cast-in-place rc coupled shear walls shear wall systems 2 • rc coupled shear wall structures are a...

Cast-in-Place RC Coupled Shear Walls:Unbonded Post-Tensioned Coupling Beams &

Debonded Starter Bars at Wall Base

NHERI Lehigh Researcher’s Workshop

Bethlehem, PA

December 5-6, 2016

Yahya C. Kurama, Ph.D., P.E.

Steven M. Barbachyn, M.S.

University of Notre Dame

Michael J. McGinnis, Ph.D.

University of Texas at Tyler

Richard Sause, Ph.D., P.E.

Lehigh University

Coupled Shear Wall Systems

2

• RC coupled shear wall

structures are a commonly

used primary lateral load

resisting system

• Two or more shear wall piers

connected by coupling (or

link) beams

• Provide large lateral strength,

stiffness, and energy

dissipation

Conventional Coupling Beams

3courtesy, Magnusson Klemencic Associates

diagonal

bar groups

wall pier

wall piercoupling

beam

• Typical coupling beams are short

• Large shear force demands under large reversed-cyclic

rotations

Conventional RC Coupling Beam

Post-Tensioned Coupling Beams

4

Partially Post-Tensioned RC Coupling Beam

(Specimen 1)

unbonded PT

steel in duct

PT anchor at

end of wall pier

energy dissipating (ED)

mild steel bar with

debonded length

Fully Post-Tensioned RC Coupling Beam

(Specimen 2)

left wall pier right wall pier

Conventional RC Coupling BeamPost-Tensioned RC Coupling Beam

LOAD

large, concentrated

cracks


5

Validation & Design Process

6

• ACI 318:“...the proposed system shall have strength and toughness equal to or exceeding those provided by a comparable monolithic reinforced concrete structure satisfying this chapter.”

• Validation and Design Documents• ACI ITG-5.1 – Acceptance Criteria for Special Unbonded

Post-Tensioned Structural Walls Base on Validation Testing and Commentary

• ACI ITG-5.2 – Requirements for Design of a Special Unbonded Post-Tensioned Shear Wall Satisfying ACI ITG-5.1 and Commentary

• ACI 318 – Building Code Requirements for Structural Concrete and Commentary

Research Objectives

7

1. To develop a validated seismic design

procedure

2. To conduct system-level experimental

evaluations

3. To validate analytical models and simulation

tools that predict system behavior

4. To create a Design Procedure Document

Presentation Outline

8

• Introduction and Objectives

• Experimental Program

• Specimen 1 Details and Behavior

• Specimen 2 Behavior and Comparisons

• Conclusions

Prototype Structure

9

elevation viewplan view

prototype

core wall

lb / hb = 4.0 (Specimen 1)

= 3.0 (Specimen 2)

• Eight-Story Office Building (coupling degree=30%)

• Designed for Seismic Category D in Los Angeles, CA

• SS = 1.50; S1 = 0.60; CS = 0.136-0.154; R = 6.0; CD = 5.0

• Base Moment for Full-Scale Core Wall ~134,000-151,000 kip-ft

10

NEES Test Setup at Lehigh Univ. (40%-scale)

11

Applied 3rd Floor Drift History

Specimen 1 Specimen 2

ACI ITG 5.1 validation drift

(3% at roof)

ACI ITG 5.1 validation drift

(3% at roof)

1.5x

ACI ITG 5.1 loading protocol

Instrumentation

Type Specimen 1 Specimen 2

load cells 29 29

displacement 123 156

rotation 46 46

strain gauges 214 250

TOTAL 412 481

LVDTplastic

slides

data

acquisition

system

12

13

Digital Image Correlation (DIC)

Type Specimen 1 Specimen 2

2D systems 11 0

3D systems 3 9

TOTAL 14 9



14





• Conclusions

elevation view

cross-section

15

overlapping

small and

large hoops

adapted

ACI ITG-5.2

to design

confinement

target εmax=0.5εsu

for unbonded

length

PT steel in two

tendons near

mid-depth

shear reinf.

satisfying

ACI 318

less

confinement

due to slab

Coupling Beam Reinforcement (Specimen 1)

16

Wall Pier Reinforcement (Specimen 1)

17

Total Base Shear versus 3rd Floor Drift

(Specimen 1)

validation-level

roof drift = ±3.0%

failure due to

buckling+fracture of

starter bars at toes

coupling beams

performed well

toe region

(designed per

ACI 318-11) flan

ge

web

Reasons for Starter Bar Fracture

18

1. Lap splices above foundation

concentratedcrack at base

lack of crackingwithin splice length

wall flange

wall flange

19

Specimen 1

Reasons for Starter Bar Fracture2. Deterioration of concrete at top of foundation

increasedunsupported

length ofstarter bars

bottom hoop in pier


20





• Conclusions

Detail Change in Wall Pier Toes

21

Specimen 1

(per ACI 318-11)

