Self-Centering Steel Frame Systems

NEESR-SG: Self-Centering Damage-Free Seismic-Resistant Steel Frame Systems

Project TeamRichard Sause, James Ricles, David Roke,

Choung-Yeol Seo, Michael Wolski, Geoff Madrazo ATLSS Center, Lehigh University

Maria Garlock, Erik VanMarcke, Li-Shiuan PehPrinceton University

Judy LiuPurdue University

Keh-Chyuan TsaiNCREE, National Taiwan University

Current NEESR Project

NEESR-SG: Self-Centering Damage-Free Seismic-Resistant

Steel Frame Systems

This material is based on work supported by the National Science Foundation, Award No. CMS-0420974, in the George E. Brown, Jr. Network for Earthquake Engineering Simulation Research (NEESR) program, and Award No. CMS-0402490 NEES Consortium Operation.

Motivation: Expected Damage for Conventional Steel Frames

Conventional Moment Resisting Frame System

(b)

(a)

Reduced beam section (RBS) beam-column test specimen with slab: (a) at 3% drift, (b) at 4% drift.

Self Centering (SC) Seismic-Resistant System Concepts

Discrete structural members are post-tensioned to pre-compress joints.

Gap opening at joints at selected earthquake load levels provides softening of lateral force-drift behavior without damage to members.

PT forces close joints and permanent lateral drift is avoided.

M

3660 to 4120 mm

6100 to 8540 mm

W24x62 (Fy 248 MPa),or W36x150 (Fy 350 MPa)

W14x311, W14x398 (Fy 350 MPa)or CFT406x406x13 (Fy 345 MPa)

PT Strands

Previous Work on SC Steel Moment Resisting (MRF) Connections

MRF Subassembly with PT Connections

-500

-400

-300

-200

-100

0

100

200

300

400

500

-150 -100 -50 0 50 100 150

Lateral Displacement - (mm)

Late

ral L

oad

- H

(K

N) FR

Stiffness with welded connection

Initial Stiffness Is Similar to Stiffness of Conventional Systems

H

PT Steel MRF

MRF subassembly with post-tensioned connections

Steel MRF subassembly with post-tensioned connections and angles at 3% drift

Lateral Force-Drift Behavior Softens Due to Gap Opening

Lateral Force-Drift Behavior Softens Without Significant Damage

• Conventional steel MRFs soften by inelastic deformation, which damages main structural members and results in residual drift

• SC steel MRF softens by gap opening and reduced contact area at joints

H

-600

-400

-200

0

200

400

600

-8 -6 -4 -2 0 2 4 6 8

Displacement, (in)

Lat

eral

Lo

ad, H

(ki

ps)

Post-Tensioned Connection

Welded Connection

Energy Dissipation from Energy Dissipation (ED) Elements

-400

-300

-200

-100

0

100

200

300

400

-150 -100 -50 0 50 100 150

Lateral Displacement, [mm]

Lat

eral

Lo

ad,

H [

KN

]

Specimen PC2L6x6x5/16, g/t = 4

Specimen PC4L8x8x5/8, g/t = 4

Steel MRF subassemblies with post-tensioned connections with different size ED elements.

H

Steel MRF

Limited, Repairable Damage

@ 4% DriftBefore testing After testing

Angle fracture

Summary of SC Seismic-Resistant Structural System Behavior

• Initial lateral stiffness is similar to that of conventional seismic-resistant systems.

• Lateral force-drift behavior softens due to gap opening at selected joints and without significant damage to main structural members.

• Lateral force-drift behavior softening due to gap opening controls force demands.

• Energy dissipation provided by energy dissipation (ED) elements, not from damage to main structural members.

NEESR-SG: Self-Centering Damage-Free Seismic-

Resistant Steel Frame Systems

• Project Scope.

• Project Goals.

• Status of Selected Research Tasks.

• Summary.

NEESR-SG: SC Steel Frame Systems Project Scope

• Develop two SC steel frame systems:– Moment-resisting frames (SC-MRFs).

– Concentrically-braced frames (SC-CBFs).

• Conduct large-scale experiments utilizing:– NEES ES (RTMD facility) at Lehigh.

– non-NEES laboratory (Purdue).

– international collaborating laboratory (NCREE)

• Conduct analytical and design studies of prototype buildings.

• Develop design criteria and design procedures.

NEESR-SG: SC Steel Frame Systems Project Goals

• Overall: self-centering steel systems that are constructible, economical, and structurally damage-free under design earthquake.

• Specific:– Fundamental knowledge of seismic behavior of SC-

MRF systems and SC-CBF systems.– Integrated design, analysis, and experimental

research using NEES facilities.– Performance-based, reliability-based seismic design

procedures.

NEESR-SG: Self-Centering Damage-Free Seismic-

Resistant Steel Frame Systems

• Project Scope.

