Building Design and Construction Handbook - Merritt - Ventech!

10.92 SECTION TEN
wood species do not produce the same quality of glue bond as other combinations,
although the same gluing procedures are used. Thus, a combination must be supported
by adequate experience with a laminator’s gluing procedure.
The only adhesives currently recommended for wet-use and preservative-treated
lumber, whether gluing is done before or after treatment, are resorcinol and phenolresorcinol
resins. Melamine and melamine-urea blends are also used for highfrequency
curing of end joints.
Glued joints may be cured with heat by several methods. Radio-frequency (RF)
curing of glue lines is used for end joints and for limited-size members where there
are repetitive gluings of the same cross section. Low-voltage resistance heating,
where current is passed through a strip of metal to raise the temperature of a glue
line, formerly was used for attaching thin facing pieces. The metal could be left in
the glue line as an integral part of the completed member. Printed electric circuits,
in conjunction with adhesive films, and adhesive films impregnated on paper or on
each side of a metal conductor placed in the glue line are other alternatives.
Preheating the wood to ensure reactivity of the applied adhesive has limited
application in structural laminating. The method requires adhesive application as a
wet or dry film simultaneously to all laminations, and then rapid handling of multiple
laminations.
Curing the adhesive at room temperature has many advantages. Since wood is
an excellent insulator, a long time is required for elevated ambient temperatures to
reach inner glue lines of a large assembly. With room-temperature curing, equipment
needed to heat the glue line is not required, and the possibility of injury to
the wood from high temperature is avoided.
10.30.3 Prefabricated Wood I Joists
Flanges of prefabricated wood I-shaped joists are either sawn lumber, visually
graded or machine stress rated, or some type of structural composite lumber product,
such as laminated veneer lumber (LVL) (Art. 10.30.4). The web members can
be either plywood or oriented strand board (OSB) although OSB is most often used
(Art. 10.12).
Numerous manufacturers produce wood I joists, and the flange and web materials
used depend on manufacturer preference. Inasmuch as these joists are proprietary
products, the manufacturer provides design information, usually in the form
of load/span tables, as well as installation and handling guidelines.
Manufacturers establish design values for these products in accordance with the
provisions of the ‘‘Specification for Establishing and Monitoring Structural Capacities
of Prefabricated Wood I-Joists,’’ ASTM D5055. Each manufacturer uses the
design values thus determined to obtain National Evaluation Service (NER) building-code
approval from model code sponsors and to generate proprietary load/span
tables. To establish consistent performance levels for I-joists, APA–The Engineered
Wood Association promulgated APA Standard PRI-400 for I-joists used in residential
floor construction. This standard has been recognized by all of the model code
agencies.
Wood I joists are available with a wide range of depths and load/span capabilities.
Although joist sizes vary with manufacturer preference, most manufacturers
produce joists with depths of 9 1 ⁄2 in or 11 7 ⁄8 in for direct substitution for 2 � 10
and 2 � 12 dimension lumber. Other depths—14 and 16 in and deeper—also are
available. The deeper products are typically used in longer-span applications, such
as for light commercial construction.