Factors in CSA O86-09 (Canadian Standard on Engineering Design in Wood) • Load Duration Factor, K D Table 4.3.2.2, CSA O86-09 . February 08, 2011 17

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10/21/11 2

Critical Characteristics for CLT

Short-term and long-term behaviour (in-plane and out-of-plane):

– bending and shear strength

– instantaneous deflection

– long-term deflection (creep deformation)

– long-term strength for permanent loading

– Load-bearing capacity (walls)

Vibration of floors

Compression perpendicular to grain strength (bearing)

In-plane strength and stiffness (diaphragms)

Fire performance

Acoustic performance

Durability in service



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Outline

Deflection

Load Duration and Creep

Vibration

Building Enclosure



10/21/11 4

Structural Design

Handbook



10/21/11 5

Deformation (Deflection)



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Degree of interaction (rigidity) between layers

Courtesy of Norsk Treteknisk Institutt



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Rolling Shear Modulus and Strength

Shear Modulus

Perpendicular to Grain

– Rolling Shear Modulus GR

Shear Strength

Perpendicular to Grain

– Rolling Shear Strength Fv,R



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Rolling Shear Modulus (GR or G90)

– Generally assumed to be 10% of the shear modulus parallel to

the grain of the boards (G)

– G E/16 for softwood species

– For example: SPF Lumber No2/No1

• Modulus of Elasticity (E): 9500 MPa

• G 9500/16 = 595 MPa

• GR 590/10 = 59.5 MPa

– Assuming GR = 50 MPa is conservative

Rolling Shear Modulus, GR



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Shear Deformation of Panels in Bending

In summary:

Deformation due to rolling shear may be significant

In addition to the rolling shear deformation, there is a shear deformation of the longitudinal boards

– Shear modulus parallel to the grain of the boards, G ( E/16)

In general, the shear deformation of CLT panels may be neglected for floor elements having a span-to-depth ratio of about 30



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Proposed Analytical Design Methods for CLT Elements used in Floor and Roof Systems

Proposed design methods:

1) Mechanically Jointed Beams Theory (Gamma Method)

– Bending Stiffness

– Bending Strength

– Shear Strength

2) Composite Theory (k Method)

– Bending Stiffness


3) Shear Analogy (Kreuzinger)

– Bending Stiffness and Shear Stiffness

4) Simplified Design Methods


– Shear Strength



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The maximum deflection max

in the middle of the CLT slab

under a uniformly distributed load (w) can be calculated as a

sum of the contribution due to bending and to shear:


where k is the shear coefficient form factor = 1.2 (Timoshenko)



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Outline

Deflection


Vibration

Building Enclosure

10/21/11 14

•CLT Handbook

February 08, 2011 15

Creep and Load Duration

•Main Factors Affecting Creep of Wood:

Magnitude, type and duration of load

Moisture content (service conditions)

Type of product (grain orientation, etc.)


Adjustment Factors in CSA O86-09 (Canadian Standard on Engineering Design in Wood)

•Load Duration Factor, KD

Table 4.3.2.2, CSA O86-09


•Service Condition Factors Glulam, Table 6.4.2, CSA

O86-09

Adjustment Factors in CSA O86-09 (Canadian Standard on Engineering Design in Wood)


Research at Graz University of Technology, Austria

Creep Tests -- CLT vs. Glulam

1-yr constant load @ 9% / 25% Ultimate Load & SC1 / SC2 (SC=Service Class)

30-40% higher creep for CLT compared to glulam

•CLT

•more likely to

behave like

plywood than

glulam

•Source: Jöbstl, R.A. and Schickhofer, G. 2007. Comparative examination of

creep of glulam and CLT slabs in bending. CIB-W19/40-12-3. Bled, Slovenia.


•Recommended Approach

KD (Table/Equation Clause 4.3.2, CSA O86-09)

KS Glulam (KS Glulam Dry SC = 1.0, Table 6.4.2, CSA O86-09)

Calculate elastic deflection due to total load using 0.75 G90

Limit: Elastic deflection < L/180 (Clause 4.5.2, CSA O86-09)

Calculate permanent deformation due to long-term load using 0.50 G90

Limit: Permanent deformation < L/360 (Clause 4.5.3, CSA O86-09)

Chapter 7: Design method for controlling vibrations in CLT floors



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Outline

Deflection


Vibration

Building Enclosure

10/21/11 21

•CLT Handbook



10/21/11 22

CLT Floor – Dynamic Behaviour

Bare

Light-weight

joisted floor

Bare

CLT floor

Bare

Steel-concrete,

concrete slab

floor

Mass/Area (kg/m2) 15-30 30-150 >150

Fundamental

Natural

Frequency (Hz)

