| SELECTING AND SPECIFYING GLULAM A glulam is made up of wood laminations, or "lams" that
are bonded together with adhesives. The grain of all laminations runs parallel with the
length of the member. Individual lams typically are 1-3/8 inches thick for southern pine
and 1-1/2 inches thick for Western species, although other thicknesses may also be used.
Glulam product typically range in net widths form 2-1/2 to 10-3/4 inches although
virtually any width can be custom produced.
Balanced and Unbalanced Beams
Glulam may be manufactured as unbalanced or balanced
members.
The most critical zone of a glulam bending member with
respect to controlling strength is the outermost tension zone. In unbalanced beams, the
quality of lumber used on the tension side of the beam if higher than the lumber used on
the corresponding c oppression side, allowing a more efficient use of the timber resource.
Therefore, unbleached beams have different bending stresses assigned to the compression
and tension zones and must be installed accordingly. To assure proper installation of
unbalanced beams, the top of the beam so clearly stamped with the word "TOP".
Unbalanced beams are primarily intended for simple span applications.
Balanced members are symmetrical in lumber quality about
the mid-height. Balanced beams are used in applications such as cantilevers or continuous
spans, where either the top or bottom of the member may be stress din tension due to
service loads. They can also be used in single span applications, although an unbalanced
beam is more efficient for this use.
Allowable Design Properties
Allowable design properties are a key factor in specifying
glulam. Bending members a typically specified on the basis of the maximum allowable
bending stress of the member. For example, a 24Fdesignation indicates a member with an
allowable bending stress of 200 psi. These different stress levels are achieved by varying
the species an percentages and grade of higher quality lumber in the beam layup.
To identify whether the lumber used in the beam is visually
or mechanically graded, the stress combination also includes a second set of designations.
For example, for an unbleached 24F layup using visually graded lumber, the layup
designation may be identified as a 24F-V4. The "V" indicates that layup uses
visually graded lumber.) The number "4" further indicates a specific combination
of lumber used to which a full set of design stresses such as horizontal shear, MOE ,
etc., are assigned.
Sizes
Glulam is available in both custom and stock sizes. Stock
beams are manufactured in commonly used dimensions and such to length when the beam is
ordered from a distributor or dealer. Typical stock beam widths include: 3-1/8",
3-1/2", 5-1/8", 5-1/2" , and 6-3/4".
Where long spans, unusually heavy loads, or other
circumstances control design, custom members are typically specified. Common custom
shapes include curved beams, pitched and curved beams, radial arches and Tudor arches.
Appearance Classification
Glulam is available in a range of appearances, all looking
different but having the same structural characteristics for a given strength grade.
Glulam appearance classifications are:
Framing: An EWS classification
that denotes the member is intended only for use in concealed applications. Beams with
this appearance classification are provided in widths designed to fit flush with 2x4 and
2x6 wall framing.
Industrial. Used for concealed
applications or there appearance is not of primary importance.
Architectural: The appearance of
choice in applications where members are exposed to view, because they have a smooth,
attractive finish. Stock beams are often supplied with this appearance so they may be
exposed to view in the finished structure.
Premium. Available only as a
custom order where finished appearance is of primary importance.
All appearance classifications permit natural growth
characteristics with varying degrees of open voids permitted. Voids are filled as required
by the appearance grade specified using inserts and wood fillers. The appearance
classification is not rallied to lumber layup requirements and thus does not affect design
values of the beam.
Section Properties and Capacities
When selecting a glulam member, the builder, designer, or
specified must t sue a member with the required section properties to satisfy the load
carrying requirements. Different load capacities are possible for different stress level
combinations of glulam. Tables giving the load carrying capacities for glulam are included
into the Engineered Wood Systems Data File: Glued Laminated Beam Design Tables,
Form EWS S475. |
