19SE07 BECKER Structural Design of Mass Timber Framing … · This presentation will provide a detailed look at the structural design processes associated with a variety of mass timber

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


Nail Laminated Decking

Images source: Structurecraft


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


Flatwise Panel Loading


MAJOR MINOR

Edgewise Panel Loading


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



Images source: CLT Handbook, Chapter 3, page 16



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


Choose 5-ply 6 7/8” (175mm) thick V2 panel, FbSeff,0 = 4,701 lb-ft/ft

Reference: NDS / CLT Handbook


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



16.5’

38 psf DL+40 psf LL

Non-homogeneous, anisotropic material



16.5’

38 psf DL+40 psf LL

Vplanar < CdCmCt (Fs(IbQ)eff)

Reference: Structurlam Design Guide


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


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)


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


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


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


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


Wall to Wall

Wood Key Steel Bracket

Connections


Wall to Wall

End Screw, Toe Screw Steel Bracket

Connections


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


NLT Design – Butt Joints – (Klayup)


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

NLT Design - Variable Depth Members (Ksection)


Deflection

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

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

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


Glu-Lam Connections per AITC / APA


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


Photo: structurlam

Beam - ColumnGlu-lam Connection Details


Photos: Chris Williamsimage: APA Glulam Design GuideDetail: Thornton Tomasetti


image: Structurecraft

Column-column connectionsconcealed steel connection


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

[email protected]

Documents

19SE07 BECKER Structural Design of Mass Timber Framing … · This presentation will provide a detailed look at the structural design processes associated with a variety of mass timber