>15 >9 <9

Damping Ratio (%) 3 1 1



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Higher is the floor frequency,

easier is the vibration control

Lower is the damping,

lower is the human tolerance to vibration




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Floor

construction Light-weight,

joists,

no topping CLT

- Heavy

steel-concrete

- Concrete slab

Guidance? -In NBCC

-In Eurocode-5

None In steel-concrete

floor design guide

Floor mass

characteristics

(kg/m2)

10-30 30-150 >150

Floor frequency

characteristics

(Hz)

>15 >9 <9




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Simple Form of the Proposed Design Method

l = vibration controlled span (m)

= density (kg/m3) Bare Floor

A = area of 1 m wide CLT (m2)

= effective apparent stiffness in span direction of 1 m wide

CLT (N-m2)



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Proposed Design Method vs. CLT Industry Experience

CLT producer’s recommendations:

L L= 2.4 kN/m2 D L = 1.5 kN/m2 UDL limit = L/400

Thickness of CLT

(mm)

Span of proposed

design method (m)

Equivalent UDL limit

of proposed design

method

140 4.75 L/417

182 5.50 L/497

230 7.00 L/606



10/21/11 27

Proposed Design Method vs. CLTdesigner

CLT thickness (mm)

FPInnovations’

design method proposed

span (m)

CLTdesigner

proposed span for 1% damping and

no topping floors (m) (Schickhofer, 2010)

100 3.58 3.53

120 3.76 3.75

140 4.50 4.43

160 4.80 4.76

180 5.16 5.14

200 5.68 5.67

220 5.84 5.89

240 6.09 6.17



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Outline

Deflection


Vibration

Building Enclosure

10/21/11 29

CLT Handbook

30

What are we expecting ?

Energy efficiency

• Heating cost

• Comfort

Durability

• Lifetime

• Easy maintenance

• Investment

31

Building with CLT

Building with CLT panels…priorities

– Prevent rain exposure during transportation and on construction site

– Prevent rain intrusion during building service – building envelope leaks and direct

exposure of CLT elements

– Drying ability and condensation prevention

– General guidelines of durability by design

• Best Practice Guide for Wood-Frame Envelopes (CMHC 1999)

• Building Enclosure Design Guide – Wood Frame Multi-Unit Residential Buildings (HPO

2010)

– Consult with building science professionals

32

Construction Moisture Management

Protection on site needed in most

climates

– Much attention paid in Europe

Simple on-site protection can make a

difference

Consider season for construction

– Try to avoid installation in rain

Assembly must allow drying in case

wetting occurs

33

34

Enclosure: Rainwater Management

Rain is usually the largest water source

Overall building design important to reduce wetting

– Overhangs and sloped roofs

Rainscreen walls proved to be effective

– Two drainage planes

• Cladding and sheathing membrane

– Air space

• Capillary break

• Pressure moderation

• Ventilation

– Provide redundancy for dry areas

35

Enclosure: Thermal Insulation Design

Solid wood provides considerable insulation

– Inherent R-value about R-1.2/per inch

• R-4.2 for 3 ” thick CLT

– Convection reduced in assembly

Exterior insulation helps keep wood warm and dry

– Thickness depends on required assembly insulation

– Cladding attachment meets structural requirements

– Insulation permeance has impact on wall performance

36

Enclosure: Vapour Flow Control

Overall principle

– Prevent condensation and facilitate drying

– Control layer on warm/high vapour pressure side

CLT is a vapour retarder/barrier

– 3 ” solid wood: 3-30 ng/Pa s m2 (0.05-0.5 US Perms)

– No need for interior vapour retarder/barrier in cold climates

CLT has big moisture storage and buffering capacity

– There is a limit for wood to safely handle moisture

– Moisture will be trapped locally such as by end grain

37

Enclosure: Air Flow Control

Important for condensation control and energy efficiency

Air tightness of CLT depends on

– Joints between boards and layers

– Edge gluing and staggered layers help

– Gap and checking may occur with moisture changes

38

Enclosure: Air Flow Control

Recommend use of a primary air barrier

– Prefer to use water-resistive barrier

– Interior drywall may also work

Continuity at interfaces critical

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Documents

Factors in CSA O86-09 (Canadian Standard on Engineering Design in Wood) • Load Duration Factor, K D Table 4.3.2.2, CSA O86-09 . February 08, 2011 17