ASD/LRFD Manual - American Wood Council

28 M5: structural glued laminated timber
M5.1 General
Products Description
Structural glued laminated timber (glulam) is a structural
member glued up from suitably selected and prepared
pieces of wood either in a straight or curved form with the
grain of all of the pieces parallel to the longitudinal axis
of the member. The reference design values given in the
NDS Supplement are applicable only to structural glued
laminated timber members produced in accordance with
American National Standard for Wood Products — Structural
Glued Laminated Timber, ANSI/AITC A190.1.
Structural glued laminated timber members are produced
in laminating plants by gluing together dry lumber,
normally of 2-inch or 1-inch nominal thickness, under
controlled temperature and pressure conditions. Members
with a wide variety of sizes, profiles, and lengths can
be produced having superior characteristics of strength,
serviceability, and appearance. Structural glued laminated
timber beams are manufactured with the strongest
laminations on the bottom and top of the beam, where the
greatest tension and compression stresses occur in bending.
This allows a more efficient use of the lumber resource
by placing higher grade lumber in zones that have higher
stresses and lumber with less structural quality in lower
stressed zones.
Structural glued laminated timber members are manufactured
from several softwood species, primarily Douglas
fir-larch, southern pine, hem-fir, spruce-pine-fir, eastern
spruce, western woods, Alaska cedar, Durango pine, and
California redwood. In addition, several hardwood species,
including red oak, red maple, and yellow poplar, are also
used. Standard structural glued laminated timber sizes
are given in the NDS Supplement. Any length, up to the
maximum length permitted by transportation and handling
restrictions, is available.
A structural glued laminated timber member can be
manufactured using a single grade or multiple grades of
lumber, depending on intended use. In addition, a mixedspecies
structural glued laminated timber member is also
possible. When the member is intended to be primarily
loaded either axially or in bending with the loads acting
parallel to the wide faces of the laminations, a single
grade combination is recommended. On the other hand,
a multiple grade combination provides better cost-effectiveness
when the member is primarily loaded in bending
due to loads applied perpendicular to the wide faces of
the laminations.
On a multiple grade combination, a structural glued
laminated timber member can be produced as either a
balanced or unbalanced combination, depending on the
geometrical arrangement of the laminations about the
mid-depth of the member. As shown in Figure M5.1-1, a
balanced combination is symmetrical about the mid-depth,
so both faces have the same reference bending design
value. Unbalanced combinations are asymmetrical and
when used as a beam, the face with a lower allowable
bending stress is stamped as TOP. The balanced combination
is intended for use in continuous or cantilevered over
supports to provide equal capacity in both positive and
negative bending. Whereas the unbalanced combination
is primarily for use in simple span applications, they can
also be used for short cantilever applications (cantilever
less than 20% of the back span) or for continuous span
applications when the design is controlled by shear or
deflection.
Figure M5.1-1
No. 2D
Unbalanced and
Balanced Layup
Combinations
Tension Lam
No. 2 No. 1
No. 2 No. 2
No. 3 No. 3
No. 3 No. 3
No. 3 No. 3
No. 2 No. 2
No. 1 No. 1
Tension Lam
Unbalanced
Tension Lam
Balanced
Structural glued laminated timber members can be
used as primary or secondary load-carrying components
in structures. Table M5.1-1 lists economical spans for
selected timber framing systems using structural glued
laminated timber members in buildings. Other common
uses of structural glued laminated timber members are
for utility structures, pedestrian bridges, highway bridges,
railroad bridges, marine structures, noise barriers, and
towers. Table M5.1-1 may be used for preliminary design
purposes to determine the economical span ranges for the
selected framing systems. However, all systems require a
more extensive analysis for final design.
American Wood Council