design for diaphragm action - newbuilds: strategic...

Design for Diaphragm Action

Chun NiTerje Haukaas

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Outline

Diaphragm Flexibility

Collective Chord Design Method for Large Diaphragm

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Diaphragm Flexibility

Flexible▫ Diaphragm load is distributed to shear walls by

tributary area

Rigid▫ Diaphragm load is distributed to shear walls by

relative stiffness of walls

Semi-rigid

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CIVL 435 Advanced Structural Analysis

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u1,floor

u2,floor

u3,floor

u1

u2

u3

F

u1

u2

u3

u2

u3

u1

u1

u2

u3

Static indeterminacy stiffness influences force distribution

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Prescriptive Method (ASCE 7)

Wood diaphragms idealized as flexible if …

▫ … walls are steel braced frames, concrete, etc.

▫ … one- and two-family dwellings

▫ … no concrete structural topping

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Calculation Method (ASCE 7 , ASCE 41)

Flexible: MDD > 2 ADVE Rigid: MDD < 1/2 ADVE Otherwise semi-rigid

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Case Studies

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NEWBuildS Funded MASc Theses

Xinlei Huang▫ Diaphragm Stiffness in Wood-frame Construction

Sepideh Ashtari▫ In-plane Stiffness of Cross-laminated Timber Floors

Sai Ganesh Sarvotham Pai▫ Force Transfer around Openings in CLT Shear Walls

www.circle.ubc.ca

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Embedment Properties, HYST (Xinlei)

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Li, M. H., Foschi, R. O., and Lam, F. (2011). "Modeling Hysteretic Behavior of Wood Shear Walls with A Protocol-Independent Nail Connection Algorithm." ASCE, 138(1), 99-108.

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Laboratory Tests (Xinlei)

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Floor Model, FLOOR2D (Xinlei)

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Bott, J. W. (2005). "Horizontal Stiffness of Wood Diaphragms". Master Degree. Virginia Polytechnic Institute and State University, Virginia, United States.

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Shear Modulus Values (Xinlei)

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Shear Wall Forces (Xinlei)

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CLT Panel Connections (Sepideh)

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Lam, F. (2011). "Cross Laminated Timber (CLT) Connection Tests for UBC Bioenergy Research & Demonstration Project Nexterra Building (Report prepared for CST Innovation

Ltd.)." Timber Engineering and Applied Mechanics (TEAM) Laboratory.

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ANSYS Model (Sepideh)

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Shear Wall Forces (Sepideh)

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Observations

Wood-frame diaphragms were relatively rigid!

CLT diaphragms were relatively flexible!

Keyword: Relatively

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Transfer Forces (Sai)

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2D Model 3D Model

Frame Model 2D Model with Tie-rods

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ICASP12

12th International Conference on Applications of Statistics and Probability in Civil EngineeringVancouver, Canada, July 12-15, 2015

Coast Plaza Hotel & Suites on Denman Streetwww.icasp12.ubc.ca

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Collective Chord Design Method for Large Diaphragm - Background Traditional diaphragm

design method▫ assumes bending moment

resisted by diaphragm perimeter members

▫ Commonly used for small & moderate diaphragms

▫ Assumed to be a conservative when applied to large diaphragms

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MT Cb

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Collective Chord Method (Lawson 2007)

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6( )( 2 )o

sMFb s b s

Assume evenly spaced girders / purlins act as collective chord

Use strain compatibility to estimate axial forces of chord members

Significantly lower chord forces

s – girder / purlin spacing

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Examine applicability of the collective chord design methods in collaboration with Clemson Univ.

Evaluate shear force distribution along the depth of diaphragm

Study on Collective Chord Method

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Study on Collective Chord Method

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Framing members Sheathing panels Connectors

▫ Panel-to-frame (P2F) –sheathing nails

▫ Frame-to-frame (F2F) –bearing contact between framing members, continuity ties, subpurlin-to-purlin connections

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Case Study Structure Single-story tilt-up concrete building Diaphragm consist of glulam girders, D.Fir lumber

purlins and sub-purlins, 15/32” OSB sheathing, 10d common nails at 6” o.c. around panel edges

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Base Case – Chord Force Distribution

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-40 -30 -20 -10 0 10 20 30 400

500

1000

1500

Axial Force (kip)

Ver

tical

Dis

tanc

e (in

)

, p ,

-0.92

0.02

0.04

1.00

-0.03

-10 -5 0 50

500

1000

Axial Force (kip)

Ver

tical

Dis

tanc

e (in

)

-0.91-0.03

0.02

0.10

1.00

-0.03

0 1 2 3 4 5 6 70

50

100

150

Displacement at Mid-span (in)

Rea

ctio

n F

orce

(ki

p)

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Base Case – Shear Flow of Sheathing Nails

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Effect of Diaphragm Aspect Ratio –Chord Force Distribution

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2:1 (Length:Width)1:24:1Collective Chord Model

2:11:2

4:1

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Effect of Diaphragm Aspect Ratio –Panel Shear Force Distribution

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Aspect Ratio 2:1 Aspect Ratio 4:1

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The distribution of chord forces and shear force is a function of the diaphragm aspect ratio

Where diaphragm aspect ratio is low, the chord force distribution approaches the assumption of the traditional method

Collective chord method works better for high aspect ratio diaphragms

In the range of the aspect ratios typical to large diaphragms, the collective chord method over-predicted the chord forces carried by the intermediate chords

Effect of Diaphragm Aspect Ratio –Summary

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Questions?

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Two-span diaphragm supported by three lateral load resisting members and subjected to a uniform load

Distribution of reaction forces with different stiffness ratios between ▫ diaphragm and supporting members ▫ central support and end supports

Study on Diaphragm Flexibility

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Study on Diaphragm Flexibility Reaction at the supports (same stiffness at supports)

True flexible diaphragm

True flexible diaphragm

True rigid diaphragm

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Study on Diaphragm Flexibility Flexible diaphragm

Center support:(a) a diaphragm with MDD /

ADVE = 2.0(b) a truly flexible diaphragm

Side supports:(a) a diaphragm with MDD /

ADVE = 2.0 (b) a truly flexible diaphragm

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Study on Diaphragm Flexibility Rigid diaphragm

Center support:(a) a diaphragm with MDD /

ADVE = 0.5(b) a truly rigid diaphragm

Side supports:(a) a diaphragm with MDD /

ADVE = 0.5 (b) a truly rigid diaphragm

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