Factors in CSA O86-09 (Canadian Standard on Engineering Design in Wood) • Load Duration Factor, K...
Transcript of Factors in CSA O86-09 (Canadian Standard on Engineering Design in Wood) • Load Duration Factor, K...
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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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10/21/11 3
Outline
Deflection
Load Duration and Creep
Vibration
Building Enclosure
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10/21/11 4
Structural Design
Handbook
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10/21/11 5
Deformation (Deflection)
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10/21/11 6
Degree of interaction (rigidity) between layers
Courtesy of Norsk Treteknisk Institutt
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10/21/11 7
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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10/21/11 9
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
– Bending Strength
3) Shear Analogy (Kreuzinger)
– Bending Stiffness and Shear Stiffness
4) Simplified Design Methods
– Bending Strength
– Shear Strength
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3) Shear Analogy (Kreuzinger)
Proposed Analytical Design Methods for CLT Elements used in Floor and Roof Systems
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3) Shear Analogy (Kreuzinger)
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:
Proposed Analytical Design Methods for CLT Elements used in Floor and Roof Systems
where k is the shear coefficient form factor = 1.2 (Timoshenko)
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10/21/11 13
Outline
Deflection
Load Duration and Creep
Vibration
Building Enclosure
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.)
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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
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•Service Condition Factors Glulam, Table 6.4.2, CSA
O86-09
Adjustment Factors in CSA O86-09 (Canadian Standard on Engineering Design in Wood)
February 08, 2011 18
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.
February 08, 2011 19
•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
Load Duration and Creep
Vibration
Building Enclosure
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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
CLT Floor – Dynamic Behaviour
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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
CLT Floor – Dynamic Behaviour
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10/21/11 25
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
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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
Load Duration and Creep
Vibration
Building Enclosure
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
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