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Disclaimer: This presentation was developed by a third party and is not funded by WoodWorks or the Softwood Lumber Board.
Structural Design of Mass Timber Framing Systems
Presented by Paul B. Becker, PE
“The Wood Products Council” is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516.
Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.
This course is registered with AIA CESfor continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.
______________________________
Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
This presentation will provide a detailed look at the structural design processes associated with a variety of mass timber products, including glu-laminated timber (glulam), cross-laminated timber (CLT), and nail laminated timber (NLT). Applications for the use of these products in gravity force-resisting systems under modern building codes will be discussed. Other technical topics will include the use of mass timber panels as two-way spanning slabs, connection options and design considerations, and detailing and construction best practices.
Course Description
1. Provide an overview of available mass timber products, design standards and structural optimization.
2. Review structural design properties and design considerations for CLT floor and roof slabs
3. Review design properties and design considerations for glulam beams and columns
4. Review connection options for CLT and glulam beams
Learning Objectives
Bath, NY circa 1880
Mass Timber Products
Albina Yards, Portland, OR circa 2016 Lever ArchitectureFour-Story, 16,000 sf
photo: Lever Architecture
Albina Yards, Portland, OR circa 2016
photo: Lever Architecture
T3, Minneapolis, MN circa 2017
photo credit: Hines Development
Michael Green Architecture/DLR GroupSeven-Story, 220,000 sf
photo: Hines Development
T3, Minneapolis, MN circa 2017
photo: Hines Development
Michael Green Architecture/DLR GroupSeven-Story, 220,000 sf
Candlewood Suites, Huntsville, AL, circa 2015
CLT Bearing Walls /Slabs3-ply walls, 5-ply slabsFour-stories 62,700 sf
photo: Lend Lease
Spaced CLT allows for MEP chase, inlay ceiling
photo: MGA
Brock Commons, Vancouver, BC
9.33’ x 13.0’ grid, 18 stories
18 stories, two-way slabs, no beams
photo: KK Law
Ascent Tower, Milwaulkee, WI 21 stories (5+16) @ 238 ft
Mass Timber Products
Nail Laminated Decking
Images source: Structurecraft
Mass Timber Products
Dowel Laminated Decking
Images source: Structurecraft
Cross-Laminated Timber
CLT Slabs
Composition• 3-ply minimum, up to 9 ply• Pieces 5/8” to 2 inch thick• Pieces 2.4 inches to 9.5 inches wide• Boards finger jointed• MSR or visual, all kiln dried• Width and length by manufacturer
Advantages• Dimensionally stable product• High strength to weight ratio• Long, wide slabs• High in-plane, out of plane strength +
stiffness• two-way action• Connector splitting resistance
Images source: CLT Handbook
Non-homogeneous, anisotropic material
Composite CLT
• proprietary mesh connector epoxied into CLT• screw connector f ield applied
Images source: Setragian/KusumaImages source: SOM
Composite Construction
Images source: SOM
