Return to index: [Subject] [Thread] [Date] [Author]

RE: Approved end-jointed lumber

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Brian Smith wrote:

>>I have been skeptical about their use; however, I received a "wood" magazine
sometime last week that discussed their use.  I still don't know if I would
allow them on one of my projects.  If you want to spec them, you need to be
aware that there are two different types.  One is designed as a direct
replacement for solid sawn members whereas the other is "axial load only."<<

I, too, am skeptical about finger joints themselves, let alone direct 
substitution.  I also received the same "wood" magazine and it appeared that 
they skirted around the direct substitution issue.

The most efficient glued joint in lumber is a scarf joint with a 1:12 slope 
or flatter and that has only a 90 percent efficiency.  (Wood Handbook, 1974, 
page 10-7)

A finger joint is supposed to simulate a scarf joint and take up considerably 
less length.  However, even making the slopes on the finger joints 1:12, 
you cannot approach the efficiency of a scarf joint as you can't have a saw 
blade with a zero width.  Therefore, the total length of a finger joint would 
be considerably less than that of an equivalent slope scarf joint due to the 
square cut at the tips and bottoms of the fingers.  The square cut at the 
tips and bottoms of the fingers are equivalent to butt joints, which have no 
predictable strength in tension. (Ibid.)  Even if the square cut could be 
eliminated and a true "V" constructed, the sharp re-entrant corner would 
create stress concentrations.

It is my understanding that there have been a number of failures in 
California of glue laminated beams 25 - 30 years old where the source of 
failure is attributed to the finger joint.  I am also aware of one failure 
here in Tucson in which the failure was at a finger joint.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona