Structural Design of Mass Timber Framing Systems
Presented by Paul B. Becker, PE
Disclaimer: This presentation was developed by a third party and is not funded by WoodWorks or the Softwood Lumber Board. “The Wood Products Council” is a This course is registered with AIA CES Registered Provider with The American for continuing professional education. Institute of Architects Continuing As such, it does not include content Education Systems (AIA/CES), Provider that may be deemed or construed to #G516. be an approval or endorsement by the AIA of any material of construction or Credit(s) earned on completion of this any method or manner of handling, course will be reported to AIA CES for using, distributing, or dealing in any AIA members. Certificates of Completion material or product. for both AIA members and non-AIA ______members are available upon request. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation. Course Description
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. Learning Objectives
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 Bath, NY circa 1880 Mass Timber Products Lever Architecture Albina Yards, Portland, OR circa 2016 Four-Story, 16,000 sf
photo: Lever Architecture Albina Yards, Portland, OR circa 2016
photo: Lever Architecture Michael Green Architecture/DLR Group T3, Minneapolis, MN circa 2017 Seven-Story, 220,000 sf
photophoto: credit: Hines Hines Development Development Michael Green Architecture/DLR Group T3, Minneapolis, MN circa 2017 Seven-Story, 220,000 sf
photo: Hines Development Candlewood Suites, Huntsville, AL, circa 2015
CLT Bearing Walls /Slabs 3-ply walls, 5-ply slabs Four-stories 62,700 sf
photo: Lend Lease Spaced CLT allows for MEP chase, inlay ceiling
photo: MGA Brock Commons, Vancouver, BC 18 stories, two-way slabs, no beams
9.33’ x 13.0’ grid, 18 stories
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 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
Non-homogeneous, anisotropic material Advantages • Dimensionally stable product • High strength to weight ratio • Long, wide slabs • High in-plane, out of plane strength + stiffness CLT Slabs • two-way action • Connector splitting resistance Images source: CLT Handbook Composite CLT
• proprietary mesh connector • screw connector field applied epoxied into CLT
Images source: SOM Images source: Setragian/Kusuma Composite Construction
Images source: SOM Mass Timber Design
Design Considerations – In-Plane and Out-of-Plane
• Bending Strength • Shear Strength strength
• Bending Stiffness • Shear Stiffness serviceability • Vibration Mass Timber Design Guides
Images source: CLT Handbook, PRG 320-2018, AWC NDS Common CLT Layups
3-ply 3 layer (3.43”-4.14”)
5-ply 5 layer 5-ply 3 layer (5.47”-6.90”) (5.47”-6.90”)
7-ply 7 layer 7-ply 5 layer (7.52”-9.66”) (7.52”-9.66”)
9-ply 9 layer 9-ply 7 layer (9.57”-12.42”) (9.57”-12.42”)
Images source: PRG 320-2018 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
MAJOR MINOR
Images source: PRG 320-18 Edgewise Panel Loading
Images source: PRG 320-18 Bending Members- Flexure (out of plane)
PRG 320- calculates capacity using extreme fiber capacity approach
From Strength of Materials/Engineering Mechanics, we know
Fb = Mc S = I M = Fb S I c
Homogeneous, isotropic material Non-homogeneous, anisotropic material Bending Members- Flexure (out of plane)
Image & Reference: CLT Handbook Bending Members- Flexure (out of plane)
Images source: CLT Handbook, Chapter 3, page 16 Bending Members- Flexure (out of plane)
Images source: PRG 320-2018 Bending Members- Flexure Design Example
Given: 16.5 ft span 38 psf DL+40 psf LL 38 psf dead load, 40 psf live load
Assume: 16.5’ one span along major axis of CLT
Analysis based on 1 ft width
Calculate: ASD dead + live load applied moment
2 2 Mb = wL /8 = (38+40 psf) (16.5 ft) /8 = 2,655 lb-ft/ft Reference: CLT Handbook Bending Members- Flexure Design Example
Choose 5-ply 6 7/8” (175mm) thick V2 panel, FbSeff,0 = 4,701 lb-ft/ft
Reference: Structurlam Design Guide Bending Members- Flexure Design Example
Mb < (FbSeff)’ Adjusted Bending Strength 38 psf DL+40 psf LL
16.5’ Mb < CD CM Ct CL (FbSeff) Per NDS
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! Reference: NDS / CLT Handbook Flatwise Shear Strength
Fs(Ib/Qeff)’ = adjusted shear strength
From Strength of Materials ѵs = V Q V = ѵs I t I t Q Reference: CLT Handbook, Images source: Scott Breneman, PE Flatwise Shear Strength
38 psf DL+40 psf LL
16.5’ Non-homogeneous, anisotropic material
Reference: NDS / CLT Handbook Flatwise Shear Strength
38 psf DL+40 psf LL
16.5’
Vplanar < CdCmCt (Fs(IbQ)eff)
Reference: NDS / CLT Handbook Flatwise Shear Strength
38 psf DL+40 psf LL
16.5’
R = 644 lb
Choose 5-ply 6 7/8” (175mm) thick V2 panel
Vallow = 1,980 lb > 644 lb OK!
