CLT STRUCTURAL DESIGN

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Nov 15, 2013 (3 years and 4 months ago)

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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: