built up wood tension members

[rittz]

How would you calculate the net effective section area of a 2 or 3 ply built-up timber strut for tension resistance e.g. Boundary beams in roof diaphragms? Thanks a bunch

 

[TLHS]

If it's connected through both so that the load is shared equally, I should expect it's the capacity of one member doubled or tripled, respectively.

 

[focuseng]

what are the fasteners for the connections to transfer the drag forces and also the multi-ply fastening?

______________
MAP

 

[rittz]

Hi TLHS This for the response
Are you saying that if we build a 2-ply long built-up tension member by half lapping 16 ft. lengths (8’) of 2 x 6 that the useful section for tension would be 3” x 5 ½”. I feel it would be true of a section for compression (axial load), but it seems counterintuitive for tension??
BTW the fasteners I would visualize as bolts or spikes from ply to ply

 

[Triangled]

Hi rittz,
I don't think that is what tlhs is saying. The terminology is very loose, (" 2 or 3 ply built-up timber strut" & "2-ply long built-up tension member by half lapping 16 ft. lengths (8’) of 2 x 6 "), admits more than one interpretation, and the interpretation I would guess for you second post leaves an available cross section of a single 2x6 less the proper allowance for fasteners as queried by focuseng.

I'd suggest you draw a sketch of what you're thinking, and I'm sure you'll obtain a crystal clear and correct response within hours.
Good luck!

 

[rittz]

Thanks for the response guys . Attaching a sketch showing laminating 2 2 x 8's for example

J

 

[Triangled]

Perfect,
What I believe was your intuition in your previous post is correct. You get, at absolute maximum, the tension value of a single 2x8 in this case, knots and all. Your system's tension value will depend upon the fasteners used and their locations as well as the wood quality. I generally prefer nails which part the wood grain in these connections, others will prefer bolts which actually remove wood material. Your engineer will look for the "weakest link", which maybe the single 2x8, or the effective net section with bolt hole material removed which depends on locations of bolts, or the fasteners themselves and their various modes of failure. The code gives explicit guidance on fastener locations and calculations.

 

[BAretired]

Do the two 8' sections add anything to strength? Do you really need a 16' long scab in order to develop the member? Why not use a scab each side so that bolts are in double shear and the member is concentrically loaded?

BA

 

[MikeHalloran]

As BA says, you're wasting almost half the wood.
If you get wood cheap, or labor and skill is expensive, that's okay.

If you were building a boat, you'd probably use a scarf joint and a good adhesive to develop the full cross section of the wood, but that is a little skill-intensive.




Mike Halloran
Pembroke Pines, FL, USA