• Project Goals.

• Status of Selected Research Tasks.

• Summary.

NEESR-SG: SC Steel Frame Systems Project Research Tasks

1. Develop reliability-based seismic design and assessment procedures.

2. Develop SC-CBF systems.3. Further develop SC-MRF systems.4. Develop energy dissipation elements for SC-MRFs and SC-CBFs.5. Develop sensor networks for damage monitoring and integrity

assessment.6. Design prototype buildings.7. Perform nonlinear analyses of prototype buildings.8. Conduct large-scale laboratory tests of SC-MRFs and SC-CBFs.9. Collaborate on 3-D large-scale laboratory tests on SC-MRF and SC-

CBF systems.

Task 2. Develop SC-CBF Systems: SC-CBF System Concept

Rocking behavior of simple SC-CBF system.

More Complex SC-CBF Configurations Being Considered

basebase

V

P01P01+P

g g

g

g

g

g

g

g

g

g

g

g

P02P02+P

V

col

roof

SC-CBF Design Criteria

Δgap

PT steel yields Member yields

Column

Decompression

PT Yielding

Significant

Yielding of

Frame

Members

Failure of

Frame

Members

DBE

MCE

Lateral Force

Roof Drift

IO CPLS

Current Work on SC-CBF Systems

• Evaluate frame configurations.

• Evaluate effect of energy dissipation (ED) elements.

• Develop and evaluate performance-based design approach.

SC-CBF Configurations Studied

Frame A Frame B12 Frame B12ED

PT

ED

PT PT

ED

Dynamic Analysis Results (DBE)

• Roof drift:– Effect of

frame configuration.

– Effect of ED elements.

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

0 5 10 15 20 25 30 35 40

Time (s)

Dri

ft (

%)

B12

B12ED2.4%

2.7%

-3.0-2.0-1.00.01.02.03.0

0 5 10 15 20 25 30 35 40

Time (s)D

rift

(%

)

AB12

1.4%

2.7%

Pushover Analysis Results

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0Drift (% )

V/V

des

AB24B12B12ED

PT Yield

Decompression

Roof Drift (%)

Base Shear (V/Vdes)

Preliminary Results for SC-CBF

• Dynamic analysis results indicate self-centering behavior is achieved under DBE.

• Frame A has lower drift capacity before PT yielding than Frame B:– PT steel is at column lines rather than mid-bay.

• Frame A also has lower drift demand.

• Energy dissipation helps to reduce drift demand and improve response.

Task 3. Further Develop SC-MRF Systems: Current Work

• Study of interaction between SC-MRFs and floor diaphragms by Princeton and Purdue.

• SC column base connections for SC-MRFs being studied by Purdue.

Interaction of SC-MRFs and Floor Diaphragms (Princeton)

Approach 1. Transmit inertial forces from floor diaphragm without excessive restraint of connection regions using flexible collectors.

gapr

gapgapgap

Collector Beams

Interaction of SC-MRFs and Floor Diaphragms (Purdue)

Approach 2. Transmit inertial forces from floor diaphragm within one (composite) bay for each frame.

SC Column Base Connections

for SC-MRFs (Purdue)Reinforcing Plate

Slotted Keeper Angle

Post-Tensioned Bars

Energy Dissipation Plate

Beam at Grade

Moment-Rotation Response at Column Base

Identifying appropriate level of column base moment capacity and connection details, leading to laboratory experiments.

Task 4. Develop Energy Dissipation Elements for SC-MRFs

• SC systems have no significant energy dissipation from main structural elements:

– Behavior of energy dissipation elements determined SC system energy dissipation.

• Energy dissipation elements may be damaged during earthquake and replaced.

• For SC-MRFs, energy dissipation elements are located at beam-column connections.

Quantifying Energy Dissipation

• Define relative hysteretic ED ratio E

E: Relative ED capacity

For SC systems: 0 ≤ E ≤ 50%

Target value: E = 25%

E = x 100(%)

Area of yellow

Area of blue

Hysteresis Loop

ED Element Assessment

• Consider several ED elements:– Metallic yielding, friction, viscoelastic,

elastomeric, and viscous fluid.

• Evaluation criteria:– Behavior, force capacity versus size,

constructability, and life-cycle maintenance.

• Friction ED elements selected for further study.

