Because large wood joist lumber is becomeing mor rare (and
thus more expensive), more an more designs are utilizing
'engineered' wood products. One such group of products are
composite wood joists. All of these designs are proprietatry and
must be approved by a Professional Engineer. (Civil) Companies
that make these products usually have an engineer on staff who
approves the use of the joist for a particular application. If you
were interested in using these joists in a construction project,
then typically you would bring a dimensioned drawing of your
application to your building supplier. The supplier would forward
the drawing to the manufacturer of the composite wood joists who
would approve a suitable selection of their products for your
application. They also would supply particulars about the details
of the installation. You would take their approved (by a
Professional Engineer) design when you go for a building
permit.
Composite wood joists are not specifically design controlled
so they exist in a variety of shapes sizes and designs. Two
typical sizes which are offered by at least two manufacturers are
shown below. Typically the top and bottom are made of 2x3 or 2x4
lumber and the flange is fabricated from plywood or OSB (oriented
strand board). Some typical allowable spans are shown for these
sizes. Although these will give a ballpark of what is possible
NEVER USE THIS TABLE FOR ACTUAL CONSTRUCTION.
These figures were arrived at by averaging data from span tables
from the manufacturres for several different cases. The actual
span lengths are dependant on many factors including; the type of
cieling, strapping, bridging, thickness of floor, whether floor is
nailed or nailed and glued, multiple or single span, type of wood
for the flanges and type of wood used in the web. The allowable
spans can vary greatly form product to product and from design to
design. For this course you may use the spans in the table below.
