ISO/TR 19623:2019

Timber structures — Glued laminated timber — Assignment of glued laminated timber characteristic values from laminate properties

This document reviews the methodologies or analytical models that have been used to develop the strength and stiffness of structural glued laminated timber (glulam) from the characteristic properties of lumber laminations. The review is limited to the methodologies used in Europe, the USA, Australia/New Zealand, and Canada as they represent different fundamental philosophies in these areas. As a result, the methodologies are not intended to be combined unless there is clear understanding of the fundamental assumptions adopted by the respective methodologies. NOTE Detailed assumptions used by the respective methodologies are available from the standards listed in the Bibliography.

Structures en bois — Bois lamellé-collé — Valeurs caractéristiques du bois lamellé-collé sur la base des propriétés des lamelles

General Information

• Status: Published
• Publication Date: 18-Jun-2019
• ICS: 91.080.20 - Timber structures
• Technical Committee: ISO/TC 165 - Timber structures
• Drafting Committee: ISO/TC 165/WG 2 - Structural glued wood products
• Current Stage: 9093 - International Standard confirmed
• Start Date: 14-May-2021
• Completion Date: 13-Dec-2025

Standards Content (Sample)

TECHNICAL ISO/TR
REPORT 19623
First edition
2019-06
Timber structures — Glued laminated
timber — Assignment of glued
laminated timber characteristic values
from laminate properties
Structures en bois — Bois lamellé-collé — Valeurs caractéristiques du
bois lamellé-collé sur la base des propriétés des lamelles
Reference number
©
ISO 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 European methodologies . 1
4.1 General . 1
4.1.1 Timber . 1
4.1.2 Related material properties . 2
4.2 V erification from classification of standardised beam lay-ups and lamination
properties of glued laminated timber . 2
4.2.1 Properties of the boards . 2
4.2.2 Strength of finger joints . 3
4.2.3 Beam lay-up and strength class . 4
4.3 Classification, verification according to method B from cross sectional layup and
properties of boards and finger joints . 6
4.3.1 Properties of the boards . 6
4.3.2 Strength of finger joints . 7
4.3.3 Determination of characteristic values for glued laminated timber . 7
4.4 Verifications from full scale tests with glulam . 8
4.4.1 Properties of the boards . 8
4.4.2 Strength of finger joints . 8
4.4.3 Strength, stiffness and density properties of glulam derived from testing . 8
4.5 Resawn glulam . 8
5 US methodologies . 9
5.1 General . 9
5.2 ASTM D3737 . 9
5.2.1 General. 9
5.2.2 I /I analysis .10
K G
5.3 Tension laminations .14
5.4 Volume factor .15
5.5 Other glulam properties.15
5.6 ANSI A190.1 .16
5.7 Performance-based standard .16
6 Australian/New Zealand methodologies .17
6.1 Direct method .17
6.1.1 Tension tests of bonded lamination pairs .17
6.1.2 Major axis bending strength of glulam assemblies .18
6.1.3 Ratio of glulam beam bending to bonded lamination pair tension strength .19
6.1.4 Comparison of glulam assembly bending strength between EN 14080 and
Formula (27) values .19
6.1.5 Depth and volume effects .20
6.1.6 Minor axis properties in bending also known as vertical glulam .20
6.1.7 Tension strength .21
6.1.8 Shear strength .21
6.1.9 Framework of AS/NZS 1328 .21
6.2 AS/NZS 1328 .21
6.2.1 Standard lamination requirements .21
6.2.2 Custom lamination requirements .23
6.2.3 Standard glulam and glued structural timber .24
6.2.4 Custom glulam and glued structural timber .25
6.3 AS/NZS 1328:2017, Appendix B .25
6.3.1 Determination of glulam major axis bending strength by computation .25
6.3.2 Beam bending stiffness (EI) .26
c
6.3.3 Direct method for major axis bending and effective stiffness .26
6.3.4 Computer-based Monte Carlo method for bending strength .27
6.4 Direct tension and curvature stresses in bending .28
6.5 AS/NZS GL grades versus EN 14080 GLh grade bending strength values .28
7 Canadian methodologies.29
7.1 General .29
7.2 Laminating lumber .29
7.3 Manufacturing .30
7.4 Layup development.30
7.4.1 Mechanics-based model .30
7.4.2 New layup confirmation by full-scale testing .30
7.4.3 Data analysis .31
7.5 Acceptance of new layup combinations .31
Bibliography .32
iv © ISO 2019 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
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expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 165, Timber structures.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
Introduction
This document was prepared in response to the growing interest in development of the strength
and stiffness of structural glued laminated timber (glulam) from the characteristic values of lumber
laminations.
Since its first introduction in 1890s, glulam has been used in timber construction for over 125 years
with excellent track record of performance. Many countries around the world, which have experience in
glulam construction, have various glulam production capabilities that are supported by methodologies
or analytical models for development of glulam strength and stiffness from the characteristic values
of lumber laminations. This document reviews methodologies from Europe, the USA, Australia/New
Zealand, and Canada that have successfully demonstrated their acceptance through years of practice
and end uses.
This document does not cover all methodologies around the world and is not intended to exclude other
methodologies that can demonstrate their capabilities of correlating the analytical results with the
actual product performance. This document will be updated with those additional methodologies when
their documentation becomes available in the future.
This document promotes the understanding of the differences between methodologies as a first step
toward an international harmonization in the process of assigning glulam characteristic values from
laminate properties.
vi © ISO 2019 – All rights reserved