Specimen 2

(with debonded bars)

10” unbonded

length

Coupling Beam Changes

lb

unbonded length

hb

ED

mild steel

barsfloor slab

unbonded PT

strands in ducts

mild steel

skin reinforcement

wallpier

confining hoops

hb

lb

= 4.0

lb

hb

floor slab

unbonded PT

strands in ducts

mild steel

skin reinforcement

wallpier

confining hoops

hb

lb

= 3.0

2 No. M13bars

Specimen 1

Specimen 2

22

flan

ge

web

23

Total Base Shear versus 3rd Floor Drift

(Specimen 2)

validation-level

roof drift = ±3.0%

strength loss due to


starter bars at flanges

failure due to


starter bars at toes

coupling beams

performed well

24

1st Story Damage Progression (Specimen 2)

wall

flange

wall

web

Comparison of Wall Pier Toe Damage

25

core wall plan view

west

pier

east

pier

Δ3,max = 3.68%

Specimen 1

Δ3,max = 2.70%Specimen 2

Coupling Beam Damage

26

west

pier

east

pier

specimen elevation view

θb,max = 10.5%

Beam End Rotations

27

Energy Dissipation

28

βh = Ad / [(E1+E1)(θ’L1+θ’L2)

Conclusions (PT Coupling Beams)

29

• Completed 2 large-scale system-level experimental tests

• Performed as predicted and validated the design approach

• Demonstrated ductile behavior up to 10.5% beam end

rotation

• Coupling beams provided adequate and stable coupling in

both specimens (30% coupling)

• Support the classification of unbonded PT coupled wall

structures as “special” RC shear walls

• Demonstrated intended behavior and advantages of the new

coupling system

• Fully-PT beams may be preferred over partially-PT beams

Conclusions (Wall Pier Bases)

30

• Lap splices of vertical starter bars above foundation resulted

in concentration of cracking at wall base (with little

distributed cracking within spliced wall height)

• There was also significant deterioration to concrete at top of

foundation

• Failure in Specimen 1 occurred due to buckling and

subsequent fracture of starter bars in wall pier toes

• Unbonding of starter bars in toes improved behavior of

Specimen 2 by delaying buckling/fracture of starter bars

• General recommendation for RC shear walls:

consider lack of cracking over splice length of starter bars

unbonding of starter bars may delay bar fracture

Acknowledgements

31

• Project funded by NSF Grant No: 1041598

• Dr. Joy Pauschke, Program Director

• This award is part of the “George E. Brown, Jr. Network for

Earthquake Engineering Simulation (NEES) Research

(NEESR)” and the “National Earthquake Hazards Reduction

Program (NEHRP)”

• Magnusson Klemencic Associates (MKA): Dave Fields, Joshua

Mouras, Amy Haaland

• Ken Bondy, structural engineer

• Concrete Donation: Essroc Cement

• Other material donations: Dayton Superior, A.H. Harris

Construction Supplies, Hayes Industries, Sumiden Wire Products

Corporation, Suncoast Post-Tension

Acknowledgements

32

• Lehigh University

– Profs. Jim Ricles, Shamim Pakzad

– Staff: Darrick Fritchman, Peter Bryan, Gary Novak, Carl

Bowman, Thomas Maurullo, Ed Tomlinson

– Graduate Students: Michelle Tillotson, Kristen Peterson,

Karim Kazemibidokhti, Golnaz Shahidi

– Undergraduate Students:

• Katie Brinkhoff, Corey Fallon

• Mathu Davis, Amy Breden, Fannie Tao, Eric Salazar (REU)

• University of Texas at Tyler

– Graduate Student: Michelle Holloman

– Undergraduate Student: Michael Lisk

ptcoupledwalls.nd.edu

33

ptcoupledwalls.nd.edu

Conventional RC Coupling BeamPost-Tensioned RC Coupling Beam

LOAD

large, concentrated

cracks

35



5

36

analytical

model

test

setup

Load Application

11

wall

web

Detail Change in Wall Pier Corners

37

flan

ge

webSpecimen 1 Specimen 2

closely-spacedU-bars


38

Comparison of Large Drift Response

wall

flange

wall

web

Comparison of Wall Pier Corner Damage

39

core wall plan view

west

pier

east

pier

Δ3,max = 3.68%

Specimen 1 Δ3,max = 2.70%

Specimen 2

Specimen 1 Movie

40

YouTube

https://youtu.be/CzafIPmRgj8?t=18s

3rd Floor Drift Components (Specimen 2)

41

Specimen 2

Beam PT Stresses

42

Energy Dissipation

43