03 Mohammad Structural Design CLT - UNB · PDF fileOne-Way or Two-Way Slab Action Two way action capability similar to a concrete slab:-ba › /2 → Slab supported on 4 sides designed

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Structural Performance and Design of CLT Building

™

Sylvain Gagnon, Eng.M. Mohammad, Ph.D., P.Eng.

CLT Symposium and WorkshopOctober 12, 2011Moncton, NB

Outline

What is Cross Laminated Timber (CLT)– Advantages– Basic Configurations & properties

Structural Design of CLT Elements/Assemblies– Floors/roof slabs– Walls– Beams and Lintels

Connections Techniques in CLT Assemblies Seismic Performance of CLT Assemblies

New generation of lightweight and prefabricated systems

Consists of wood strips (planks) stacked crosswise on top of each other (glued or nailed)

Thicknesses of the panels vary from50 to 600 mm

•••••

•Cross-Laminated Timber

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© 2009 FPInnovations. All rights reserved. Copying and redistribution prohibited.

27/10/2011

CLT Assemblies

A series of prefabricated CLT panels connected together on site to form an assembly or a building

Quick on-site erection due to- High degree of prefab. - Ease of assembly using conventional & innovative fasteners

Source: Kevin MeechanCourtesy WoodWorks

•••••

•Advantages of CLT Panels

X-lamination minimizes swelling & shrinkage in the board plane

Relatively high in-plane (seismic) & out-of-plane (wind) strength and stiffness

Good seismic & fire resistance –heavy timber construction (i.e., inherent fire resistance)

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Why CLT is Different than Glulam?!

•Beam-like member•Thick Orthotropic Plate

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•••••

•Come in a Variety of Configurations….Single or multiple

surface layers1, 2, 3, 4

transverse layers


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One-Way or Two-Way Slab Action

Two way action capability similar to a concrete slab:- b/a › 2 → Slab supported on 4 sides designed in one direction- b/a ≤ 2 → Slab supported on 4 sides designed in 2 directions


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Lumber Properties

Dimensions

– Thickness of boards varies from ~15 mm to ~50 mm

– Width of boards may varies considerably (63 ~ 235 mm)

MC: 12% ± 3%


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Lumber Properties for Floor/Roof Panels

– Longitudinal Layers• MSR 1650Fb-1.5E (or Better)• Visual Grade No1/No2• SCL (proprietary)

– Transverse Layers• Visual Grade No3/Stud (or

Better)• SCL (proprietary)


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Lumber Properties for Wall Panels

Lumber grades for walls

– Vertical Layers

• Visual Grade No1/No2• SCL (proprietary)

– Horizontal Layers/Transverse

• Visual Grade No3/Stud• SCL (proprietary)


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Ultimate Limit State• Bending capacity (in & out-of plane)

• Shear (transverse direction)

• Comp. perp. to grain strength

Serviceability Limit State• Deflection

• Creep

• Vibration

Fire, acoustics, durability, etc.

Principle Design Properties of CLT Panels (i.e., Floor or Roof Slab or Wall Applications)


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Structural Design Principles of

CLT Elements

•Source: Structurlam

February 08, 201114

Wood = low rolling shear Significant deformations

•Stresses due to rolling shear

Rolling Shear Strength & Modulus

Rolling Shear failure

Rolling Shear strength & stiffness of transverse layers may control the design of CLT floor or wall


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Rolling Shear Strength (Fv,R)

– Rolling shear strength varies between 18% to 28% of parallel-to-grain shear values (≈ 0.3 to 0.6 MPa)

(Source: Wood Handbook)

Roughly ≈ Tension Perpendicular to Grain Strength

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 (// to grain)

– Assuming GR = 50 MPa is conservative

Rolling Shear Modulus, GR

FPInnovations CLT Handbook

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• Provides information on how to determine the engineering properties of CLT elements by using the properties of individual laminations


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Determination of Design Properties of CLT

Different methods adopted in Europe

Some methods are experimental, while others are analytical

Experimental: Flexural properties determined by testing sections or full-size panels: configuration specific

Analytical methods more versatile and less costly alternative once verified

Analytical approach can predict strength & stiffness properties of CLT element based on lamina properties

No analytical approach has been universally accepted by CLT manufactures in Europe as yet


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

1) Mechanically Jointed Beams Theory (Gamma Method)– Bending Strength & Stiffness – Shear Strength