TECHNICAL REPORT ISO/TR 19623:2019(E)
Timber structures — Glued laminated timber —
Assignment of glued laminated timber characteristic
values from laminate properties
1 Scope
This document reviews the methodologies or analytical models that have been used to develop the
strength and stiffness of structural glued laminated timber (glulam) from the characteristic properties
of lumber laminations. The review is limited to the methodologies used in Europe, the USA, Australia/
New Zealand, and Canada as they represent different fundamental philosophies in these areas. As a
result, the methodologies are not intended to be combined unless there is clear understanding of the
fundamental assumptions adopted by the respective methodologies.
NOTE Detailed assumptions used by the respective methodologies are available from the standards listed in
the Bibliography.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
4 European methodologies
EN 14080 provides three different routes for producer to classify the glulam and all of them are related
to properties in structural sizes. The glulam standard covers only properties related to nominal 12 %
moisture content (in lamination, joint, and structural specimen tests, moisture content may be 12 ± 3 %
without mandatory adjustments).
4.1 General
Mechanical resistance of glulam is intended to be determined from and declared:
— on the basis of geometrical data (e.g. cross-sectional sizes of laminations and layups) and material
properties (strength, stiffness and density properties of laminations and strength properties of
finger joints); or
— from tests.
4.1.1 Timber
Timber is strength graded according to EN 14081-1.
4.1.2 Related material properties
The characteristic strength, stiffness and density properties of glulam are verified either:
a) from classifications from layups and lamination properties (this route is a direct result of the
calculation procedure implemented in 4.3);
b) from calculations taking into account the cross-sectional layup and documented properties of
boards and finger joints according to 4.3, or
c) from full scale tests according to 4.4.
The characteristic strength, stiffness and density properties may be declared by reference to a strength
class according to Table 3 or 4 or to a manufacturer’s specific strength class. For glulam having an
asymmetrical layup, “ca” should be added to the class name, e.g. GL28 ca. The class name of resawn
glulam is marked by “s”, e.g. GL24 cs.
The characteristic bending strength should be valid for glulam with a depth h of 600 mm and a
lamination thickness of t = 40 mm. If the lamination thickness is less than 40 mm, the bending strength
may be multiplied by k as given in Formula (1). For lamination thicknesses 40 mm < t ≤ 45 mm, it is not
necessary to take any strength modification into account.
01,

 

 
k =min (1)
t

 

