Thesis Defense Presentation


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Thesis Defense Presentation

  1. 1. Enhanced Seismic Performance of Multi-Story Special Concentrically Braced Frames Using a Balanced Design Procedure Masters Thesis Defense December 8, 2009 Eric J. Lumpkin Masters Candidate Charles W. Roeder Professor – Principal Investigator (PI) University of Washington Dawn E. Lehman Associate Professor – Co-PI University of Washington Po-Chien Hsiao PhD Candidate University of Washington K.C. Tsai International Collaborator NCREE Laura N. Lowes Associate Professor – Committee University of Washington
  2. 2. <ul><li>SCBFs – Special Concentrically Braced Frame </li></ul><ul><ul><li>High seismic regions </li></ul></ul><ul><ul><li>Economic (less steel and fabrication) </li></ul></ul><ul><li>Traditional design philosophy </li></ul><ul><ul><li>Inherently stiff and inverted truss systems </li></ul></ul><ul><ul><li>Transfer forces through brace axial loads </li></ul></ul><ul><li>Design requirements in AISC seismic manual and specification </li></ul>Overview of SCBFs
  3. 3. <ul><li>Majority of seismic response dominated by brace buckling and tensile yielding </li></ul><ul><li>Brace to framing element connections </li></ul><ul><ul><li>Gusset plate – pinned ends </li></ul></ul><ul><ul><li>Moment connection – fixed ends </li></ul></ul>Overview of SCBFs
  4. 4. Current Design May Lead to… Overly large gusset plates Undesirable failure mechanisms Excessive beam/column damage Soft Stories
  5. 5. Summary of Project University of California - Berkeley NCREE University of Minnesota - MAST University of Washington Objective To improve the seismic design of SCBF systems and connections through advanced models and full scale system testing Elliptical Clearance Mode Balanced Design Procedure Investigation of Midspan Gussets
  6. 6. <ul><li>Investigation of unique gusset plate configurations </li></ul><ul><ul><li>Chevron configuration </li></ul></ul><ul><ul><li>Gussets on either side of a beam </li></ul></ul><ul><li>Modification of middle gusset plate clearances </li></ul><ul><li>Eliminate middle gusset edge stiffeners </li></ul><ul><li>Lead to Tomorrows Concentrically Brace 2 (TCBF2) </li></ul>Extension to Three Story Testing
  7. 7. TCBF 2 Overview TCBF 2-HSS TCBF 2-WF TCBF 2-IP Construction Drawings Moment Connection Shear Connection Reusable Test Frame
  8. 8. Experimental Setup Test Specimen OOP Frame OOP Kicker Actuators Strong Wall Strong Floor 7.8” Composite Slabs Most Accurate SCBF Specimens Tested
  9. 9. <ul><li>TCBF2-HSS </li></ul><ul><ul><li>OOP buckling HSS5x5x3/8 brace shape (U.S.) on each story </li></ul></ul><ul><ul><li>3/8” middle and corner gusset plates </li></ul></ul><ul><li>TCBF2-WF </li></ul><ul><ul><li>OOP buckling H175x175x7.5x11 brace shape (Japan) on each story </li></ul></ul><ul><ul><li>10-mm middle and corner gusset plates </li></ul></ul><ul><li>Similar braces sizes to UW specimens </li></ul>TCBF2 Specimens
  10. 10. <ul><li>TCBF2-IP </li></ul><ul><ul><li>HSS125x125x9 In-plane buckling braces on each story </li></ul></ul><ul><ul><ul><li>In-plane buckling achieved with a 20-mm knife plate </li></ul></ul></ul><ul><ul><ul><li>18-mm gusset plate to remain elastic </li></ul></ul></ul><ul><ul><ul><li>Majority of inelastic action in knife plate hinge region </li></ul></ul></ul>TCBF2 Specimens
  11. 11. <ul><li>Loading Protocol </li></ul><ul><ul><li>Pseudo-static </li></ul></ul><ul><ul><li>Cyclic </li></ul></ul><ul><ul><li>Increasing amplitude based on yield displacement ( Δ y ) </li></ul></ul><ul><ul><li>Adopted from SAC recommended protocol </li></ul></ul>Experimental Setup Δ y