2) Composite Theory (k Method)– Bending Stiffness & Stiffness

3) Shear Analogy (Kreuzinger)– Bending Stiffness – Shear Stiffness

4) Simplified Design Methods– Bending Strength– Shear Strength


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1) Mechanically Jointed Beams Theory (Gamma Method)

– Most common approach in Europe– Based on Annex B of Eurocode 5 (EN 1995:2004)– Developed for beams connected together with

mechanical fasteners with stiffness k uniformly spaced at a certain distance s

– Only layers acting in the direction of loading are used– For L/d ratio ≥ 30, shear deformation of longitudinal

layers is neglected

Analytical Design Methods for CLT Slab Elements (i.e., Floor and Roof Systems)

Fastener (k)


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Mechanically Jointed Beams Theory (Gamma Method)

– Method takes into account the rolling shear stiffness (GR) of the cross layers using “imaginary fasteners”

– Longitudinal layers are taken as beam elements connected with “imaginary” fasteners with a

Stiffness = rolling shear stiffness of cross layers

– Recommended for 3 & 5 layers CLT panels– Applied to simply supported beams/panels



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Normal Stress Distribution in Composite Sections: Degree of interaction (rigidity) between layers

No interaction between members

γ =0

Some interaction(semi‐rigid)

0 < γ < 1

Full interaction(rigid)

γ = 1

Courtesy of Norsk Treteknisk InstituttDepends on stiffness of interface/connection

Connection Efficiency

factor (γ)


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Mechanically Jointed Beams Theory (Gamma Method)

n

iiiiiiieff aAEIEEI

1

2 )()(

Where 0 < γ ≤ 1

γ = 1 for rigid connection and γ = 0 for no connection

But typically γ varies from 0.85 to 0.99

Effective Bending Stiffness:



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2) Composite Theory: K-Method (Blass & Fellmoser)– Based on a theory adopted for plywood but modified– However, stiffness of all layers is used in this case

• Stiffness of cross layers is taken as E90 = E0 / 30– Composition factors (ki) are determined for certain loading

configurations – Effective values of strength and stiffness are calculated

using a composition factor ki

– For L/d ≥ 30, shear deformation is neglected



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3) Shear Analogy (Kreuzinger)- Most Precise..

– Consider the different modulus of elasticity and shear modulus of single layers (both directions)

– The effect of shear deformation in both directions is NOT neglected

– Stiffness of cross layers is taken as E90 = E0 / 30

– NOT limited by the No. of layers– Multi-layer CLT panels are considered as two virtual beams A and B

Beam A

Beam B



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3) Shear Analogy (Kreuzinger)

n

iiii

n

i

iiiBAeff zAEhbEEIEIEI

1

2

1

3

12)()()(

bGh

bGh

bGh

aGA

n

nn

i ii

ieff

22

)(1

21

1

2


Beam A Beam B

Proposed CLT Standard for North America

A harmonized CLT draft product standard developed by ANSI/APA CLT Standard Committee “Standard for Performance Rated CLT ANSI/APA PRG 320”

5 stress grades for Canada & 7 grades for the US Shear Analogy method has been adopted Properties are given for major and minor axis of the panel

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4) Simplified Design Methods – Based on CSA O86

Factored Bending Capacity:

tot

effbr hEEI

FM5.01)(

1

tot

effbr h

IFM

5.0

For E1=E2=E3 (longitudinal layers)



27/10/2011 29

CLT wall elements under combined axial in-plane & out-of-plane loads

1) Theory of Mechanically Jointed Columns (Eurocode 5)

2) CSA O86-09 Approach Combined with Mechanically Connected Beams Theory

Proposed Analytical Design Methods for CLT Wall Elements


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CSA O86-09 Approach Combined with Mechanically Connected Beams Theory

Only the layers oriented parallel to the axial force carry the load



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Using CSA O86-09 Clause 5.5.6.2.2, the slenderness ratio Cc for rectangular CLT wall can be calculated as:

effc r

HdHC

32 eff

effeff A

Ir where

i

ieffeff hbhbAb

Many producers in Europe limit the panel slenderness ratio (H/reff ) to 150



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Axial loading

• The design procedure for determining the buckling strength follows Clause 5.5.6 of CSA O86-09 • Substituting the cross section area A with Aeff , and the total thickness d with the effective thickness heff :

CZCeffCr KKAFP



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Axial and out of plane/P-∆ effect

Resistance can be determined as per Section 5.5.10 of CSA O86-09:

1,2

r

Pf

r

f

MM

PP



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Proposed Analytical Design Methods for CLT Elements used as Beams and Lintels

CLT beams/lintels under Axial in-plane Loads

F


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Two Methods Proposed

1) Simplified Design Method for Calculating Bending Strength

2) Composite Theory – k Method

Analytical Design Methods for CLT Elements used as Beams and Lintels


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When the modulus of elasticity of all longitudinal layers areequal (i.e., E1 = E2, etc..)