10, 5

where t is the lamination thickness, in mm.
The characteristic tensile strength parallel to the grain should be valid for glulam with depth h of
600 mm or width b of 600 mm.
The characteristic tensile strength perpendicular to the grain should be valid for glulam with a stressed
volume of 0,01 m .
The 5 %-fractile of a shear modulus or a modulus of elasticity should be estimated from the mean value
by applying the ratio of G /G = 5/6 and E /E = 5/6, respectively.
g,k g,mean 0,g,k 0,g,mean
For glulam members made of at least 10 laminations the product (E G ) may be increased by a
0,g,k g,k
factor k = 1,40.
For rectangular glued laminated timber with depths in bending or widths in tension less than 600 mm,
the characteristic values for f and f may be increased by the factor k given by
m,k t,0,k h
01,

600

 
k =min (2)
h

h  

11,

where h is the depth for bending members or width for tensile members, in mm.
4.2 Verification from classification of standardised beam lay-ups and lamination
properties of glued laminated timber
4.2.1 Properties of the boards
The requirements of the boards given in Table 1 should be fulfilled. The essential material properties
needed for the EN 14080 model are tension strength, modulus of elasticity, and density of the unjointed
laminations and further finger joint tension or bending strength (see Table 1). Laminations up to
T-class T18 can be graded visually according to several European grading standards and then assigned
2 © ISO 2019 – All rights reserved

to T-classes, provided the respective classification reports on the basis of EN 384 exist. (issue of flatwise
and edgewise bending needs to be addressed).
In case no information exists, the effort to group laminations based on tension tests according to
EN 408 into a certain T-class (similar as for C class) is
— 40 specimens from 5 growth areas: no reduction in evaluation, based on mean of the 5 % quantiles
of all 5 samples or 1,2 times of the sample with the lowest 5 % quantile (the lesser value is relevant)
— 40 specimens from 3 growth areas; penalization by factor of 0,89
— 40 specimens from 1 growth area; penalization by 0,77
Table 1 — Characteristic strength and stiffness properties for T-classes in N/mm and densities
in kg/m for boards or planks for glued laminated timber
a
T-class of boards f E ρ
t,0,l,k t,0,l,mean l,k
T8 (C14) 8 7 000 290
T9 9 7 500 300
T10 (C16) 10 8 000 310
T11 (C18) 11 9 000 320
T12 (C20) 12 9 500 330
T13 (C22) 13 10 000 340
T14 (C24) 14 11 000 350
T14,5 14,5 11 000 350
T15 15 11 500 360
T16 (C27) 16 11 500 370
T18 (C30) 18 12 000 380
T21 (C35) 21 13 000 390
T22 22 13 000 390
T24 (C40) 24 13 500 400
T26 26 14 000 410
T27 (C45) 27 15 000 410
T28 28 15 000 420
T30 (C50) 30 15 500 430
a
The C-classes according to EN 338:2009 meet at least the required values of the respective T-classes.
4.2.2 Strength of finger joints
The declared or necessary finger joint strength values depend on the different glulam classification
approaches.
— For classification approach (A), fixed values need to be met.
— For classification/verification approach (B), i.e. the calculation method, values in a certain bandwidth
can be declared.
The required characteristic values of the flatwise bending strength of finger joints f in laminations
m,j,k
for glulam classification approach (A) should be taken from Table 2 or 3. If the finger joints are tested in
tension the required characteristic value of the tensile strength of finger joints should be taken as
ff= /,14 (3)
t,0,j,km,j,k
4.2.3 Beam lay-up and strength class
Provided the beam lay-up is in accordance with Table 2 or 3, the glulam fulfils the requirements of a
strength class given in Table 4 or 5.
The zones of the cross section are defined in Figure 1.
Figure 1 — Example of a beam lay-up of combined glulam
— For combined glulam, the outer zones of lamination grades (see Figure 1) should be at least the
proportion given in Table 2, but at least two laminations for glulam with more than 10 laminations
and at least one lamination for glulam with up to 10 laminations.
Table 2 — Beam lay-up of combined glued laminated timber and minimum values for bending
strength of finger joints in laminations in N/mm
Glued Outer zones of laminations Intermediate zones of lamina- Inner zone of laminations
laminated tions
timber
Strength Strength Propor- Strength Propor- Strength Propor-
a
class class tion f class tion f class tion f
m,j,k m,j,k m,j,k
2 2 2
[%] [N/mm ] [%] [N/mm ] [%] [N/mm ]
GL 20c T13 2 × 33 21 — — — T8 34 18
GL 22c T13 2 × 33 26 — — — T8 34 18
GL 24c T14 2 × 33 31 — — — T9 34 19
GL 26c T16 2 × 33 34 — — — T11 34 22
GL 28c T18 2 × 25 37 — — — T14 50 28
GL 28c T21 2 × 17 36 — — — T14 66 26
GL 28c T21 2 × 17 38 — — — T13 66 25
GL 28c T21 2 × 25 35 — — — T11 50 22
GL 28c T21 2 × 20 35 T14 2 × 20 28 T11 20 22
GL 28c T22 2 × 20 35 — — — T13 60 25
4 © ISO 2019 – All rights reserved