  12. 12. <ul><li>Designed using a combination of two methods </li></ul><ul><ul><li>Balanced design procedure and limit states </li></ul></ul><ul><ul><ul><li>Used to ensure strength and ductility </li></ul></ul></ul><ul><ul><li>Non-linear, finite element modeling (Hsaio, 2009) </li></ul></ul><ul><ul><ul><li>Used to establish connection clearances </li></ul></ul></ul>TCBF2 Design
  13. 13. <ul><li>AISC Design </li></ul><ul><ul><li>Ensures strength with resistance factors (φ) for varying limit states (φR n ) </li></ul></ul><ul><ul><li>φ factors based strength, safety and statistically extreme considerations </li></ul></ul><ul><ul><li>Higher φ factors for limit states that are easier to predict or outcome is less severe </li></ul></ul><ul><ul><li>Connection is stronger than brace </li></ul></ul>Balanced Design Procedure <ul><li>Balanced Design Procedure </li></ul><ul><ul><li>Ensures strength AND ductility with balance factor (β) for varying limit states ( β R n ) </li></ul></ul><ul><ul><li>β factors based on achieving ductility while preventing undesirable failure mechanisms </li></ul></ul><ul><ul><li>Higher β factors for limit states that increase connection ductility (gusset plate yielding) </li></ul></ul><ul><ul><li>Connection stronger than brace </li></ul></ul>
  14. 14. <ul><li>Vertical offset (Nt p ) from the beam clearance to allow for gusset plate ductility and rotation (TCBF2-HSS, WF) </li></ul>TCBF2 Design - Middle Gusset Plates 2t p 4t p 6t p
  15. 15. <ul><li>FEM used to establish hinge region dimensions (Hsiao, 2009) </li></ul><ul><ul><li>Parametric study </li></ul></ul>TCBF2 Design – Knife Plates Nt kp 260-mm x 20-mm (1t kp ) 210-mm x 25-mm (2t kp ) 260-mm x 20-mm (2t kp )
  16. 16. TCBF 2 Connection Summary Corner Gusset Plates Middle Gusset Plates <ul><li>8t elliptical clearance </li></ul><ul><li>Based on single story, single bay UW testing </li></ul><ul><li>Balanced design procedure </li></ul><ul><li>6t linear offset from beam </li></ul><ul><li>Close proximity to beam eliminates need for edge stiffeners to control buckling </li></ul><ul><li>Balanced design procedure </li></ul>Knife Plates <ul><li>2t linear clearance from gusset plate </li></ul><ul><li>Balanced design procedure </li></ul>8t 6t 2t
  17. 17. <ul><li>Test results broken into four parts </li></ul>Test Results HSS OOP vs. WF OOP TCBF2-HSS vs. TCBF2-WF HSS OOP vs. HSS In-Plane TCBF2-HSS vs. TCBF2-IP Overall Test Comparison Beam/Column Demands
  18. 18. Test Results – Overall Backbone Frame Response Sustained Drifts Maximum Resistance 1 st Story 2 nd Story 3 rd Story Frame TCBF2-HSS 4.38% 4.32% 2.13% 3.82% TCBF 2-WF 5.21% 5.56% 2.99% 4.86% TCBF2-IP 3.99% 4.56% 1.99% 3.48% kip TCBF2-HSS 516.4 TCBF 2-WF 490 TCBF2-IP 449
  19. 19. <ul><li>Test results broken into four parts </li></ul>Test Results HSS OOP vs. WF OOP TCBF2-HSS vs. TCBF2-WF HSS OOP vs. HSS In-Plane TCBF2-HSS vs. TCBF2-IP Overall Test Comparison Beam/Column Demands
  20. 20. <ul><li>OOP buckling specimens hysteretic response </li></ul><ul><ul><li>WF brace achieve higher story drifts, however prone to loss of resistance </li></ul></ul><ul><ul><li>WF braces have a more “pinched” hysteretic behavior </li></ul></ul>Test Results-HSS vs. WF TCBF2-HSS TCBF2-WF Frame Drift :-2.43% to 2.43% Frame Drift :-1.74% to 2.08%
  21. 21. <ul><li>Energy dissipation </li></ul><ul><ul><li>On an equal drift basis, HSS braces dissipate more energy than WF braces </li></ul></ul><ul><ul><li>WF braces dissipate more energy overall </li></ul></ul>Test Results-HSS vs. WF TCBF2-HSS TCBF2-WF
  22. 22. <ul><li>Brace fracture achieved in both specimens </li></ul>Test Results – HSS vs. WF TCBF2-HSS TCBF2-WF