HI

FM effbr 5.0

Simplified Design Method


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Simplified Design Method

H

ii

effeff hHHhI

1212

33



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Load Duration Factor KD

Service Condition Factor KS

System Factor KH

Treatment Factor KT

Lateral Stability Factor KL (beams and lintels)

Size Factor for Bending KZb

Strength Adjustment Factors in CSA O86-09

February 08, 2011 39

Typically, creep rupture is attributed to the load duration effect.

Recommendations are to use those given for lumber & glulam in Table 4.3.2.2, CSA O86-09

Load Duration Factor (KD)


February 08, 2011 40

Service Condition Factor (KS)

Different adjustment factors for different mechanical properties

DRY or WET conditions(Dry: Avg. EMC ≤ 15% & MC < 19%)

Recommendations to use Ks for glulam as given in Table 6.4.2, CSA O86-09


Shake Table Tests on CLT Assemblies

•Coupled CLT Walls

•Single storey •3-storey building

Seismic Performance of CLT Assemblies


27/10/2011

IVALSA SOFIE Project7-Storeys High CLT Building on a Shake Table

Seismic Performance of CLT Assemblies

Recommendations for Seismic Force Factors are

Rd=2.0, Ro=1.5

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● Research findings from Europe, Japan and Canada has shown that CLT construction can have adequate seismic performance when nails or slender screws are used with the steel brackets and/or hold-downs

● Connections control seismic performance


27/10/2011

Maintain structural integrity

Provide ductility for lateral load design (e.g., seismic & wind)

Affect the serviceability design (vibration, acoustics, etc.)

May affect the fire safety design

Importance of Connections in CLT AssembliesImportance of Connections in CLT Assemblies

Source: M. Augustin /ITE

18/10/2011 45

Common Connection Techniques in CLT

Current CLT Connections Practice in Europe

CarpentryUsing CNC technology to create various types of interlocking profiles (Dovetail connections)

•Source: G. Traetta

● Traditional fastenersBearing or dowel type fasteners, i.e., nails, wood screws, lag screws & bolts, in combination with metal plates, brackets and ties.

● InnovativeSelf-tapping screws & dowels, glued in rods, bearing-type systems, metal hooks, etc.

Source: SFSIntec


27/10/2011

•Wood and Self-Tapping Screws

Extensively used in Europe

Easy to install & provide high lateral & withdrawal capacity

Come in a variety of sizes and features

- Diameters from 4mm to 12mm

- Lengths up to 600mm

Do not require predrilling in most cases, (unlike traditional lag screws)

Used for WW or WS connections

Source: Log &Timber Connections

Source: SFSIntec


•© 2009 FPInnovations. All rights reserved. • Copying and redistribution prohibited.

•27/10/2011

CLT Panel to Panel Connection DetailsTraditional Fasteners (Screws, Nails)

Single surface splineSource: G. Traetta

Half-lapped

Double surface splineSingle Internal spline


•27/10/2011

CLT Panel to Panel Connection DetailsTraditional Fasteners (Screws, Nails)

Double internal spline



•27/10/2011

Innovative Systems for CLT Panel to Panel Connections

Tube connection system

(Source:Traetta & Schickhofer , TU of Graz)

Hook type of connection system


27/10/2011

•Development of Innovative Connection Systems Concepts in CLT @ FPInnovations

Design Concepts Developed.. Testing is underway..

Capitalize on the high bearing resistance of wood

Concept 1: Bearing washer (Prototype)


27/10/2011

•Development of Innovative Connection Systems Concepts in CLT

Adopting CNC technology & allow for quick assembly/disassembly

Reduce to steel-to-steel connections

Concept 2

Concept 5

Concept 3Concept 4

www.fpinnovations.ca

Structural Performance and Design of CLT Building

™

© 2010 FPInnovations. All rights reserved. Copying and redistribution prohibited. ™ FPInnovations, its marks and logos are trademarks of FPInnovations.

Sylvain Gagnon, Eng.M. Mohammad, Ph.D., P.Eng.

Documents

03 Mohammad Structural Design CLT - UNB · PDF fileOne-Way or Two-Way Slab Action Two way action capability similar to a concrete slab:-ba › /2 → Slab supported on 4 sides designed