Table 2 (continued)
Glued Outer zones of laminations Intermediate zones of lamina- Inner zone of laminations
laminated tions
timber
Strength Strength Propor- Strength Propor- Strength Propor-
a
class class tion f class tion f class tion f
m,j,k m,j,k m,j,k
2 2 2
[%] [N/mm ] [%] [N/mm ] [%] [N/mm ]
GL 30c T22 2 × 17 40 — — — T15 66 27
GL 30c T22 2 × 17 41 — — — T14 66 28
GL 30c T22 2 × 20 40 T14 2 × 20 30 T11 20 22
GL 30c T22 2 × 17 42 T14 2 × 23 31 T11 20 22
GL 32c T24 2 × 17 44 — — — T18 66 31
GL 32c T26 2 × 17 45 — — — T14 66 26
GL 32c T26 2 × 10 48 T18 2 × 20 32 T11 40 22
Table 3 — Beam lay-up of homogeneous glued laminated timber and minimum values for
bending strength of finger joints in laminations in N/mm
Strength class glued laminated timber Strength class laminations f
m,j,k
GL 20h T10 25
GL 20h T11 22
GL 22h T13 25
GL 24h T14 30
GL 26h T16 33
GL 28h T18 36
GL 30h T21 38
GL 30h T22 37
GL 32h T24 41
GL 32h T26 38
2 3
Table 4 — Characteristic strength and stiffness properties in N/mm and densities in kg/m for
combined glulam
Glulam strength class
a
Property Symbol GL 20c GL 22c GL 24c GL 26c GL 28c GL 30c GL 32c
Bending strength f 20 22 24 26 28 30 32
m,g,k
Tensile strength f 15 16 17 19 19,5 19,5 19,5
t,0,g,k
f 0,5
t,90,g,k
Compression strength f 18,5 20 21,5 23,5 24 24,5 24,5
c,0,g,k
f 2,5
c,90,g,k
Shear strength
f 3,5
v,g,k
(shear and torsion)
Rolling shear strength f 1,2
r,g,k
Modulus of elasticity E 10 400 10 400 11 000 12 000 12 500 13 000 13 500
0,g,mean
E 8 600 8 600 9 100 10 000 10 400 10 800 11 200
0,g,05
E 300
90,g,mean
E 250
90,g,05
a
Properties given in this table have been calculated on the basis of the layups given in Table 2. If different layups for a
certain strength class lead to different characteristic values the lowest values are given here.
b
Calculated as the weighted mean of the densities of the different lamination zones.
Table 4 (continued)
Glulam strength class
a
Property Symbol GL 20c GL 22c GL 24c GL 26c GL 28c GL 30c GL 32c
Shear-modulus G 650
g,mean
G 540
g,05
Rolling shear modulus G 65
r,g,mean
G 54
r,g,05
b
Density ρ 355 355 365 385 390 390 400
g,k
ρ 390 390 400 420 420 430 440
g,mean
a
Properties given in this table have been calculated on the basis of the layups given in Table 2. If different layups for a
certain strength class lead to different characteristic values the lowest values are given here.
b
Calculated as the weighted mean of the densities of the different lamination zones.
2 3
Table 5 — Characteristic strength and stiffness properties in N/mm and densities in kg/m for
homogeneous glulam
Glulam strength class
Property Symbol GL 20h GL 22h GL 24h GL 26h GL 28h GL 30h GL 32h
Bending strength f 20 22 24 26 28 30 32
m,g,k
Tensile strength f 16 17,6 19,2 20,8 22,3 24 25,6
t,0,g,k
f 0,5
t,90,g,k
Compression strength f 20 22 24 26 28 30 32
c,0,g,k
f 2,5
c,90,g,k
Shear strength
f 3,5
v,g,k
(shear and torsion)
Rolling shear strength f 1,2
r,g,k
Modulus of elasticity E 8 400 10 500 11 500 12 100 12 600 13 600 14 200
0,g,mean
E 7 000 8 800 9 600 10 100 10 500 11 300 11 800
0,g,05
E 300
90,g,mean
E 250
90,g,05
Shear modulus G 650
g,mean
G 540
g,05
Rolling shear modulus G 65
r,g,mean
G 54
r,g,05
Density ρ 340 370 385 405 425 430 440
g,k
ρ 370 410 420 445 460 480 490
g,mean
4.3 Classification, verification according to method B from cross sectional layup and
properties of boards and finger joints
4.3.1 Properties of the boards
If the boards comply with one of the relevant strength classes, the strength, stiffness and density
properties may be taken from Table 1.
If the boards or planks do not comply with Table 1, the characteristic values of the tensile strength
parallel to the grain f , the mean modulus of elasticity parallel to the grain E and the
t,0,l,k t,0,l,mean
characteristic density ρ should be derived from tests according to EN 408 and calculated in accordance
l,k
with EN 384 as outlined in 4.2.1 (there also specimen numbers are given).
6 © ISO 2019 – All rights reserved

