3rd Annual EHKS Retreat March 10, 2007 Allerton Park

1. Controlled Rocking Steel Frames with Replaceable Energy-Dissipating Fuses Matt Eatherton, MS SE University of Illinois at Urbana-Champaign 3rd Annual EHKS Retreat March 10, 2007 Allerton Park

2. My Background • Born in Kansas City, 1975 • BS in CE - University of Missouri at Columbia, 1997 • MS in CE - University of Missouri at Columbia, 1999 • Master’s research involved instrumenting and monitoring four prestressed bridge girders with over 150 gages during construction and for one year in service. • 2 years structural design experience in Kansas City • 5 years structural design experience in San Francisco • Volunteer Structural Engineering Activities • Build Change – improving seismic resistance of housing in developing countries • SEAONC subcommittee – investigated diaphragm forces • Steel Plate Shear Walls – several projects, conference articles, design examples, and other involvement • Began PhD program at UIUC in fall 2006 with the goal of getting an academic position afterward

3. Organization • Introduction • Controlled Rocking System • Parametric Study & Prototype Building • UIUC Half-Scale Test Program • Conclusions

4. Expected Building Performance Two story steel-framed office building in Santa Clarita suffered residual drift in the first story due to the Northridge Earthquake. Building with a Red Tag restricting access after the Northridge Earthquake Industrial Structure that experienced brace buckling and residual drift during Loma Prieta From EERI Earthquake Recon. Report, Jan. 1996 & May 1990

5. Controlled Rocking System Component 3 – Replaceable energy dissipating fuses take majority of damage Component 2 – Post-tensioning strands bring frame back down during rocking Component 1 – Stiff braced frame, designed to remain essentially elastic - not tied down to the foundation. Bumper or Trough

6. Rocked Configuration • Corner of frame is allowed to uplift. • Fuses absorb seismic energy • Post-tensioning brings the structure back to center. Result is a building where the structural damage is concentrated in replaceable fuses and virtually no residual drift!

7. Design Equations - Overturning Resistance FPT = Initial post-tension force Vp = Fuse yield strength in shear Overturning moment = Resistance comes from Post-Tensioning and Fuses: In an LRFD context use a resistance factor to design: Can also include gravity loads

8. Design Equations - Self-Centering Mechanism In the rocked configuration, the fuses resist self-centering. The restoring moment due to P/T must overcome the restoring resistance: • Other sources of resistance not considered in this equation include: • Stiffness of gravity system • Stiffness of interior partitions that have undergone inelastic damage • P-delta effect • Can also include effect of gravity load in restoring force.

9. Fuse Shear Strain Demand Using small angle assumption: Shear strain in the fuses is amplified compared to the roof drift ratio (RDR). Fuse Shear Strain, g = Example:

10. Controlled Rocking – Hysteretic Response • FLAG SHAPED HYSTERESIS • Begin Loading • Frame Uplifts • Fuses Yield • Load reversal. If pushed far enough P/T would yield • Zero force in fuses • Fuses yield in other direction • Frame sets back down and forces in the frame relax. • Elastic strain energy remains in frame and fuses 3 4 5 2 7 1 6 8

11. Other Design Considerations • Global Overturning (FPT > Vp) • Initial P/T stress: Stressing the P/T strands 0.4 Fu may require special procedures to anchor post-tensioning (post-blocking). • P/T strain capacity: If performance criteria includes not replacing P/T after a severe earthquake then ensure adequate strain capacity. Preventing Global Overturning

12. Prototype Structure Use prototype structure to apply controlled rocking to a realistic structure Based on SAC Building configuration Tests and analysis simulate the controlled rocking frames in this structure.

13. Prototype – Controlled Rocking Frames

14. UIUC Half Scale Tests • Goals: • To test and improve details – post-tensioning and base connections are not typical to steel structures. • Study the forces realized in the fuses and distribution of force between fuses. Geometric nonlinearity and indeterminacy creates complexity. • Examine effect of out-of-plane motion while rocking. • Determine whether typical P/T strands and anchorage can be stressed to yield without fracturing or slipping. • Establish whether there is inelasticity or relaxation in the P/T that would require replacement or re-stressing. • Investigate whether inelasticity occurs in the frame. VERIFY THE PERFORMANCE OF THE SYSTEM FOR IMPLEMENTATION IN PRACTICE

15. UIUC Half Scale Tests Front View Side View

16. Test Matrix

17. Mixed Mode Control The horizontal movement of the Left LBCB would be used to control the test. The Right LBCB will match the horizontal force in the left LBCB. This will apply the same amount of load to both frames, but allow differential rocking between the frames.

18. Summary • Seismic loads prescribed in current building codes assume considerable inelasticity in the structure during a severe earthquake. This can result in structural damage and residual drift that cannot be economically repaired. • To provide a building that is relatively easy to repair after an earthquake, two attractive performance criteria are: • Eliminate residual drift. • Concentrate bulk of structural damage in replaceable fuses. • The controlled rocking system satisfies these performance goals. • The controlled rocking system consists of three major components: • Stiff steel braced frame designed to remain essentially elastic, but not tied down to the foundation. • Post-tensioning that provides self-centering capability. • Highly ductile energy dissipating fuses. • A multi-institution, international research project is underway to examine, improve, and validate the performance of this innovative system.

19. Summary • A parametric study was conducted to optimize A/B ratio, OT ratio, and SC ratio. • Some considerations in the design of the controlled rocking system include: • Proportioning fuses and P/T to resist overturning, but still self-center. • Insuring enough P/T strain capacity. • Using fuses with enough shear strain capacity based on frame geometry (fuse shear strain is amplified compared to roof drift ratio). • Preclude global overturning. • Half-scale tests will be conducted later this year at the UIUC MUST-SIM Facility to improve details and validate the performance of the controlled rocking system for implementation in practice. • Hybrid simulation tests will further validate the system performance and demonstrate the self-centering and repairability of the controlled rocking system when subjected to a realistic ground motion.

20. Research Team PI Greg Deierlein – Project Manager, Stanford University Co-PI Sarah Billington – ECC & HPFRCC Fuses, Stanford Unviersity Co-PI Jerome Hajjar – Simulation and Half-Scale Tests, University of Illinois Helmut Krawinkler, Stanford University Mitsumasa Midorikawa – E-Defense, Building Research institute in Japan David Mar - Industry Collaborator, Tipping and Mar Engineers Current Graduate Students: Xiang Ma (Stanford), Matt Eatherton (UIUC) Past Graduate Students: Paul Cordova (Post-Doc at Stanford), Eric Borchers (Stanford), Kerry Hall (UIUC), Project is funded by a grant from NSF - NEESR-SG E-DEFENSE JAPAN