  23. 23. <ul><li>Comparison of first story brace buckling </li></ul>Test Results - HSS vs. WF TCBF2-HSS TCBF2-WF
  24. 24. Test Results – HSS vs. WF TCBF2-HSS TCBF2-WF <ul><li>Larger initial spike in WF brace displacement </li></ul><ul><li>HSS buckling more gradual </li></ul><ul><li>WF brace achieve higher OOP displacements </li></ul>1.2% SD 2.1% SD Wide Flange HSS
  25. 25. Test Results – HSS vs. WF TCBF2-HSS TCBF2-WF Corner Gussets Corner Gussets Middle Gussets Middle Gussets
  26. 26. <ul><li>6t p vertical clearance permits substantial OOP gusset rotation </li></ul><ul><li>So does 8t p elliptical clearance </li></ul>Test Results – HSS vs. WF TCBF2-HSS TCBF2-WF
  27. 27. <ul><li>Interface weld tearing </li></ul>Test Results – HSS vs. WF TCBF2-HSS TCBF2-WF 2.15% SD 3.05% SD
  28. 28. <ul><li>Test results broken into four parts </li></ul>Test Results HSS OOP vs. WF OOP TCBF2-HSS vs. TCBF2-WF HSS OOP vs. HSS In-Plane TCBF2-HSS vs. TCBF2-IP Overall Test Comparison Beam/Column Demands
  29. 29. <ul><li>OOP buckling specimens </li></ul><ul><ul><li>Similar hysteretic behavior </li></ul></ul><ul><ul><li>Identical energy dissipation capabilities </li></ul></ul>Test Results-In Plane Buckling TCBF2-HSS TCBF2-IP Frame Drift :-1.74% to 1.74% Frame Drift :-1.74% to 2.08%
  30. 30. Test Results-In Plane Buckling
  31. 31. Test Results – In Plane Buckling <ul><li>In-plane buckling connections are long – resulting in shorter braces </li></ul><ul><li>In-plane braces achieve slightly higher normalized buckled displacements than OOP buckling HSS braces </li></ul>
  32. 32. <ul><li>Knife plate connections are fundamentally different than gusset plate connections </li></ul><ul><ul><li>Inelastic rotation and yielding is isolated to a confined region as opposed to over an area. </li></ul></ul>Test Results – In Plane Buckling
  33. 33. <ul><li>In-plane rotating knife plates achieve substantial inelastic rotations </li></ul>Test Results - In Plane Buckling Knife Plate Gusset Plate
  34. 34. <ul><li>3NBr buckled OOP instead of the intended In-plane direction </li></ul><ul><ul><li>Construction error resulted in a hinge region that was 1.55t kp (31-mm) instead of 2t kp (40-mm) </li></ul></ul><ul><ul><li>Indicates 2t kp is on the threshold of what is permissible to initiate in-plane buckling </li></ul></ul><ul><li>Inelastic tearing in hinge region </li></ul><ul><ul><li>Large rotational demands in confined region </li></ul></ul>Test Results – In Plane Buckling
  35. 35. <ul><li>Test results broken into four parts </li></ul>Test Results HSS OOP vs. WF OOP TCBF2-HSS vs. TCBF2-WF HSS OOP vs. HSS In-Plane TCBF2-HSS vs. TCBF2-IP Overall Test Comparison Beam/Column Demands
  36. 36. <ul><li>Two predominant forces – axial and moment </li></ul><ul><li>Axial due to dead weight of structure and vertical component of brace forces (P c , P t ) </li></ul><ul><li>Inelastic moments arise when column bending increases as braces buckle and fracture </li></ul>Test Results – Column Demands P t P t P t P c P c P c
  37. 37. <ul><li>Results in column axial forces that are 50% nominal tensile yield capacity </li></ul><ul><li>Inelastic bending demands are very high, moments slightly exceed M p (1.1M p ) </li></ul>Test Results – Column Demands Brace Buckling Moment Axial Load
  38. 38. <ul><li>Predominant force in SCBF beams are moments, due to: </li></ul><ul><ul><li>Unbalanced brace vertical resultants as buckling and fracture occurs – especially at third story beam </li></ul></ul><ul><ul><li>Frame action </li></ul></ul>Test Results – Beam Demands P t P t P t P c P c P c