4.3.2 Strength of finger joints
The characteristic flat wise bending strength or tensile strength of the finger joints should be declared
by the glulam manufacturer. The declared strength of finger joints should be verified by tests in
accordance with Annex E of ISO 10983:2014 (30 specimens per species, grade, strength class) and
evaluation according to EN 14358.
4.3.3 Determination of characteristic values for glued laminated timber
The strength and stiffness properties of homogeneous glulam should be determined from the strength
and stiffness properties of the laminations using the formulae given in Table 6.
The characteristic bending strength, the characteristic tensile and compression strengths parallel to
the grain, the mean modulus of elasticity and the characteristic density of a combined glulam should be
determined from the respective values of the different lamination zones considered as homogeneous
glulam by means of the elastic composite beam theory.
For combined glulam, the outer zones of lamination grades should be at least two laminations for glulam
with more than 10 laminations and at least one lamination for glulam with up to 10 laminations.
The strength verification should be made at all relevant points of the cross section.
2 3
Table 6 — Characteristic strength and stiffness properties in N/mm and densities in kg/m of
homogeneous glued laminated timber
Property Characteristic values
Bending strength f The characteristic bending strength should be calculated using the following
m,g,k
(N/mm ) expression.
0,,65
f
 
m,j,k
07, 5
ff=−22,,++25 15, −+f 6
 
m,g,k t,0,l,k t,0,l,k
14,
 
The expression should only be used for a characteristic flat wise bending
strength of the finger joint in the range:
1,4 f ≤ f ≤ 1,4 f + 12
t,0,l,k m,j,k t,0,l,k
The formula is also applicable to glulam without finger joints provided f is
m,j,k
taken as:
f = 1,4 f + 12
m,j,k t,0,l,k
Tensile strength f The characteristic tensile strength should be taken as 80 % of the character-
t,0,g,k
(N/mm ) istic values of the bending strength f .
m,g,k
f 0,5
t,90,g,k
Compression strength f The characteristic compression strength should be taken as f in N/
c,0,g,k m,g,k
2 2
(N/mm ) mm where f is the characteristic bending strength of the glued lami-
m,g,k
nated timber.
f 2,5
c,90,g,k
Shear strength f 3,5
v,g,k
(N/mm )
f 1,2
r,g,k
Modulus of elasticity E The mean modulus of elasticity should be taken as E = 1,05 E .
0,g,mean 0,g,mean t,0,l,mean
(N/mm )
E 300
90,g,mean
Shear modulus G 650
g,mean
(N/mm )
G 65
r,g,mean
Density (kg/m ) ρ 1,1 ρ
g,k l,k
ρ ρ
g,mean l,mean
Glulam may have an asymmetrical layup. In that case, the verification of the bending strength in the
outer compressive zone may be disregarded if the followings conditions are met:
— the difference in nominal bending strength between the outer compressive zone and the adjacent
zone of laminations (see Figure 1) does not exceed 8 N/mm ;