Bottom Flange Friction Device

Friction bolts with Belleville

washers

Anchorage

Section A-A

Reinforcing plate

Slotted plate welded to beam bottom flange

Built-up angles

bolted to column Brass friction plate at

slotted plate/angle interface

Shim Plates

(a)

PT strands

Beam

Column BFFD

A

Reinforcing plate

(b)

A

Slotted keeper angle

PT strands

Built-up angles

Col. angle bolts

Friction bolts

Column angle

Slotted plate

Friction PL

W36x300

BFFD Moment Contribution

• BFFD contribution to connection moment capacity

MFf = Ff∙rMFf

+ = Ff ∙r+

MFf- = Ff ∙r -

|MFf+ | > |MFf

- |

MFf+

MFf-

r

r

Ff Ff

r+

r-

MFf+

MFf-

r

r

Ff Ff

r+

r-

COR+

COR-

Test Setup

Test specimen subassembly

N S

W21x111

V

Hydraulic actuator

Beam

BFFD

Column

PT strands

12’-

2 ½

”

– Denotes lateral bracing

Reaction wall

Strong floor

Clevis plates

Test specimen subassembly

N S

W21x111

V

Test specimen subassembly

N S

W21x111

V

Hydraulic actuator

Beam

BFFD

Column

PT strands

12’-

2 ½

”

– Denotes lateral bracing

Reaction wall

Strong floor

Clevis platesHydraulic actuator

Beam

BFFD

Column

PT strands

12’-

2 ½

”

– Denotes lateral bracing

Reaction wall

Strong floor

Clevis platesHydraulic actuator

Beam

BFFD

Column

PT strands

12’-

2 ½

”

– Denotes lateral bracing

Reaction wall

Strong floor

Clevis platesHydraulic actuator

Beam

BFFD

Column

PT strands

12’-

2 ½

”

– Denotes lateral bracing

Reaction wall

Strong floor

Clevis plates

Test setup elevation Test setup

Bra

cing

Bra

cin

g

BFFD

Beam

PT Strands

Column

3/5 Scale

W21x111

Test Matrix

CS: Cyclic Symmetric EQ: Chi-Chi MCE Level Earthquake Response

Test No.

Loading Protocol

r,max (rads)

Experimental

Parameter

1 CS 0.035 Reduced Friction Force

2 CS 0.030 Design Friction Force

3 CS 0.030 Fillet Weld Repair

4 EQ 0.025 Response to EQ Loading

5 CS 0.065 Effect of Bolt Bearing

6 CS 0.035 Assess Column L Flex., CJP

7 CS 0.065 Effect of Bolt Bearing, CJP

Test 2: Design Friction Force

Beginning of Test 2

r = +0.03 rads r = -0.03 rads

Test 2: Response

-1.0

-0.5

0.0

0.5

1.0

-0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04

Rotation, r (rads)

No

rma

lize

d M

om

en

t, M

/Mp

,n

theoretical decompression

theoretical decompression

stiffness reduction

stiffness reduction

imminent gap opening

imminent gap opening

M+

M-r-

r+

Test 2: Comparison with Simplified Model

-1.0

-0.5

0.0

0.5

1.0

-0.04 -0.02 0.00 0.02 0.04

Rotation, r (rads)

No

rmal

ized

Mo

men

t, M

/Mp

,n

experiment

model

M+

M-r-

r+

(b) Test 2

Md + M+Ff

2M-Ff

2M+Ff

1

Axial stiffness of PT strands & beam

Md + M-Ff

Results for ED Elements for SC-MRFs

• Friction ED element:– Reliable with repeatable and predictable behavior.– Large force capacity in modest size.

• BFFD:– Provides needed energy dissipation for SC-MRF

connections.– When anticipated connection rotation demand is

exceeded, friction bolts can be designed to fail in shear without damage to other components.

Task 8. Conduct Large-Scale Laboratory Tests

• Two specimens, one SC-MRF and one SC-CBF, tested at Lehigh NEES ES (RTMD facility).

• 2/3-scale 4 story frame.

• Utilize hybrid test method (pseudo dynamic with analytical and laboratory substructures).

• Utilize real-time hybrid test method, if energy dissipation elements are rate-sensitive.

9. Collaborate on 3-D Large-Scale Laboratory Tests

• Large-scale 3-D SC steel frame system tests at NCREE in Taiwan under direction of Dr. K.C. Tsai.

• Interaction of SC frame systems with floor diaphragms and gravity frames will be studied.

• 3-D tests are part of Taiwan program on SC systems.

• Project team is collaborating with Taiwan researchers:

– US-Taiwan Workshop on Self-Centering Structural Systems, June 6-7, 2005, at NCREE.

– 2nd workshop planned for October 2006 at NCREE.

Summary

• Two types of SC steel frame systems are being developed:– Moment-resisting frames (SC-MRFs).– Concentrically-braced frames (SC-CBFs).

• Research plan includes 9 major tasks:– Significant work completed on 7 tasks.– Numerous conference publications available from

current project.• Large-scale experiments utilizing NEES ES at

Lehigh are being conducted.• Ongoing collaboration with NCREE in Taiwan.

Self-Centering Steel Frame Systems

Thank you.