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RE:eave blocking in high wind and seismic zones

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Martin,

Yes we require this blocking "Outside" (Alaskan's term for the rest of the
world, particularly the Lower 48).  And that requirement generates more
phone calls and complaints from the contractors than everything else in the
plans combined!  We typically ventilate with the holes in the blocking that
you describe, nail the roof diaphragm to the blocks, and use A35's or
toe-nails to the top plate depending on the loads.  Another solution I allow
is to block every other bay.  This only ventilates if you have cross
ventilation between the truss bays which we usually do.  Also, every truss
is supported, and you can still get the shear transfer you need unless you
have a very high load.
I'm not sure what you mean with half height blocking.  Do they only fill the
bottom half of the truss bay gap with wood?  How do you provide roof
diaphragm edge nailing?  If this is really what you mean, it seem that it
would not be worth the effort to include any blocking at all!
The problem with blocking the truss tails is again, no diaphragm edge
nailing.
I always worry about a discontinuity in the diaphragm for a ridge vent.
This is occurring at the point of maximum shear in the diaphragm.  I usually
try to talk the designer into cap vents, but if that fails, I specify a
special double 2X blocking attached to plywood gussets, and inserted at
every bay at the truss ridges.  The cost of this detail often results in the
ridge vent being converted to cap vents.
The issue of eave blocking seems to cause the most strain on the
contractor/engineer relationship, which seems really strange, given the cost
of the detail relative to the overall construction.

Dmitri Wright, P.E.
Portland, Oregon




Dear Seaint:

Background
This last winter in Anchorage, Alaska we observed an increase in ice =
damming on sloped roofs due to a seasonally large snow fall and recent =
construction practices. The snow on the roof melts and freezes on the =
unheated eaves.  This can cause leaks by backing up melt water under the =
roof covering and causes unsightly icicles on the eaves.  Although many =
buildings did not have problems, one suspected factor was insufficient =
ventilation at the eaves.   Most residential construction locally is =
engineered and does not conform to the prescriptive conventional light =
frame construction provisions.  It is current standard practice to =
require full height 2x blocks between truss heels to provide a lateral =
force load path from roof sheathing to exterior shear wall top plates. =
And we require a minimum 9" energy heel with boundary nailing into the =
blocking thru the roof diaphragm and normally we see A35's specified for =
the blocking to shear wall top plate connection.  The trusses/rafters =
are braced at the gable end and every 20' thereafter.   We are seismic =
design category D and wind speeds zones from 100 mph to 125 mph for =
3-second gust (seismic zone 4 and wind speed 80-110 in UBC). =20

Several suggestions from the home builders include increasing the area =
of venting at the eave by eliminating the solid blocking required by =
code (see '97 UBC 2320.12.8, 2000 IBC 2308.10.6).  Note we have a =
handout to allow alternate half height blocking with 3-1 =BD" holes in =
the full height blocking or full height blocking with 3-1 1/2" holes per =
truss bay.  Our problem with eliminating the blocking all together is of =
course shear transfer and cross grain bending (CGB). They want us to =
apply common sense to the problem because there are lots of older houses =
without blocking that have withstood the test of time and of course "in =
the lower 48 nobody blocks at the eave."   Additionally they say the =
calculations do not consider repetitive members and one or two trusses =
may fail in CGB but a whole series with sheathing on them would not.  I =
would appreciate your input.

1. Do you require eave blocking? =20
2. If the forces at the roof diaphragm were greater or less than about =
200 plf would your answer to #1 change?
3. Are there any test results available with half height blocking in =
high wind areas and or high seismic zone?
4. Would flat or vertical blocking at the truss/rafter tail eliminate =
cross grain bending?
5.   Do you see any problems with a discontinuous roof diaphragm at =
the roof peak because it was cut for ridge vent placement?
I thank you in advance for your time and comments...Martin




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