Mass Timber Design
Design Considerations – In-Plane and Out-of-Plane
• Bending Strength• Shear Strength
• Bending Stiffness• Shear Stiffness
• Vibration
strength
serviceability
Mass Timber Design Guides
Images source: CLT Handbook, PRG 320-2018, AWC NDS
Common CLT Layups
Images source: PRG 320-2018
3-ply 3 layer(3.43”-4.14”)
5-ply 5 layer(5.47”-6.90”)
7-ply 7 layer(7.52”-9.66”)
9-ply 9 layer(9.57”-12.42”)
9-ply 7 layer(9.57”-12.42”)
5-ply 3 layer(5.47”-6.90”)
7-ply 5 layer(7.52”-9.66”)
CLT Stress Grades
Stress Grade Major Axis Minor Axis
E1 1950f-1.7E MSR SPF #3 Spruce Pine Fir
E2 1650f-1.5E MSR DFL #3 Doug Fir Larch
E3 1200f-1.2E MSR Misc #3 Misc
E4 1950f-1.7E MSR SP #3 Southern Pine
V1 #2 Doug Fir Larch #3 Doug Fir Larch
V2 #1/#2 Spruce Pine Fir #3 Spruce Pine Fir
V3 #2 Southern Pine #3 Southern Pine
CLT Stress Grades
Images source: PRG 320-2018
Flatwise Panel Loading
Images source: PRG 320-18
MAJOR MINOR
Edgewise Panel Loading
Images source: PRG 320-18
Bending Members- Flexure (out of plane)
PRG 320- calculates capacity using extreme fiber capacity approach
Fb = Mc S = I M = Fb SI c
From Strength of Materials/Engineering Mechanics, we know
Non-homogeneous, anisotropic materialHomogeneous, isotropic material
Image & Reference: CLT Handbook
Bending Members- Flexure (out of plane)
Bending Members- Flexure (out of plane)
Images source: CLT Handbook, Chapter 3, page 16
Images source: PRG 320-2018
Bending Members- Flexure (out of plane)
Reference: CLT Handbook
Bending Members- Flexure Design Example
Given:16.5 ft span
38 psf dead load, 40 psf live load
Assume:one span along major axis of CLT
Analysis based on 1 ft width
Calculate:ASD dead + live load applied moment
Mb = wL2/8 = (38+40 psf) (16.5 ft)2/8 = 2,655 lb-ft/ft
16.5’
38 psf DL+40 psf LL
Reference: Structurlam Design Guide
Bending Members- Flexure Design Example
Choose 5-ply 6 7/8” (175mm) thick V2 panel, FbSeff,0 = 4,701 lb-ft/ft
Reference: NDS / CLT Handbook
Bending Members- Flexure Design Example
Mb < (FbSeff)’
Mb < CD CM Ct CL (FbSeff)
Mb = 2,655 lb-ft/ft < (1.0) (1.0) (1.0) (1.0) 4,701 lb-ft/ft
Mb = 2,655 lb-ft/ft < 4,701 lb-ft/ft
Flexure Strength is OK!
16.5’
38 psf DL+40 psf LL
Per NDS
Adjusted Bending Strength
Flatwise Shear Strength
Reference: CLT Handbook, Images source: Scott Breneman, PE
From Strength of Materialsѵs = V Q V = ѵs I t
I t Q
Fs(Ib/Qeff)’ = adjusted shear strength
Reference: NDS / CLT Handbook
Flatwise Shear Strength
16.5’
38 psf DL+40 psf LL
Non-homogeneous, anisotropic material
Reference: NDS / CLT Handbook
Flatwise Shear Strength
16.5’
38 psf DL+40 psf LL
Vplanar < CdCmCt (Fs(IbQ)eff)
Reference: Structurlam Design Guide
Flatwise Shear Strength
16.5’
38 psf DL+40 psf LL
R = 644 lb
Choose 5-ply 6 7/8” (175mm) thick V2 panel
Vallow = 1,980 lb > 644 lb OK!
Reference: CLT Handbook
Flatwise Flexural Stiffness (Deflection)
• Bending Stiffness• Shear Stiffness
16.5’
38 psf DL+40 psf LL
R = 644 lb
G = Shear Modulus
Images source: CLT Handbook
Flatwise Flexural Stiffness (Deflection)
Using Shear Analogy Method
Because Shear deflections can be a significant in CLTs, adjust the effective bending stiffness to an apparent bending stiffness.