  39. 39. Test Results – Beam Demands First Story Beam Moments due to uneven brace buckling and brace fracture Third Story Beam Moments due to unbalanced vertical brace forces Second Story Beam Moments due to frame action
  40. 40. <ul><li>AISC recommends designing chevron beams for full unbalanced load (non-composite capacity) </li></ul><ul><li>However, no significant damage or deflections were noted during testing </li></ul><ul><li>Deflections did not exceed 4-mm </li></ul>Test Results – Beam Demands P t 0.3P n
  41. 41. <ul><li>General </li></ul><ul><ul><li>All specimens achieved increased drift ranges when compared to current AISC design SCBFs </li></ul></ul><ul><ul><li>Balanced design procedure maximized system ductility by extending inelastic behavior beyond brace </li></ul></ul><ul><ul><li>Behavior of single story, single bay corner gusset is also applicable to multi-story SCBFs </li></ul></ul><ul><ul><li>Specimens exhibited good distribution of inelastic behavior over first two stories </li></ul></ul><ul><ul><li>Decreased contribution from third story </li></ul></ul>Conclusions
  42. 42. <ul><li>Connection clearances </li></ul><ul><ul><li>6t p vertical clearance on middle gusset plates </li></ul></ul><ul><ul><ul><li>Provides for substantial ductility and OOP rotational ability </li></ul></ul></ul><ul><ul><ul><li>Controls gusset plate buckling and eliminates need for edge stiffeners </li></ul></ul></ul><ul><ul><li>8t p elliptical clearance </li></ul></ul><ul><ul><ul><li>Allows for substantial OOP rotation on corner gusset plates </li></ul></ul></ul><ul><ul><ul><li>Valid only for corner gusset plates because stiffening is required on midspan gussets </li></ul></ul></ul><ul><ul><li>2t kp clearance performed well however may require modification </li></ul></ul>Conclusions
  43. 43. <ul><li>Brace type (HSS vs. Wide-flange) </li></ul><ul><ul><li>Type of brace has a large effect on SCBF performance </li></ul></ul><ul><ul><li>Wide flange braces more ductile (can achieve approximately 25% higher story drifts than HSS braces) </li></ul></ul><ul><ul><li>However, wide flange braces: </li></ul></ul><ul><ul><ul><li>Increase connection demand and damage </li></ul></ul></ul><ul><ul><ul><li>More rapid loss of compressive load after buckling than HSS braces </li></ul></ul></ul><ul><ul><ul><li>May increase risk for a soft story </li></ul></ul></ul><ul><ul><ul><li>Have a more pinched hysteretic behavior than HSS braces and thus dissipate less energy on an equal drift basis </li></ul></ul></ul>Conclusions
  44. 44. <ul><li>In-plane buckling HSS braces </li></ul><ul><ul><li>Viable option for SCBFs </li></ul></ul><ul><ul><li>Achieve comparable drifts to OOP buckling HSS braces </li></ul></ul><ul><ul><li>Energy dissipation equal for OOP and in-plane buckling braces </li></ul></ul><ul><ul><li>Increased post-earthquake serviceability </li></ul></ul><ul><ul><li>Buckling not impeded by walls, exteriors or studs </li></ul></ul>Conclusions
  45. 45. <ul><li>Investigation of larger knife plate clearances (2.5t kp – 3t kp ) </li></ul><ul><li>Test SCBF specimen with a pseudo-dynamic loading protocol with varying brace sizes at each level </li></ul><ul><ul><li>More accurate representation of seismic loading </li></ul></ul><ul><li>Further evaluate chevron brace configuration to assess demands on beam </li></ul>Future Work
  46. 46. Acknowledgements <ul><li>K.C. Tsai and the NCREE staff </li></ul><ul><li>My advisors – Charles Roeder and Dawn Lehman </li></ul><ul><li>Po-Chien Hsiao, Kelly Clark, Jake Powell </li></ul><ul><li>NEES and NSF </li></ul><ul><li>Our advisory committee </li></ul>
  47. 47. Questions/Comments?