— the ratio of the moduli of elasticity E of the outer tensile and compressive zone of laminations,
0,g,mean
respectively, does not exceed 1,25.
The density of a combined glulam should be taken as the weighted densities of the lamination zones
estimated as the densities of homogeneous glulam according to Table 6.
4.4 Verifications from full scale tests with glulam
4.4.1 Properties of the boards
The characteristic values of the tensile strength parallel to the grain f or the bending strength
t,0,l,dc,k
f , the mean modulus of elasticity parallel to the grain E and the characteristic density
m,l,dc,k t,0,l,dc,mean
ρ of the boards should be estimated and declared by tests according to Annex E of ISO 10983:2014.
l,dc,k
4.4.2 Strength of finger joints
The characteristic flatwise bending strength of the finger joints f should be estimated and
m,j,dc,k
declared by tests according to Annex E of ISO 10983:2014.
The declared characteristic flatwise bending strength of the finger joints f should be not less than
m,j,dc,k
1,4 f .
t,0,j,dc,k
4.4.3 Strength, stiffness and density properties of glulam derived from testing
4.4.3.1 Combined glulam
Combined glulam should be assigned to one of the strength classes given in Table 4 or to any other
manufacturer specific strength class if the characteristic bending strength parallel to the grain f ,
m,g,k
the mean modulus of elasticity parallel to the grain E and the characteristic density derived
0,g,mean
from full-scale tests according to Annex F of ISO 10983:2014 and the characteristic tensile strength
f and the compression strength f parallel to the grain tested according to EN 408 and derived
t,0,g,k c,0,g,k
according to EN 14358 are not less than the declared values. Characteristic tensile strength f and
t,0,g,k
compression strength f parallel to the grain may be taken as the values for the lamination zone
c,0,g,k
having the lowest characteristic tensile strength parallel to the grain f
t,0,l,k.
The other strength and stiffness properties of a manufacturer specific strength class should be
calculated using the expressions given in Table 6.
4.4.3.2 Homogeneous glulam
Homogenous glulam should be assigned to one of the strength classes given in Table 5 or to any other
manufacturer specific strength class if the characteristic bending strength parallel to the grain f ,
m,g,k
the mean modulus of elasticity parallel to the grain E and the characteristic density ρ derived
0,g,mean g,k
from full scale tests according to Annex F of ISO 10983:2014 are not less than the declared values.
The other strength and stiffness properties of a manufacturer specific strength class should be
calculated using the formulae given in Table 6.
4.5 Resawn glulam
Glulam may be sawn perpendicular to the glue lines into 2 or 3 parts (resawn glulam).
Each part should have a minimum width b of 38 mm and a maximum depth to width ratio of h/b ≤ 8.
s s
8 © ISO 2019 – All rights reserved

