Glulam
Glue-laminated Timber
Glue-laminated timber (glulam) is the name given to large solid wood members manufactured by gluing many smaller pieces together. Glulam is an engineered structural material consisting of a number of graded, kiln dried and selected full length laminations, bonded with proven adhesives, to form a solid member of practically any length, shape or size. The main reason for laminating is to produce larger size members than possible in solid sawn timber.
There is also an increase in strength. In laminating, the weakest point of one piece of timber is bonded to the higher strength of adjoining pieces, thus forming a homogeneous structural component of great efficiency.
Glulam is a most versatile construction material.
Performance
The information provided below has been taken from the New Zealand Timber Design Guide 2007. Designers using the GL grades should use the characteristic stresses from the table below, taken from AS1720. The GL grades are “strength classes” to be used by designers.
Characteristic stresses for dry GL grades of glulam.
|
Grade |
Bending |
Compression parallel |
Tension |
Shear in beams |
Modulus of elasticity |
Modulus of rigidity |
|
|
fb |
fc |
ft |
fs |
E |
G |
|
|
(MPa) |
(MPa) |
(MPa) |
(MPa) |
(GPa) |
(MPa) |
|
GL10 |
22 |
26 |
11 |
3.7 |
1000 |
10.0 |
|
GL8 |
19 |
24 |
10 |
3.7 |
8000 |
8.0 |
Design
In addition to factors k1 to k5 and k8 for sawn timber, the following k factors apply specifically to glue-laminated timber.
Curvature factor k23
The curvature factor allows for the additional stress induced in laminations that are bent to a tight radius to form curved glulam members. It is not applied to straight members with a slight camber. k23 is applied to the bending strength of curved members and is given by:
where tl = lamination thickness
R= radius of curvature
Lamination and size factors k6 and k24
The previous lamination factor k6 and the size factor k24 have been deleted with the introduction of GL grades because the GL grades are performance grades which are specifically manufactured to give the assigned characteristic stresses.
Radial stresses in curved or tapered members
The strength of curved glulam flexural members needs to be checked to ensure that failure does not occur due to stresses perpendicular to the grain. If the bending tends to increase the radius of curvature (bending in the opening mode), the stresses will be in tension perpendicular to the grain (splitting). If the bending tends to decrease the radius of curvature (bending in the closing mode), the stresses will be in compression perpendicular to the grain (crushing).
The design equation for curved members stressed in the opening mode is:
The design equation for curved members stressed in the closing mode is:
where
ø is the strength reduction factor
k1 is the duration of load factor for strength
k4 is the load sharing factor for number of beams
fs is the characteristic shear stress
fp is the characteristic bearing stress perpendicular to the grain
Ris the radius of curvature at mid-depth of the section
b is the breadth of the section
d is the depth of the section
Design equations for tapered or pitch cambered beams are given in NZS3603.
