I would prefer the single piece solution, but practically, the difference could be small if the two pieces are tightly bind together without the worry of slippage.

BTW, where is the 2t comes from?

 

[Pawel1234]

Thank you for your replies.

Would you also be confident that double shear is achieved if the inner ply was formed with say 3 or 4 No. timbers? Do you think there is a way to determine whether fixing of the plies, for example, using nails along the member length is adequate?

 

[msquared48]

Retired13:

T + T = 2T

Look at the whole diagram...

Mike McCann, PE, SE (WA, HI)

 

[r13]

I don't see the difference of number of ply in between the hangers, but slip must not occur. I think you can achieve it through lamination, or nailing, though the later seems questionable for its effectiveness.

 

[r13]

Mike,

Thanks for your response. So it is for efficient use of the lumber, is it? Can I have 1t in the middle with smaller hangers on sides, that have same "t" (1-2x2 + 2-2x1, say)?

 

[KootK]

The more plies you add to the middle, the less I like it. At what exact point have you officially pulled into goofnut town? Who knows.

For each of the potential dowel failure modes that you investigate, it will make sense to assume a different distribution of tension to the various parts of the cross section. Some of the modes will surely load the interior member disproportionately at the outsides. I think that you would need to study that and let it be your guide with respect to the fastening requirements. Obviously, the axial load in the plies that don't participate at the connection would need to get transferred over to those that do.

I also don't know where this 2t requirement comes from other than to say that, if both members are of the same grade and species and are both fully taxed, that's about what one would expect to shake out of the design.

 

[jayrod12]

I agree with Koot about the 2t requirement. I've never seen it in writing, however it certainly would make sense from a member capacity standpoint that if they are the same grade that double shear would be of no benefit if they are the same size since the single ply would be the controlling factor still. In timber connections, rarely have I come across a situation where the bolt strength governs the connection design, normally it is the strength of the wood.