  48. 48. <ul><li>Balanced design procedure </li></ul><ul><ul><li>Broken into yield mechanisms and failure mechanisms </li></ul></ul><ul><ul><li>Yield mechanisms – increase ductility and are encouraged </li></ul></ul><ul><ul><ul><li>Whitmore yielding ( β =1.0) </li></ul></ul></ul>TCBF2 Design
  49. 49. <ul><li>Balanced design procedure </li></ul><ul><ul><li>Failure mechanisms – result in total loss of load carrying capacity – should be discouraged </li></ul></ul><ul><ul><li>Net section fracture ( β =0.90) </li></ul></ul>TCBF2 Design
  50. 50. <ul><li>Interface weld tearing </li></ul><ul><ul><li>Failure mechanism – however limited ductile tearing is not a bad thing </li></ul></ul><ul><ul><li>Design for full capacity of gusset plate instead of brace ( β =0.65) </li></ul></ul>TCBF2 Design
  51. 51. <ul><li>Third story drift approximately 50% lower than first and second story </li></ul><ul><li>Third story (with braces) elastic stiffness is not overly large compared to other stories </li></ul><ul><li>Due to increased bare frame stiffness of the third story </li></ul><ul><ul><li>Bounded by moment connected beams </li></ul></ul><ul><ul><li>Larger third story beam </li></ul></ul><ul><li>Less demand on brace due to stiffer framing elements </li></ul>Test Results – 3 rd Story Drift 1083 1017 1296 146 80 101 Elastic Stiffness (kip/% rad)
  52. 52. <ul><li>Subhash Goel (1982, 1985, 1989) </li></ul><ul><ul><li>Behavior of double angle braces and gusset plates </li></ul></ul><ul><ul><li>Establish criteria for gusset plate ductility </li></ul></ul><ul><ul><li>Investigate single story, X-brace configuration </li></ul></ul><ul><li>Issues </li></ul><ul><ul><li>Atypical brace sections for seismic regions </li></ul></ul><ul><ul><li>Pinned end, reusable test frame </li></ul></ul>Past Research on SCBFs
  53. 53. <ul><li>Minoru Wakabayshi (1977, 1979) </li></ul><ul><ul><li>Impact of varying brace cross-sections </li></ul></ul><ul><ul><li>In-plane vs. OOP brace buckling </li></ul></ul><ul><ul><li>Hysteretic responses of brace sections </li></ul></ul><ul><li>Issues </li></ul><ul><ul><li>Slender brace sections (< 5-cm deep) </li></ul></ul><ul><ul><li>Reusable, pinned end test frame </li></ul></ul>Past Research on SCBFs
  54. 54. <ul><li>Steve Mahin (2004) </li></ul><ul><ul><li>Behavior of multi-level, chevron SCBF </li></ul></ul><ul><ul><li>Validation for computer models </li></ul></ul><ul><ul><li>System behavior of SCBFs </li></ul></ul><ul><li>Issues </li></ul><ul><ul><li>Weak column phenomenon </li></ul></ul><ul><ul><li>No slab </li></ul></ul><ul><ul><li>Formation of soft story </li></ul></ul><ul><ul><li>Low </li></ul></ul>Past Research on SCBFs
  55. 55. <ul><li>Overview of SCBFs </li></ul><ul><li>Past research – Why? </li></ul><ul><li>Summary of project </li></ul><ul><li>TCBF2 Overview </li></ul><ul><ul><li>Specimens </li></ul></ul><ul><ul><li>Setup </li></ul></ul><ul><ul><li>Design </li></ul></ul><ul><li>TCBF2 Results </li></ul><ul><ul><li>Varying brace shapes </li></ul></ul><ul><ul><li>Buckling directions </li></ul></ul><ul><ul><li>Framing element demands </li></ul></ul><ul><li>Conclusions and Recommendations </li></ul>Agenda
  56. 56. Past Research on SCBFs Subhash Goel (1982, 1985, 1989) Minoru Wakabayshi (1977, 1979) Steve Mahin (2003)