5 US methodologies
The US building codes require that all glulam be trademarked as being in conformance with
ANSI A190.1, and Section 4.3.6 of ANSI A190.1 requires that grade combinations for glulam be
developed in accordance with ASTM D3737, or should be obtained by performance testing and analysis
in accordance with recognized standards, such as ASTM D7341. ANSI 117 provides many glulam lay-up
combinations that were developed based on the methodologies reviewed in this clause.
5.1 General
Since glulam was a new product in the US, the US Forest Products Laboratory (FPL) in Madison, WI
undertook a series of extensive tests on glulam arches beginning in 1934. These included tests of glulam
to check for such factors as design formulas, working stresses and the effect on strength of curvature,
end joints and knots and was reported in Reference [24]. With the great demand during World War
II for heavy timbers, the development of glulam was greatly hastened and significant research was
conducted in the areas of adhesives, lumber quality, and the testing of full-size laminated beams and
columns to supplement and confirm the work reported in Reference [24]. This work was published in
Reference [16] and formed the basis for the development of characteristic design stresses for glulam
which is still used today.
The basic premise of Reference [16] is that the strength of glulam is dependent on the knot characteristics
and their distribution in the glulam member. This led to the development of the “I /I ” model that forms
K G
the basis for ASTM D3737. This standard has undergone numerous changes over the years including
a) incorporating provisions for using full-scale beams tests to determine bending and horizontal
shear properties,
b) adding special provisions for tension laminations, and
c) introducing the volume effect factor.
In addition to being the basis for ASTM D3737, Reference [16] was also the predecessor of the
manufacturing and fabrication requirements for glulam used today in the US. The first manufacturing
standard for glulam was US. Department of Commerce Commercial Standard CS 253-63 published in
1963 and it was subsequently revised and published as Department of Commerce Standard PS 56-73.
At the same time, it was also promulgated as ANSI A190.1-1973, which has been superseded by A190.1-
1983, A190.1-1992, A190.1-2002, A190.1-2007, and ANSI A190.1-2017.
5.2 ASTM D3737
5.2.1 General
Since ASTM D3737 is relatively complicated, this document is focused on the determination of bending
strengths for a glulam member loaded perpendicular to the wide face of laminations (called X-x-axis).
The basic concept of ASTM D3737 is that clear wood strength properties and their expected variation
for small clear, straight-grained specimens of green lumber based on ASTM D2555 can be used to
develop stress index values for the various strength properties. For example, for a bending member
loaded on the X-x-axis, the bending stress index can be determined by calculating the fifth percentile of
modulus of rupture in accordance with ASTM D2555, multiplying by an appropriate adjustment factor
and multiplying by 0,743 to adjust to a 300-mm deep, uniformly loaded simple beam with a 21:1 span-
to-depth ratio. As an alternative, results of testing and analysis of large glulam beams of Douglas Fir-
Larch, Southern Pine and Hem-Fir are also used to establish stress indexes.
These stress indexes are then multiplied by stress modification factors developed based on the effects
of strength reducing characteristics of knots, slope of grain and density. For example, the bending stress
modification factor for knots, (SMF ) is given by Formula (4):
bx knots
I I I
K K 3 K
SMF =+()13 ()11−−() (4)
bxknots
I I 2I
G G G
5.2.2 I /I analysis
K G
The “I /I ” analysis that forms the basis for ASTM D3737 is based on determining how strength
K G
reducing characteristics including knot sizes and slope of grain of the individual laminations and their
distribution within the glulam member affect the overall strength of the finished glulam. The following
definitions are used.
I is the sum of the moments of inertia of the cross-sectional areas of all knots within 150 mm of a single
k
cross section at the 99,5 percentile.
I is the moment of inertia of the full of gross cross section.
g
Defining X-bar as the average of the sum of all knot sizes within any 300-mm length along the piece of
lumber, h is the difference between the 99,5 percentile knot size and X-bar.
Thus, it is necessary to define the knot characteristics, X-bar and h, for all combinations of species
and lumber grades to apply the “I /I ” principles. Annex A6 of ASTM D3737 provides guidance on the
K G
determination of these knot dependent values. There are 8 types of knots that are measured and these
are shown in Figure 2. Note that there is no knot type 8 by committee decision. All knots of 6 mm or
greater are measured and the projected cross-sectional area for each knot type is determined.
Any linear regression routine that determines the parameters of the regression line and the value of the
99,5 percentile can be used to emulate the procedure of plotting the sum of knots cumulative frequency
data on arithmetical probability paper and drawing a straight line through the data, which was the
method used in Reference [16]. The underlying assumption for using this procedure is that an analysis,
which handles the knot data as normally distributed, is satisfactory.
Figure 2 — Knot types for used in I /I model
K G
The following steps summarize the provisions of Annex A4 of ASTM D3737 to determine the bending
strength (F ) for horizontally laminated beams assuming the use of several zones based on different
bx
grades and or species of laminating lumber throughout depth of the member. This can be used for
symmetric layups or for unsymmetrical layups such as shown in Figure 3. The grade combination in
10 © ISO 2019 – All rights reserved

