CSA S6 Butt Joint Stiffness Factor

[MagicFarmer]

Good morning,

I am working on the design of a nail-laminated bridge deck and don't understand the Butt Joint Stiffness Factor, kb.

kb = (Nb - 1)/Nb

Where "Nb = a measure of the frequency of butt joints in laminated wood decks, being for any 1.0 m wide band perpendicular to the laminates, the minimum number of laminates without joints adjacent to a laminate having a but joint."

I have attached an image of my interpretation of this definition, am I understanding the wording correctly? The yellow laminate would suggest a value of Nb of 3, yet the pink laminate would suggest a value of Nb of 0... which doesn't work with the equation for kb.

Any guidance would be greatly appreciated.

Regards,
MF

 

[OldBldgGuy]

Maybe I'm wrong, but I see it as a convoluted way of saying the minimum number of laminates without a joint which would be 2 in your example. This can be manipulated by taking your 1m band in a selective location as well, either conservatively or liberally depending upon your bent.

 

[MagicFarmer]

@OldBldgGuy

That was my first thought... After sleeping on it I've come to the conclusion that the code tossed this clause in in a sloppy, ill explained way. I believe that the code covers the "selective 1.0 m strip" approach by requiring detailed fabrication drawings. It also notes that the fabrication requirements shall be noted on the plans... placing the burden on the contractor to develop a fabrication plan that meets the requirements of butt-joint spacing.

Secondly, the value of Nb should be treated as an afterthought, and back calculated as consequence of moment demand, such that:

kb x Mr > Mf

-> (Nb-1)/Nb > Mf/Mr

So, your deck notes should include a sentence stating that no butt-joints shall occur within Nb laminates within any given 1.0 m wide longitudinal strip.

 

[SMR8611]

There is this NLT design guide.

https://www.thinkwood.com/products-and-systems/nltguide

Keep in mind that some reduction factor are not correct.
This guide it taken from some European research and the redaction factor for butt jointed lamination are too high in some cases it reduce the bending stiffness by 70%. Doesn't sound right!
We have run some tests for deflection and doesn't really match with this Guide. But as an overall guide it works in most cases.

Cheers