Seismic of Older
Concentrically 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 SCBFs
Post-1988/Beyond (SCBF)
Pre-1988 (NCBF)
• Brace
– Kl/r <~ 100
– b/t – seismically
compact (1997)
• Gusset
– Designed for brace
material overstrength
– Accommodate out-ofplane rotation of brace
• Conventional: linear
• Improved: elliptical
• Brace
– No limit on KL/r
– No limit on b/t
• Gusset
– Nominal tension capacity
of the brace (lateral forces)
– No provisions
accommodating out-ofplane rotation of the brace
Comparing SCBFs and NCBFs
1. Experimental Results
2. Analytical Modeling
3. Performance Evaluation
Experimental Results
UW : Single-Story SCBF
Load Beam
Strong
Floor
Actuator
HSS 5x5x3/8
Brace
W12x72
Columns
Strong Wall
W16X45 Beams
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-plate
connection
• Relative to SCBF:
– Shorter brace-togusset length
– Gusset and
associated
connections are
typically weaker than brace
Comparison of
Three Tests
• Current AISC Design
Procedure
• Improved
(Balanced) Design
• Older (Pre-1988)
Design
Improved SCBF Response: Brace
1. Hinging at Center
3. Tearing
2. Cupping
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
1.2
Yield Force Ratio
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-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
Yield Force Ratio
Drift Ratio (%)
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-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 (%)
Response of pre-1988 CBF
Analytical Modeling of CBFs
Composite fiber sections
Rigid
elements
Spring-type
model of
gussets
Increased
strength
element
Simple
connection
10 beam-column
elements with initial
imperfection
through entire
length
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
SCBF Model
Well-Discretized
Fiber Cross
Section
Minimum of 10
Elements along
Brace Length
(b)
(a)
be
HSS
Concrete slab
(c)
(b)
(a)
be
Concrete slab
Wide
Flange
Giuffé-MenegottoPinto model
Overview of SCBF Model
Model
Spring-type
of Shear Tab
Proposed model
of gusset plate connections
Rigid Links
Brace
Fracture
Connection Model
SCBF: Connection Model
• Out-of-plane
rotation of
gusset plate
WW t 2
Mp =
sy
6
• Rigid offsets:
brace, beam &
column
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
Basis of Model
• 44 Specimens
• 16 Test programs
• Wide range of
slenderness(34-167),
compactness (7-28),
& strengths
( range )max.   t ,max   c,max
(a)
be
Concrete slab
 c,max
 t ,max
Limit State Calibration
Max. εrange
0.08
3~6
7~8
9
10
0.06
11
12
0.05
13
14
15
16
Cal.
Cal.εrange
εrange
0.07
0.04
( range )max.   t ,max   c,max
x
0.03

0.02
0.01

range max. ,cal
 w
 0.1435 
t
0
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
εrange
0.4
 KL 


 r 
0.3
 E 


 Fy 


0.4
Model Implementation: NCBF
Model
Connection Model
Proposed spring-type model of
gusset plate connections
combined with axial fracture
model of brace-to-gusset
connections.
Connection
Fracture
Load
Fracture triggered
Ke
Dlimit
Disp.
(Ke and Dlimit were
calibrated by NCBF32.)
Axial Fracture
Model of
Connection
Calibrated by
NCBF32
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
Building Height
100 100 100
100
Percentage (%)
100
3F (R=6)
3F (R=3)
9F (R=6)
80
80
2/50
60
50
40
30
25
20
15
20
5
5
5
0
5
0
5
0
0
> DS1
> DS2
> DS3
> DS4
Br. Frac.
SD>5%
Impact of building height as or more significant than R
SCBF vs. NCBF
_
100
100
95
100
Bal-SCBF
95
Percentage (%)
AISC SCBF
80
Pre-1988
NCBF
60
10/50
40
VS.
20
10
0
0
0
0
0
Brace
buckled or
yielded
Fracture of
brace or
connection
Potential
collapse
_
100 100 100
100
Bal-SCBF
100
Percentage (%)
NCBF vs. SCBF
AISC SCBF
80
Pre-1988
NCBF
60
2/50
35
40
25
25
20
20
15
0
Brace
buckled or
yielded
Fracture of
brace or
connection
Potential
collapse
Spectral Acceleration (g)
Evaluation of SCBF and NCBF:
FEMA P-695 Analysis
Collapse Level
Ground Motions
ŜCT
CMR
SMT
MCE Ground
Motions
1.5R
Cs
1.5Cd
SDMT/1.5R
CMR
SDMT SDCT
Spectral Displacement
Incremental Dynamic Analysis
Pre-1988 NCBF
ŜCT
ŜCT
SMT
SCBF
SMT
NCBF
Results
Frame System
R Factor
Bal-SCBF
6
Pre-1988 NCBF
8
Drifts limits
SMT (g)
ŜCT (g)
5%
1.76
1.88
10%
1.76
2.77
5%
1.12
2.36
10%
1.12
3.91
CMR
SSF
ACMR
(CMR x SSF)
ACMR 20%
Pass/Fail
1.07
1.3
1.58
1.3
2.11
1.15
3.50
1.15
1.39
2.05
2.75
4.55
1.73
Fail
1.73
Pass
1.73
Pass
1.73
Pass
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