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Seismic of Older Concentrically Braced Frames

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

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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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