1. 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

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) 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

4. Comparing SCBFs and NCBFs 1.Experimental Results 2.Analytical Modeling 3.Performance Evaluation

5. Experimental Results

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

7. SCBF: Clearance types

8. NCBF: Connection Variations Extensive! Some Examples…

9. Example Pre-1988 Connection Bolted end-plate connection Relative to SCBF: – Shorter brace-to- gusset length – Gusset and associated connections are typically weaker than brace

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

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

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-2t p and E-8t p

14. Response of pre-1988 CBF

15. Analytical Modeling of CBFs Composite fiber sections 10 beam-column elements with initial imperfection through entire length Spring-type model of gussets Simple connection Rigid elements Increased strength element

16. 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.

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

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

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

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 approach used local measure of maximum strain. Local Pinching Initial Tearing Brace Fracture

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

23. Limit State Calibration

24. Fracture triggered (K e and D limit were calibrated by NCBF32.) Disp. Load KeKe D limit Model Implementation: NCBF Connection Fracture 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 Model

25. Comparison of Three Frames ImprovedCurrent 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. CsCs S MT Ŝ CT SD MT SD CT SD MT /1.5R CMR 1.5C d 1.5R MCE Ground Motions Collapse Level Ground Motions Spectral Displacement Spectral Acceleration (g) Evaluation of SCBF and NCBF: FEMA P-695 Analysis

33. S MT Ŝ CT S MT Ŝ CT Pre-1988 NCBF Incremental Dynamic Analysis SCBF NCBF

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