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Seismic of Older Concentrically Braced Frames. Charles Roeder (PI) Dawn Lehman, Jeffery Berman (co-PI) Stephen Mahin (co-PI nees@berkeley ) Po- C hien Hsiao (GSR) University of Washington. Seismic Vulnerability of CBFs.

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Seismic of older concentrically braced frames

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
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
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-of-plane 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-of-plane rotation of the brace

Comparing scbfs and ncbfs
Comparing SCBFs and NCBFs

  • Experimental Results

  • Analytical Modeling

  • Performance Evaluation

Seismic of older concentrically braced frames

Load Beam




HSS 5x5x3/8




Strong Wall

W16X45 Beams

UW : Single-Story SCBF

Scbf clearance types
SCBF: Clearance types

Straight line (2t) Elliptical (8t)

(AISC Recommendation)

Elliptical clearance allows a more compact plate

Ncbf connection variations
NCBF: Connection Variations

  • Extensive!

  • Some Examples…

Example pre 1988 connection
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
Comparison of Three Tests

  • Current AISC DesignProcedure

  • Improved (Balanced) Design

  • Older (Pre-1988)Design

Improved scbf response brace
Improved SCBF Response: Brace

1. Hinging at Center

2. Cupping

4. Fracture

3. Tearing

Improved scbf extensive yielding in gusset
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 2t p and e 8t p
Comparison of L-2tp and E-8tp

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

Analytical Modeling of CBFs

Required properties of scbf model
Required Properties of (SCBF) Model

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

  • Significant deformation of the gusset plate connectionsand included in model. Variations in the design are important.

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

Nonlinear model
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
SCBF Model

Well-Discretized Fiber Cross Section

Minimum of 10 Elements along Brace Length

Giuffé-Menegotto-Pinto model


Wide Flange

Overview of scbf model
Overview of SCBF Model


Connection Model

Spring-type of Shear Tab

Proposed model of gusset plate connections

Rigid Links



Scbf connection model
SCBF: Connection Model

  • Out-of-plane rotation of gusset plate

  • Rigid offsets: brace, beam & column

Modeling brace fracture
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 approachused local measure of maximum strain.

Brace Fracture

Initial Tearing

Local Pinching

Basis of model
Basis of Model

  • 44 Specimens

  • 16 Test programs

  • Wide range of slenderness(34-167), compactness (7-28),& strengths

Model implementation ncbf


Fracture triggered




(Ke and Dlimit were calibrated by NCBF32.)

Model Implementation: NCBF


Connection Model

Proposed 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


Comparison of three frames
Comparison of Three Frames



Pre-1988 (NCBF)

Dynamic response analysis
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
Building Height

Impact of building height as or more significant than R

Evaluation of scbf and ncbf fema p 695 analysis
Evaluation of SCBF and NCBF:FEMA P-695 Analysis

Collapse Level Ground Motions




MCE Ground Motions

Spectral Acceleration (g)








Spectral Displacement

Incremental dynamic analysis
Incremental Dynamic Analysis

Pre-1988 NCBF








  • 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.