Figure 3 for a six-zone beam also shows that different species as well as different lumber grades can be
intermixed.
Figure 3 — Grade combination and transformed section
1) The location of the neutral axis of the transformed section is determined using Formula (5), and
the distance from the neutral axis to the edges of each grade zone in the beam is determined using
Formulae (6) and (7).
n
E
 
j
2 2
yy
−
∑ ()
jj()−1
 
j=1
y = (5)
n
Ey − y 
()
∑
 jj ()j−1 
j=1
where
is the distance from bottom of beam to neutral axis;
y
E is the long span modulus of elasticity for jth zone;
j
y is the distance from bottom of beam to top of jth zone;
j
y is the distance from bottom of beam to bottom of jth zone;
( j−1)
n is the total number of zones in beam.
Ny=− y (6)
()
jj
Ny=− y (7)
()
()jj−−11()
where
N is the distance from neutral axis to upper edge of jth zone;
j
N is the distance from neutral axis to lower edge of jth zone.
( j−1)
2) The transformed moment of inertia for each zone about the neutral axis is calculated using
Formula (8), and the moment of inertia of the transformed section is calculated using Formula (9).
3 3
NN−
E ()
  jj(1− )
j
Ib= (8)
 
j
E 3
 
T
where
I is the transformed moment of inertia of jth lam about neutral axis;
j
E is the modulus of elasticity of transformed section;
T
b is the un-transformed width of laminations.
n
II= (9)
Tj∑
j=1
where I transformed moment of inertia of the section.
T
3) The moment of inertia of the un-transformed (gross) section is calculated using Formula (10).
bD
I = (10)
g
where
I is the gross moment of inertia of the section;
g
D is the depth of the section.

4) An I /I ratio is calculated for each zone using Formula (11).
K G
 
j j
 
 
 
E E
i  2 i 
 
x Oh+ P
 () ()
∑ i ii∑ ii
   
 
 
E E
j  
i=1    i=1
j
 
 
I
 
k
= (11)
 
 
I
2d
g
 
j
j
12 © ISO 2019 – All rights reserved

where
x is the average knot size, expressed in decimal fraction of width, for the grade of lamina-
j
tion in the jth zone;
h is the difference between the 99,5 percentile and average knot size, expressed in decimal
j
fraction of the width, for the grade of lamination in the jth zone;
d is the distance between the outermost edge of the jth zone and the neutral axis.
j
5) The stress modification factor for knots, SMF , is calculated for each zone using Formula (12).
bx knots j
    
     
I I I
k k k
    
SMF =+13  1−  1−  ≥SR (12)
bx knots j bx min j
     
    
I I 2I
g g g
     
j j j
    
6) The stress modification factor for slope of grain, SMF , is
...