Education - GL Design

GL Design

Design and Construction Best Practice

The design of structures incorporating timber laminated members, which will be fully exposed to the elements, should include measures to mitigate exposure to direct sunlight and moisture ponding and promote rapid shedding of moisture.

The following detailing and design practices are desirable with regard to enhancing the structure’s service life:

Joint detailing should comply with the following:

ensure moisture entering the joint is not trapped but can run away freely
keep horizontal contact areas to a minimum, favouring self-draining vertical surfaces
use non-corroding fasteners which do not cause splitting during installation
minimise use of morticed joints.

Beams should be provided with adequate ventilation.

Damp proof membranes should be used where timber members are in contact with masonry.

Metal or plastic shields on the top and ends of laminated timber beams can exclude moisture and sunlight.

Arrised edges on timber members help prevent the failure of coating systems at sharp corners.

Building overhangs will provide protection from moisture and direct sunlight.

Engineering Design

The engineering design of Glulam timber can have two differing approaches:

Straight (off-the-shelf) glulam products can be designed using many of the same philosophies used in designing sawn timber.
Elements with complex shapes must be designed as special units, and there will need to be dialog with the manufacturer in order to detail an appropriate shape and specify appropriate materials.

Table 1 below is adapted from AS 1720.1, and gives the design properties for glulam material.

Table 1 – Characteristic Strengths and Elastic Moduli for

Horizontally Laminated Glulam Grades.

Stress Grade

Characteristic Strengths

(Mpa)

Elastic Moduli

(Mpa)

Bending

Tension parallel to grain

Shear in beam

Compression parallel to grain

Short duration average modulus of elasticity parallel to grain

Short duration average modulus of rigidity for beams

(f’_b)

(f’_t)

(f’_s)

(f’_c)

(E)

(G)

GL 18 (Hw)

GL 17 (Hw)

GL 13 (Sw)

GL 12 (Hw/Sw)

GL 10 (Sw)

GL 8 (Sw)

5.0

3.7

18500

16700

13300

11500

10000

8000

1230

1110

900

770

670

530

Design of straight glulam members

AS1720.1 section 7 is the primary reference for the structural design of glulam elements. Glued laminated timber that complies with its product standard AS/NZS1328 will carry a stamp that indicates its compliance and can be designed using the provisions and stress grade data in Section 7 of AS1720.1.

Glulam bending members

The most common glulam element is the bending member.

The normal strength limit state capacity equation is given in Clause 3.2.1.1 of AS1720.1,

Capacity factor () can be found in Table 2.5, and has high values relative to sawn timber, because of the high levels of quality control required in the manufacture of structural glulam to AS/NZS1328.
Duration of load (k₁), Partial seasoning (k₄) and temperature factors (k₆) are defined in Section 2 of AS1720.1 and are found in exactly the same way for glulam as for sawn timber.
For Glulam timber, the strength sharing factor (k₉) is 1.0. This is appropriate as there is already a high level of strength sharing between the laminates in the glulam beam.
For glulam beams, the size factor for bending (k₁₁) is also 1.0. Because the production of Glulam materials is a manufacturing process, it is possible for each manufacture to have controls in place to ensure that the design properties can be achieved regardless of the size.
The stability factor (k₁₂) is found using the normal slenderness calculations given in AS1720.1 Clause 3.2. The material constant ( _b) for the normal Glulam grades (GL grades) is given in Table 7.2(A).
The bending strength of the glulam material is a function of the grade awarded to the glulam. The normal glulam grades are given in Table 7.1 with the design strengths and design MoE.
Minimum design dimensions (after allowing for tolerances) should be used to find the section modulus (Z).

The strength limit state shear capacity equation is given in Clause 3.2.5 of AS1720.1,

Capacity factor (), duration of load factor (k₁), partial seasoning factor (k₄) and temperature factors (k₆) are all found in the normal way using Section 2 of AS1720.1
For glulam beams, the size factor for shear (k₁₁) is 1.0. This is common with the size factor for shear in most other timber materials.
The shear strength of the glulam material is a function of the grade awarded to the glulam. The normal glulam grades are given in Table 7.1 with the design strengths and design MoE.
Minimum design dimensions (after allowing for tolerances) should be used to find the shear area (A_s). For timber, the shear area is 2/3 the cross sectional area (Clause 3.2.5 of AS1720.1).

The strength limit state bearing capacity of timber is only a function of the species of the timber, the grade or manufacture of the material is not important.

The strength limit state bearing capacity equation is given in Clause 3.2.6 of AS1720.1,

- applicable for bearing of beams.

Capacity factor (), duration of load factor (k₁), partial seasoning factor (k₄) and temperature factors (k₆) are all found in the normal way using Section 2 of AS1720.1 (The capacity factor for all timber materials is the same – it is independent of grade or method of production.)
(k₇) is a new factor for the length and position of bearing. It is defined in Clause 2.4.4 of AS1720.1 and is used in the same way for all timber materials.
The bearing strength of the glulam material (and all other timber elements) is only a function of the species of the bearing layers. Table 2.1 or Table 2.2 in AS1720.1 is used to define a strength group to the species, and the bearing strength of the timber is given in Table 2.3(A) in AS1720.1.
Minimum design dimensions (after allowing for tolerances) should be used to find the bearing area (A_p).

