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Re: Parallel chord wood floor trusses

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Chris,

I have not read the article either, but I will soon as it pertains to a
three-story motel building that we will submit in the near future.

Three comments:

1.  Consider how often the trusses may actually experience the design loads.
I do not know what occupancy you are dealing with that calls for 100psf live
load;  my motel occupancy calls for 100psf  for "exit walkways."   100psf
is people crammed very closely together.  I do not reallistically expect
these motel exit walkways to EVER see 100psf LL.  If your trusses support
the lobby in a theatre or some other waiting area where they may frequently
be loaded to a good percentage of their design loads, I would start to worry
more.

2.  You may want to discuss deflection with the architect, and restrict
it to less than L/360.  Owners seem to get as worked up over tile that is
cracking RIGHT NOW than they do about trusses that may possibly fail and
injure dozens of people sometime in the future.   Goes along with the "but
we never have earthquakes (hurricanes/tornadoes/ice-storms/floods/heavy
snow/etc.) around here" mentality.  You could then blame this apparent fit
of conservatism on the architect.  ;-)

3.  Research for another project led me to NDS Table 7.3.3--Wet Service
Factors for Connections  (1991 NDS, still cited by the California Bldg.
Code).  The adjustment factor for metal connector plates installed in
Partially Seasoned or Wet wood that will be DRY or WET in service gives you
80% of the full load for the connector plate.  Around here it is very likely
that trusses will be fabricated from wood with moisture content of 19%
(partially seasoned) or more.  In all the truss calcs that I have reviewed,
I have seen plate stress increases of 1.25 for roof Live Load on virtually
all roof truss jobs, but
NEVER an adjustment of 0.80.   I would (will) specify that trusses be
manufactured from lumber below 19% M.C. (stamped "S-DRY" on the grade
stamp), or that the designer include the 0.80 adjustment factor for the
connector plate design.  If you will load-test trusses, you may also want to
have the moisture content checked with a moisture meter--or at least verify
that all the lumber used has  "S-DRY" in the grade-stamp.

Looking forward to reading the article, which may prove the above comments
totally worthless.  (Specify first, ask questions later....  :-)  )

Thor Matteson





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