CLT STRUCTURAL DESIGN
Reference:
CLT Handbook
, Ch. 3
1.
Considerations for CLT’S used as roof/floor:

Short/long term behavior out

of

plane: bending, shear, instantaneous deflection,
long

term deflection (creep), long term strength due to permanent loading

Vibrations

Compression perpendicular to grain strength (bearing)

In

plane strength and stiffness

Fire performance

Acoustic performance

Durability

Two

way action similar to concrete slab may be achieved for span to width ratio
of less or equal to 2 for sing
le

board diaphgrams (up to 3
–
4m)
2.
Considerations for CLT’s used as walls:

Load

bearing capacity

In

and out

of

plane strength and stiffness

Fire performance

Acoustic performance

Durability
3.
Properties of CLT Boards
:

Thickness = 15 mm

50 mm (5/8”
–
2
”)

Width = 63 mm

250
mm (2.5”
–
10”)

Moisture Content = 12%
±
2
%

Properties may differ between tr
ansverse and longitudinal layers

CLT’s as floors/roofs
: longitudinal boards of visual grade No1/No2, transverse
boards of visual grade No3/Stud (or
better), thickness 45

500mm, up to 3m
width, 16m

18m long.

CLT’s as walls
:
vertical boards of visual grade No1/No2, horizontal boards of
visual grade No3/Stud, thickness 45

500mm, up to 3m width, 16m

18m long.
4.
Interaction Between Layers/Rolling Shea
r Modulus:

Layers achieve some interaction due to rolling shear modulus

Two sources of deflections of panels: shear along longitudinal boards, and shear
perpendicular to grain (rolling shear)

Deflections due to rolling shear may be significant

Shear deflec
tions may be neglected for span

to

depth ratio of 30
Shear Modulus
≈
for softwood species
Rolling Shear Modulus = G
rolling
Rolling Shear Strength
=
F
V,R
= 18%

20
% of parallel

to

grain values (
0.3 to
0.6 MPa = 44 psi
–
87 psi
)
5.
Analytical
Design Methods
For Floor and Roof Systems
:
Gamma Method:

Only layers acting in direction of loading are considered

Longitudinal layers are connected by imaginary fasteners that have stiffness
equal to the rolling shear deformation of cross
layers.

It is typically used for 3
–
5 layers

She
ar deformations are neglected for
span

to

width ratio of 30

This method considers deformation between layers,
= 0.85
–
0.99
(
)
∑
where
distance from NA of panel to c
entroid of board/imaginary fastener
1 for rigid connection
0 for no connection
.85
–
0.99, typ
Composite Theory Method:

Based on theory used
for plywood analysis

The relationship of moduli of elasticity for
transverse and longitudinal layers
can be expressed as: E
90
°
= E
0
°
/30

Shear deformation is neglected for span do depth ratio
≥
30

Composition factors,
k
i
are tabulated for certain loading combinations
Bending Stiff
ness
Where
(
)
Shear Analogy Method:

Shear deformation is considered

Different moduli of shear and elasticity considered for single layers in both
directions

The relationship of moduli of elasticity for transverse and longitu
dinal layers
can be expressed as: E
90
°
= E
0
°
/30

Multi

layer
CLT is simplified as
two interconnected
beams each contributing to
the overall system’s stiffness: (EI)
eff
= (EI)
Bm A
+ (EI)
Bm B
(
)
∑
∑
(
)
[
(
)
(
∑
)
(
)
]

Maximum deflection is due to bending and shear

For a uniformly loaded slab, the maximum deflection in the middle of the slab
is:
(
)
(
)
W
here
shear coefficient form factor =
(Timoshenko)
Simplified Method:

Bending Strength:
(
)

Shear Strength:
Where
and
6.
Analytical Design Methods for Wall Systems:
Theory of Mechanically Jointed Columns (Eurocode 5)
CSA 086

09 Approach Combined with Mechanically Connected Beams Theory

Only layers parallel to the axial force carry the load

Slenderness ratio for rectangular CLT walls:
√
Where
√
∑

Many producers in Europe limit panel slenderness ratio H/r
eff
to 150

Compressive resistance of panels shall satisfy
interaction equation with
combined axial and out

of

plane bending considered with bending moment
including P

effects

Bending moment should include: out

of

plane bending (
f
), bending due to axial
load eccentricity (
e
0
=
t
panel
/6
), and b
ending due to initia
l imperfections
(
0
=wall
height/500)
(
Where
P
e
=
Euler buckling load in plane of the bending moment using I
eff
and E
0
5
of boards
parallel to the axial load

Axial load capacity should account for shear deformations:
(
)
7.
Analytical Design Methods for Beams and Lintels Design:

Beams and lintels: for this application, glulams are more cost effective
Simplified Design Methods:

Bending strength is dependent upon longitudinal layer stiffness
Where
∑
8.
Modification Factors:

Load duration factor (K
D
), service condition factor (K
S
), system factor (K
H
),
treatment factor (K
T
), lateral stability factor (K
L
) for beams and lintels, size factor
fo
r bending (K
Zb
)

Factors influencing creep:
load
magnitude, moisture, product (orientation)

Creep glulam vs. CLT: CLT higher by 30

40% (consider as plywood)

deformations in the order of 0.75G
90
& 0.5G
90
(permanent deformation)
9.
Approximation of Minimum
Thickness for Floor Panels:
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