Ks = constant based on loading and end conditions per Table 2, Chapter 3 CLT Handbook (simple, pinned Ks = 11.5)
Deflection Under Long-Term Loading
reference: CLT Handbook Chapter 3
Design Example
16.5’
38 psf DL+40 psf LL
ΔDL = from 38 psf = 0.19 in
ΔLL = from 40 psf = 0.20 in
Δmax = (.19 ) + ( .20 ) = 0.39 in (L/507)
app
Deflection Under Long-Term Loading
Images source: CLT Handbook Chapter 6
ΔT = Kcr ΔLT + ΔST
Kcr = Time Dependent Creep Factor, 2.0 for dry service CLT
ΔLT = Immediate Deflection Due to Long Term Component of load
ΔST = Deflection due to Short Term or Normal Component of load
ΔT = 2.0 (ΔLT)+ (ΔST)
Deflection Under Long-Term Loading
Images source: CLT Handbook Chapter 6
Design Example
16.5’
38 psf DL+40 psf LL
ΔLT = from 38 psf = 0.19 in
ΔST = from 40 psf = 0.20 in
Δtotal = 2.0 ( 0.19 ) + (0.20 ) = 0.58 in (L/341)
app
Floor Vibration
Occupant perception of vibration is significant design consideration,Human sensitivity to vibration @ frequency between 4 – 8Hz
Natural Frequency not best measure of vibration performance; accelerations felt is better measure but difficult to calculate accurately given the variability of inputs esp, damping
Vibration response to footfall is dependent on natural frequency and damping
Mass Timber Floors – 2.5% to 3.5% modal damping
Ignore composite action of concrete topping unless floor designed and detailed as such
Simplified Analysis per CLT Handbook, AISC Design Guide 11, CSA 086 Annex A, ISO 10137
Floor Vibration
Images source: Scott Breneman, PE, reference CLT Handbook Chapter 7
Human sensitivity to Vibration - frequency between 4 – 8Hz
`
Floor Vibration – Span Limitations
reference: CLT Handbook Chapter 7
16.5’
38 psf DL+40 psf LLFPI Method Floor Span Limitations
Frequency f > 9.0 Hz
Human sensitivity to Vibration - frequency between 4 – 8Hz
Span L Using Spreadsheet and Iterate1. Estimate L2. Calculate EIapp
3. Calculate L limit4. Repeat until convergence
Floor Vibration Controlled Span - Example
reference: CLT Handbook Chapter 7
FPI Vibration Span Limits for Std Grades / Layups
reference: Woodworks
• These are approximate only!• Verify based on manufactures
properties• Verify based on project specifics• Check strength and deflection
Floor Vibration
reference: CLT Handbook Chapter 7
Recommendation seems conservative
Mass Timber Floor Vibration Guide coming 2020 – more specific design info
Two-Way Action
Flexure• Manufactures provide flexure and shear capacity in both major axis
• NDS 2018 Chapter 10 (CLT) - Adjustment factors to check capacity against the design code using ASD / LRFD design checks
• NDS 2018 C3.9.2-2 could check the combined loading condition - very conservative approach
• Laminations in orthogonal directions share load based upon their relative stiffness
• Treat the major / minor direction flexural and shear checks independently. This is the common state of practice in European design
Two-Way Action
Bearing Capacity at Supports NDS 2018 Chapter 10 includes design provisions for the bearing capacity of CLT panels to loads on the surface
Local Punching Shear at SupportsThis limit state is critical for point supported CLT panels.
