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Seismic of Older Concentrically Braced Frames

This study focuses on improving the seismic performance of older Concentrically Braced Frames (CBFs), specifically those designed prior to 1988. It evaluates the vulnerability of these frames using Performance-Based Seismic Engineering (PBSE) and ATC P-695 methodologies. Key findings include the redesign of gusset plates that can significantly enhance drift capacity. Experimental and analytical modeling results reveal that pre-1988 CBFs are prone to premature connection failure and significant damage under seismic events. Recommendations for retrofitting and improved design practices are discussed.

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Seismic of Older Concentrically Braced Frames

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  1. 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

  2. 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.

  3. 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

  4. Comparing SCBFs and NCBFs • Experimental Results • Analytical Modeling • Performance Evaluation

  5. Experimental Results

  6. Load Beam Strong Floor Actuator HSS 5x5x3/8 Brace W12x72 Columns Strong Wall W16X45 Beams UW : Single-Story SCBF

  7. SCBF: Clearance types Straight line (2t) Elliptical (8t) (AISC Recommendation) Elliptical clearance allows a more compact plate

  8. NCBF: Connection Variations • Extensive! • Some Examples…

  9. Example Pre-1988 Connection • Bolted end-plateconnection • Relative to SCBF: • Shorter brace-to-gusset length • Gusset and associated connections aretypically weaker than brace

  10. Comparison of Three Tests • Current AISC DesignProcedure • Improved (Balanced) Design • Older (Pre-1988)Design

  11. Improved SCBF Response: Brace 1. Hinging at Center 2. Cupping 4. Fracture 3. Tearing

  12. Improved SCBF: Extensive Yielding in Gusset • Brace buckling and yielding • Extensive yielding and OOP rotation of gusset plate • Yielding of beams and columns

  13. Comparison of L-2tp and E-8tp

  14. Response of pre-1988 CBF

  15. 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

  16. 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.

  17. 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

  18. SCBF Model Well-Discretized Fiber Cross Section Minimum of 10 Elements along Brace Length Giuffé-Menegotto-Pinto model HSS Wide Flange

  19. Overview of SCBF Model Model Connection Model Spring-type of Shear Tab Proposed model of gusset plate connections Rigid Links Brace Fracture

  20. SCBF: Connection Model • Out-of-plane rotation of gusset plate • Rigid offsets: brace, beam & column

  21. 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

  22. Basis of Model • 44 Specimens • 16 Test programs • Wide range of slenderness(34-167), compactness (7-28),& strengths

  23. Limit State Calibration

  24. Load Fracture triggered Ke Dlimit Disp. (Ke and Dlimit were calibrated by NCBF32.) Model Implementation: NCBF Model 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 ConnectionFracture

  25. Comparison of Three Frames Improved Current Pre-1988 (NCBF)

  26. Predicting Performance of CBFs

  27. Performance States (ATC)

  28. 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

  29. Building Height Impact of building height as or more significant than R

  30. SCBF vs. NCBF VS.

  31. NCBF vs. SCBF

  32. Evaluation of SCBF and NCBF:FEMA P-695 Analysis Collapse Level Ground Motions ŜCT CMR SMT MCE Ground Motions Spectral Acceleration (g) 1.5R Cs 1.5Cd CMR SDMT/1.5R SDMT SDCT Spectral Displacement

  33. Incremental Dynamic Analysis Pre-1988 NCBF ŜCT ŜCT SMT SMT NCBF SCBF

  34. Results

  35. 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.

  36. Overview of New NEES Project

  37. Thank You

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