ASD/LRFD Manual - American Wood Council

ASD/LRFD MANUAL FOR ENGINEERED Wood Construction
47
Sloped Bearing Conditions
Sloped bearing conditions require design considerations
different from conventional lumber. An example is
a birdsmouth bearing cut (notches in the bottom flange, see
Figure M7.4-6). This type of bearing should only be used
on the low end bearing for wood I-joists. Another example
is the use of metal joist support connectors that attach only
to the web area of the joist and do not provide a bottom
seat in which to bear. In general, this type of connector
is not recommended for use with wood I-joists without
consideration for the resulting reduced capacity.
The birdsmouth cut is a good solution for the low end
bearing when the slope is steep and the tangential loads
are high (loads along the axis of the joist member). This
assumes the quality of construction is good and the cuts
are made correctly and at the right locations. This type of
bearing cut requires some skill and is not easy to make,
particularly with the wider flange joists. The bearing capacity,
especially with high shear capacity members, may
be reduced as a result of the cut since the effective flange
bearing area is reduced. The notched cut will also reduce
the member’s shear and moment capacity at a cantilever
location.
An alternative to a birdsmouth cut is a beveled bearing
plate matching the joist slope or special sloped seat
bearing hardware manufactured by some metal connector
suppliers. These alternatives also have special design considerations
with steep slope applications. As the member
slope increases, so does the tangential component of
reaction, sometimes requiring additional flange to bearing
nailing or straps to provide resistance. Figure M7.4-6
shows some examples of acceptable low end bearing
conditions.
Figure M7.4-6
Sloped Bearing Conditions (Low End)
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M7: PREFABRICATED WOOD I-JOISTS
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