Image: Gafner, Jackson WCTE 2016
Bearing Walls
Image: PRG 320-2018
Fc’Aparallel = Fc*Aparallel = FcCpAparallel
Bearing Walls
Image: PRG 320-2018 Source: NDS 2018
Bearing Walls
Source: PRG 320-2018
Pparallel < PcCpAparallel
EIapp-min = 0.5184 EIapp
Connections
Image: Code Unlimited-Block 76W CLT Connections Engineering Judgement Report
Panel to Panel at floors, walls and roof
Connections
Image: CLT Handbook Chapter 5
Wall to Wall
End Screw Toe Screw
Connections
Image: CLT Handbook Chapter 5
Wall to Wall
Wood Key Steel Bracket
Connections
Image: CLT Handbook Chapter 5
Wall to Wall
End Screw, Toe Screw Steel Bracket
Connections
reference: CLT Handbook Chapter 5
Panel to Panel at floors, walls and roof
Double Steel Bracket Steel Bracket
Connections
Image: Code Unlimited Block 76W CLT Connections Engineering Judgement Report
Panel to GLB
Images source: Greg Kingsley, PhD, PE, KL&A Engineers and Builders
Mass Timber Products - NLT
Images source: Structurecraftdownload –rethinkwood.com
NLT Design
Images source: NLT Guide V1.0
Treat NLT as built-up beam per NDS provisions
Beam Stability Factor (CL) =1
Size factor (CF) based on individual lam thickness (blam)
Repetitive Member (Cr) =1.15
Layup Factor (Klayup) adjusts bending and stiffness
NLT Design
Images source: NLT Guide V1.0
NLT Design – Butt Joints – (Klayup)
Images source: NLT Guide V1.0
NLT Design - Variable Depth Members (Ksection)
Images source: NLT Guide V1.0
• Nails do not provide necessary stiffness for full composite
• All lams do not reach maximum bending capacity
• Deeper lams reach full capacity first• Shallow lams reach only a portion of their
capacity• Based on relative stiffness
NLT Design - Variable Depth Members (Ksection)
Images source: NLT Guide V1.0
Deflection
Glu-Lams
reference: CLT Handbook Chapter 7
• Fabricated from conventional sawn lumber
• Laminations glued together to create a built-up beam
• Typically Southern Pine or Douglas Fir– high architectural quality
Glu-lams
reference: CLT Handbook Chapter 7
• Specified by size, grade and finish (architectural versus industrial)
• Beam design• Unbalanced layup – 24F-V3 or 24F-1.8E• Higher grade laminations at bottom, weaker grade
laminations at middle and top• Conventional layup - works for simple spans
• Balanced layups – 24F-E4 or 24F-1.8E balanced• Same grade laminations at top and bottom• Cantilevers and continuous beams
Glu-lams
reference: APA Glulam Design Guide
Balanced vs. Unbalanced Layup
Image: Thornton Tomasetti
- “Off the shelf”
- Stock beams: Power Beams by Anthony Forest Product, X-beam by Rosboro, etc..
- Size up to 7”x28 7/8” (Anthony Powers); 8 ¾”x30” (Rosboro)
- Available as architectural grade and treated for exterior- Connection made in the field
- Specialized manufacturer
- High quality, custom product: Unalam, CLT list of Manuf
- Shop fabricated, connection design, CNC
- Curved!
- Can be treated for exterior
- Cost is a function of width - 2 beams cheaper?
Glulam – Manufacturer options
- Values included for width (Table 5.1.3), Section Properties (Supplement Table 1C and 1D) and Reference Design Values (Supplement Table 5a)
- Check with local manufacturer
Glulam Beams – Design (NDS Chapter 5)
Images: American Wood Council
- Design as standard wood member using NDS Table 5.3.1 adjustment factors
Glulam Beams – Design
Image: American Wood Council
APA design guide for “first pass” tables
Glulam Columns - Design
reference: APA Glulam Design Guide
Glu-Lam Connections per AITC / APA
reference: APA Glulam Design Guide
Glu-Lam Connection Details
Photos: Chris Williams
Glu-lam Connection Details
Photo Chris Williams reference: APA Glulam Design Guide
reference: MiTiCon
Beam - Beam
5,800-29,400 lb capacity
Glu-lam Connection Details
Photo: structurlam
Beam - ColumnGlu-lam Connection Details
Glu-lam Connection Details
Photos: Chris Williamsimage: APA Glulam Design GuideDetail: Thornton Tomasetti
Glu-lam Connection Details
image: Structurecraft
Column-column connectionsconcealed steel connection
Glu-lam Connection Details
images: Structurecraft
Brock Commons, Vancouver, BC
concealed steel connection Column-column connections
What’s Possible?
image: APA Glulam Design Guide
image: Sidewalk Labs
This concludes The American Institute of Architects Continuing Education Systems Course
QUESTIONS?
Paul B. Becker, PE
Thornton Tomasetti