Seismic of OlderConcentrically Braced Frames

Charles Roeder (PI)Dawn Lehman, Jeffery Berman (co-PI) Stephen Mahin (co-PI nees@berkeley)

Po-Chien Hsiao (GSR) University of Washington

Seismic Vulnerability of CBFs• Current research has focused on improving

seismic performance of Special Concentrically Braced Frames (SCBFs).

• Redesign of gusset plate can double drift capacity.

• Prior to 1988, modern capacity-design principles were not in place.

• Preliminary study to evaluate the vulnerability of older CBFs using PBSE and ATC P695.

Changing the Design of SCBFsPost-1988/Beyond (SCBF)• Brace

– Kl/r <~ 100– b/t – seismically

compact (1997)• Gusset

– Designed for brace material overstrength

– Accommodate out-of-plane rotation of brace

• Conventional: linear• Improved: elliptical

Pre-1988 (NCBF)• Brace

– No limit on KL/r– No limit on b/t

• Gusset– Nominal tension capacity

of the brace (lateral forces)– No provisions

accommodating out-of-plane rotation of the brace

Comparing SCBFs and NCBFs

1. Experimental Results

2. Analytical Modeling

3. Performance Evaluation

Experimental Results

UW : Single-Story SCBF

W12x72 Columns

W16X45 Beams

HSS 5x5x3/8 Brace

Actuator

Strong Wall

Strong Floor

Load Beam

SCBF: Clearance types

Straight line (2t) Elliptical (8t) (AISC Recommendation)

Elliptical clearance allows a more compact plate

NCBF: Connection Variations• Extensive!• Some

Examples…

Example Pre-1988 Connection

• Bolted end-plateconnection

• Relative to SCBF:– Shorter brace-to-

gusset length– Gusset and

associated connections aretypically weaker than brace

Comparison of Three Tests

• Current AISC DesignProcedure

• Improved (Balanced) Design

• Older (Pre-1988)Design

Improved SCBF Response: Brace

1. Hinging at Center 2. Cupping

3. Tearing 4. Fracture

Improved SCBF: Extensive Yielding in Gusset

Brace buckling and

yielding

Extensive yielding and OOP rotation of gusset plate

Yielding of beams and columns

Comparison of L-2tp and E-8tp

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

Drift Ratio (%)

Yie

ld F

orc

e R

atio

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

Drift Ratio (%)

Yie

ld F

orc

e R

atio

Response of pre-1988 CBF

Analytical Modeling of CBFsComposite fiber sections

10 beam-column elements with initial imperfection through entire length

Spring-type model of gussets

Simple connection

Rigid elements

Increased strength element

Required Properties of (SCBF) Model

1.Buckling behavior of the brace is a key elements in the SCBF seismic response.

2.Significant deformation of the gusset plate connections and included in model. Variations in the design are important.

3.Local yielding of the beams and columns must be simulated.

Nonlinear Model• OpenSees was selected as analysis platform.

• Fiber-type (nonlinear beam-column) element for braces, beams and columns.

• Custom connection element(s) developed.

• Model response beyond brace/connection failure to collapse

Giuffé-Menegotto-Pinto model

(a)

(c)

(b)

(c)

SCBF ModelWell-Discretized

Fiber Cross Section

HSS

Wide Flange

Minimum of 10 Elements along

Brace Length

Overview of SCBF ModelModel

BraceFracture

Connection ModelSpring-type of Shear TabProposed model of gusset plate connections

Rigid Links

• Out-of-plane rotation of gusset plate

• Rigid offsets: brace, beam & column

SCBF: Connection Model

Modeling Brace Fracture• Fracture results from low-cycle fatigue at middle

of brace• Equivalent plastic strain limit used for continuum

analyses; not available from OpenSees analysis approach used local measure of maximum strain.

Local Pinching Initial Tearing Brace Fracture

max,c

max,t

max,max,.max)( ctrange

(a)

(c)

Basis of Model• 44 Specimens • 16 Test programs• Wide range of

slenderness(34-167), compactness (7-28),& strengths

Max. εrange

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

εrange

Cal

. εra

nge

3~6 7~8

9 10

11 12

13 14

15 16

Cal

. εra

nge

x

4.0

3.04.0

,.max1435.0

ycalrange F

E

r

KL

t

w

max,max,.max)( ctrange

Limit State Calibration

Fracture triggered

(Ke and Dlimit were calibrated by NCBF32.)

Disp.

Load

Ke

Dlimit

Model Implementation: NCBF

ConnectionFracture

Connection ModelProposed spring-type model of gusset plate connections combined with axial fracture model of brace-to-gusset connections.

Axial Fracture Model of

Connection Calibrated by

NCBF32

Model

Comparison of Three Frames

Improved Current Pre-1988 (NCBF)

Predicting Performance of CBFs

Performance States (ATC)

Dynamic Response Analysis• 3, 9 and 20 story buildings (SAC SMRF) buildings• Emphasis on 3-story building model. • 40 Seattle ground motions (scaled)• 2% and 10% in 50 yr. events

100 100

80

3025

15

100

50

5 5 5 5

100

20

50 0 0

0

20

40

60

80

100

> DS1 > DS2 > DS3 > DS4 Br. Frac. SD>5%

Per

cen

tage

(%

)

3F (R=6) 3F (R=3) 9F (R=6)

2/50

Building Height

Impact of building height as or more significant than R

95

0 0

95

0 0

100 100

10

0

20

40

60

80

100

Bracebuckled or

yielded

Fracture ofbrace or

connection

Potentialcollapse

Per

cent

age

(%)

_

Bal-SCBF

AISC SCBF

Pre-1988NCBF

10/50

SCBF vs. NCBF

VS.

100

25

15

100

2520

100 100

35

0

20

40

60

80

100

Bracebuckled or

yielded

Fracture ofbrace or

connection

Potentialcollapse

Per

cent

age

(%)

_

Bal-SCBF

AISC SCBF

Pre-1988NCBF

2/50NCBF

vs. SCBF

Cs

SMT

ŜCT

SDMT SDCTSDMT/1.5R

CMR

CMR1.5Cd

1.5R

MCE Ground Motions

Collapse Level Ground Motions

Spectral Displacement

Spec

tral

Acc

eler

atio

n (g

)

Evaluation of SCBF and NCBF:FEMA P-695 Analysis

SMT

ŜCTSMT

ŜCT

Pre-1988 NCBFIncremental Dynamic Analysis

SCBF NCBF

Frame System Bal-SCBF Pre-1988 NCBFR Factor 6 8

Drifts limits 5% 10% 5% 10%SMT (g) 1.76 1.76 1.12 1.12 ŜCT (g) 1.88 2.77 2.36 3.91CMR 1.07 1.58 2.11 3.50 SSF 1.3 1.3 1.15 1.15

ACMR (CMR x SSF) 1.39 2.05 2.75 4.55

ACMR 20% 1.73 1.73 1.73 1.73Pass/Fail Fail Pass Pass Pass

Results

Conclusions• Pre-1988 CBF vulnerable to “premature”

connection failure.• Retrofit methods untested; largely absent in

ASCE-31• Connection model is critical to accurate

response and performance prediction of all CBFs. Move beyond “pinned” or “fixed”.

• Pre-1988 CBF sustains significant damage at lower levels of seismic excitation, yet exceeds performance of SCBF from FEMA 695 evaluation. Careful(re-)consideration of this approach as a design basis is needed.

Overview of New NEES Project

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