Reference: Structurlam Design Guide Flatwise Flexural Stiffness (Deflection)
• Bending Stiffness 38 psf DL+40 psf LL • Shear Stiffness
16.5’
R = 644 lb
G = Shear Modulus
Reference: 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)
Images source: CLT Handbook Deflection Under Long-Term Loading
Design Example 38 psf DL+40 psf LL
16.5’ app
ΔDL = from 38 psf = 0.19 in
ΔLL = from 40 psf = 0.20 in
Δmax = (.19 ) + ( .20 ) = 0.39 in (L/507)
reference: CLT Handbook Chapter 3 Deflection Under Long-Term Loading
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)
Images source: CLT Handbook Chapter 6 Deflection Under Long-Term Loading
Design Example 38 psf DL+40 psf LL
16.5’ app
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)
Images source: CLT Handbook Chapter 6 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
Human sensitivity to Vibration - frequency between 4 – 8Hz
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Images source: Scott Breneman, PE, reference CLT Handbook Chapter 7 Floor Vibration – Span Limitations
Human sensitivity to Vibration - frequency between 4 – 8Hz 38 psf DL+40 psf LL FPI Method Floor Span Limitations
Frequency f > 9.0 Hz 16.5’
Span L Using Spreadsheet and Iterate 1. Estimate L 2. Calculate EIapp 3. Calculate L limit 4. Repeat until convergence
reference: CLT Handbook Chapter 7 Floor Vibration Controlled Span - Example
reference: CLT Handbook Chapter 7 FPI Vibration Span Limits for Std Grades / Layups
• These are approximate only! • Verify based on manufactures properties • Verify based on project specifics • Check strength and deflection
reference: Woodworks Floor Vibration
Recommendation seems conservative
Mass Timber Floor Vibration Guide coming 2020 – more specific design info
reference: CLT Handbook Chapter 7 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 Supports This limit state is critical for point supported CLT panels.
Image: Gafner, Jackson WCTE 2016 Bearing Walls
Fc’A parallel = Fc*Aparallel = FcCpAparallel
Image: PRG 320-2018 Bearing Walls
Image: PRG 320-2018 Source: NDS 2018 Bearing Walls
EIapp-min = 0.5184 EIapp
Pparallel < PcCpAparallel
Source: PRG 320-2018 Connections Panel to Panel at floors, walls and roof
Image: Code Unlimited-Block 76W CLT Connections Engineering Judgement Report Connections Wall to Wall
End Screw Toe Screw
Image: CLT Handbook Chapter 5 Connections Wall to Wall
Wood Key Steel Bracket
Image: CLT Handbook Chapter 5 Connections Wall to Wall
End Screw, Toe Screw Steel Bracket Image: CLT Handbook Chapter 5 Connections Panel to Panel at floors, walls and roof
Double Steel Bracket reference: CLT Handbook Chapter 5 Steel Bracket Connections Panel to GLB
Image: Code Unlimited Block 76W CLT Connections Engineering Judgement Report Images source: Greg Kingsley, PhD, PE, KL&A Engineers and Builders Mass Timber Products - NLT
download –rethinkwood.com Images source: Structurecraft NLT Design
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
Images source: NLT Guide V1.0 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)
• 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
Images source: NLT Guide V1.0 NLT Design - Variable Depth Members (Ksection)
Deflection
Images source: NLT Guide V1.0 Glu-Lams
• Fabricated from conventional sawn lumber
• Laminations glued together to create a built-up beam
• Typically Southern Pine or Douglas Fir– high architectural quality
reference: CLT Handbook Chapter 7 Glu-lams
• 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
reference: CLT Handbook Chapter 7 Glu-lams
reference: APA Glulam Design Guide Balanced vs. Unbalanced Layup
Image: Thornton Tomasetti Glulam – Manufacturer options
- “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 Beams – Design (NDS Chapter 5)
- 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
Images: American Wood Council Glulam Beams – Design
- Design as standard wood member using NDS Table 5.3.1 adjustment factors
Image: American Wood Council Glulam Columns - Design
APA design guide for “first pass” tables
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 Glu-lam Connection Details Beam - Beam
5,800-29,400 lb capacity
reference: MiTiCon Glu-lam Connection Details Beam - Column
Photo: structurlam Glu-lam Connection Details
Detail: Thornton Tomasetti image: APA Glulam Design Guide Photos: Chris Williams Glu-lam Connection Details
Column-column connections concealed steel connection
image: Structurecraft Glu-lam Connection Details
Column-column connections concealed steel connection
Brock Commons, Vancouver, BC images: Structurecraft What’s Possible?
image: APA Glulam Design Guide
image: Sidewalk Labs QUESTIONS?
This concludes The American Institute of Architects Continuing Education Systems Course
Paul B. Becker, PE Thornton Tomasetti [email protected]