  57. 57. TCBF2 Overview <ul><li>Most accurate SCBF specimens to date </li></ul><ul><ul><li>Tested vertically </li></ul></ul><ul><ul><li>Additional demands from concrete slab (7.87-in) </li></ul></ul><ul><ul><li>Full scale </li></ul></ul><ul><ul><li>Representative beam/column sizes </li></ul></ul><ul><li>Reused frame between tests </li></ul><ul><ul><li>Simply removed and replaced gusset plates and braces </li></ul></ul><ul><ul><li>Allowed for rapid testing </li></ul></ul><ul><li>Used U.S. beam/column shapes </li></ul>
  58. 58. <ul><li>Single story, single bay </li></ul><ul><ul><li>12-ft x 12-ft specimen </li></ul></ul><ul><ul><li>HSS5x5x3/8 brace </li></ul></ul><ul><ul><li>Establish a balanced design procedure </li></ul></ul><ul><ul><li>Determine effects of varying parameters on system and connection performance </li></ul></ul><ul><ul><li>New gusset plate clearance limit </li></ul></ul><ul><ul><ul><li>Elliptical model </li></ul></ul></ul><ul><ul><li>30 test completed to date </li></ul></ul><ul><ul><li>Results used to design TCBF specimens </li></ul></ul>University of Washington
  59. 59. <ul><li>TCBF 1 (Tomorrows Concentrically Braced Frame 1) </li></ul><ul><ul><li>Kelly Clark </li></ul></ul><ul><ul><li>International testing done at NCREE in Taiepi, Taiwan </li></ul></ul><ul><ul><li>First phase was composed of three tests - 2008 </li></ul></ul>TCBF1 TCBF 1-1 (8t,R) TCBF 1-2 (8t,WF) TCBF 1-2 (2t,T)
  60. 60. <ul><li>Design of middle gusset plates in TCBF1 </li></ul><ul><ul><li>Used elliptical clearance </li></ul></ul>TCBF1 No Vertical or Edge Stiffeners Only Vertical Stiffeners Vertical and Edge Stiffeners
  61. 61. <ul><li>Test frame reused between tests </li></ul><ul><ul><li>A lot of frame damage occurred in TCBF2-HSS </li></ul></ul><ul><ul><li>This lead to a decrease in elastic stiffness in later tests </li></ul></ul>Test Results – Test Frame Beam/Column Yielding Slab Cracking Elastic Stiffness (kip/ % rad) TCBF2-HSS TCBF2-WF TCBF2-IP 1235.6 1349.6 1116.1
  62. 62. <ul><li>Yielding not easily indicated in framing elements </li></ul><ul><li>No strain hardening, lower maximum resistance in TCBF2-WF and TCBF2-IP </li></ul>Test Results – Test Frame
  63. 63. <ul><li>Energy dissipation </li></ul><ul><ul><li>On an equal drift basis, OOP and in-plane buckling HSS braces dissipate similar energy </li></ul></ul><ul><ul><li>Difference is 3NBr buckled OOP </li></ul></ul>Test Results – Global TCBF2-HSS TCBF2-IP
  64. 64. <ul><li>Framing element demands </li></ul><ul><ul><li>Large demands are placed on SCBF columns from: </li></ul></ul><ul><ul><ul><li>Axial forces from vertical brace resultant forces </li></ul></ul></ul><ul><ul><ul><li>Inelastic bending moments as less load is carried by braces </li></ul></ul></ul><ul><ul><li>Demands are placed on SCBF beams from: </li></ul></ul><ul><ul><ul><li>Unbalanced vertical brace resultant forces </li></ul></ul></ul><ul><ul><ul><li>Brace fracture </li></ul></ul></ul><ul><ul><ul><li>Frame action </li></ul></ul></ul>Conclusions
  65. 65. <ul><li>Traditional AISC design </li></ul><ul><ul><li>Uses φ factors to ensure resistance against various limit states ( φ R n ) </li></ul></ul><ul><ul><li>φ factors based on strength, safety and statistically extreme considerations </li></ul></ul><ul><ul><li>Ensures connection is stronger than brace with no consideration for connection ductility </li></ul></ul><ul><li>Balanced design procedure </li></ul><ul><ul><li>Used on all TCBF2 specimens </li></ul></ul><ul><ul><li>Uses β factor to ensure ductility instead of resistance </li></ul></ul><ul><ul><li>Allows for yielding in elements that increases ductility </li></ul></ul><ul><ul><li>Discourages failure mechanisms </li></ul></ul>TCBF2 Design-Balanced Design