Deflections of glulam members can be calculated using the same techniques as other timber structural beams. Some glulam members are built with a camber (A small upwards deflection).

Camber:

Camber should always be installed upwards – cambered beams cannot be installed either way up, usually the top is marked with “TOP”.
Camber is intended so that when the self-weight and other permanent structural loads are applied, the beam deflects so that it is flat.
Camber affects the final position of a beam under load – so it can be quite useful in stopping beams resting on partitions or elements underneath them.
Camber does not affect the displacement under imposed actions (live loads) alone. It is the relative deflection of the loaded beam compared with the member when unloaded that is important here, and camber does not affect that at all.
Vibrations under repetitive loads are not influenced at all by camber.

Glulam compression members

Glulam compression members can be used in trusses, as large cross section columns or in reticulated arches or domes.

The normal strength limit state capacity equation is given in Clause 3.3.1.1 of AS1720.1,

Capacity factor () can be found in Table 2.5, and has high values relative to sawn timber, because of the high levels of quality control required in the manufacture of structural glulam to AS/NZS1328.
Duration of load (k₁), Partial seasoning (k₄) and temperature factors (k₆) are defined in Section 2 of AS1720.1 and are found in exactly the same way for glulam as for sawn timber.
For glulam compression members, the size factor for bending (k₁₁) is 1.0 (in this case, this is the same as the value for F-graded products).
The stability factor (k₁₂) is found using the normal slenderness calculations given in AS1720.1 Clause 3.3.3. The material constant ( _c) for the normal Glulam grades (GL grades) is given in Table 7.2(B).
The compression strength of the glulam material is a function of the grade awarded to the glulam. The normal glulam grades are given in Table 7.1 with the design strengths.
Minimum design dimensions (after allowing for tolerances) should be used to find the compression area (A_c).

Glulam tension members

Glulam tension members are really only found in very large trusses, though designers may have to calculate tension capacity to check compression members that may experience load reversal, or for combined actions (such as portal frames).

The normal strength limit state capacity equation is given in Clause 3.4.1 of AS1720.1,

Capacity factor () can be found in Table 2.5, and has high values relative to sawn timber, because of the high levels of quality control required in the manufacture of structural glulam to AS/NZS1328.
Duration of load (k₁), Partial seasoning (k₄) and temperature factors (k₆) are defined in Section 2 of AS1720.1 and are found in exactly the same way for glulam as for sawn timber.
For glulam beams, the size factor for tension (k₁₁) is given in Clause 7.4.4 of AS1720.1. The value found is the same one for F-graded timber products. This is likely to be quite conservative, but because tensile failures are brittle, that is wise.
The tension strength of the glulam material is a function of the grade awarded to the glulam. The normal glulam grades are given in Table 7.1 with the design strengths.
Minimum design dimensions (after allowing for tolerances) should be used to find the tension area (A_t). The tension area is the minimum net cross sectional area. For large or complex connections, this may involve calculation of area of cross section removed for large connectors (such as split rings or shear plates), bolts or dowels, and any embedded steel plates.

Design of complex, curved or tapered glulam members

As well as complying with the clauses of AS1720.1 section 7, <link to above text> curved and tapered glulam elements must also satisfy other provisions in AS1720.1 Appendix E12 and Appendix E13. Manufacturers may also have some practical limitations on the manufacture such as span length, maximum width of laminations or minimum radius of curvature.

Tapered cuts of one or both edges cause the flexural stresses (that are parallel to the edge of the member) to be at an angle to the grain. The calculations to model this behaviour are given in Clause E12 in AS1720.1.

Members with curved centre-lines can easily be fabricated in a curved clamping frame <link to examples of curved glulam >. The curves introduce residual stresses in the timber as each laminate is locked into a bent profile. As well, moments that cause opening of the curved shape can induce tension perpendicular to the grain near the centre-line of the curved member. Calculations to model these behaviours are given in Clause E13 in AS1720.1

The serviceability calculations of curved or tapered members follow a similar methodology to those for straight members, though some adjustment should be made for the slope of grain at the edge of tapered members.

This can be approximated by using Hankinson’s formula and by assuming that the MoE of wood perpendicular to grain is about 1/50 the value of MoE parallel to grain. Hankinson’s formula is:

with

E_l MoE parallel to grain (as given in Table 7.1 of AS1720.1)

E_p MoE perpendicular to grain (estimated as 1/50 to 1/30 E_l)

q angle of slope of grain (taper angle at the edge)

Other guidance and some examples of the calculations required for curved and tapered members are given in “Timber Design Handbook” HB108 from Standards Australia <link to Standards Australia site >

Manufacturers’ Information

Product specific information is available from individual glulam manufacturers.