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Self-Centering Steel Frame Systems

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1 Self-Centering Steel Frame Systems
NEESR-SG: Self-Centering Damage-Free Seismic-Resistant Steel Frame Systems Project Team Richard Sause, James Ricles, David Roke, Choung-Yeol Seo, Michael Wolski, Geoff Madrazo ATLSS Center, Lehigh University Maria Garlock, Erik VanMarcke, Li-Shiuan Peh Princeton University Judy Liu Purdue University Keh-Chyuan Tsai NCREE, National Taiwan University

2 Current NEESR Project NEESR-SG: Self-Centering Damage-Free Seismic-Resistant Steel Frame Systems
This material is based on work supported by the National Science Foundation, Award No. CMS , in the George E. Brown, Jr. Network for Earthquake Engineering Simulation Research (NEESR) program, and Award No. CMS NEES Consortium Operation.

3 Motivation: Expected Damage for Conventional Steel Frames
Conventional Moment Resisting Frame System The motivation for our project is the expected performance of conventional steel frames, which as is well-documented, will experience some damage under the design level earthquake, perhaps like this upper photo, even when they perform as intended. Our goal is to develop steel frame systems without such damage under the design earthqake. Reduced beam section (RBS) beam-column test specimen with slab: (a) at 3% drift, (b) at 4% drift.

4 Self Centering (SC) Seismic-Resistant System Concepts
Discrete structural members are post-tensioned to pre-compress joints. Gap opening at joints at selected earthquake load levels provides softening of lateral force-drift behavior without damage to members. PT forces close joints and permanent lateral drift is avoided. M The systems we are developing use the following concepts that we initially applied to precast shear walls about 10 years ago:

5 Previous Work on SC Steel Moment Resisting (MRF) Connections
W24x62 (Fy 248 MPa), or W36x150 (Fy 350 MPa) W14x311, W14x398 (Fy 350 MPa) or CFT406x406x13 (Fy 345 MPa) PT Strands 3660 to 4120 mm Current project is built upon some previous work at Lehigh on SC steel MRFs with these post-tensioned connections MRF Subassembly with PT Connections 6100 to 8540 mm

6 Initial Stiffness Is Similar to Stiffness of Conventional Systems
D H PT Steel MRF Stiffness with welded connection MRF subassembly with post-tensioned connections The next few slides summarize some of the observed behavior. Here you see a typical hysteresis loop from a SC system, with the self-centering behavior, the force-deformation behavior always returning to the origin. The slide shows that before decompression and gap opening, the initial stiffness is similar to that of a conventional system.

7 Lateral Force-Drift Behavior Softens Due to Gap Opening
Steel MRF subassembly with post-tensioned connections and angles at 3% drift This slide shows the gap opening behavior and the result that there is essentially no damage at 3% drift, even though there is a softening in the force-deformation behavior as shown in the previous slide.

8 Lateral Force-Drift Behavior Softens Without Significant Damage
Conventional steel MRFs soften by inelastic deformation, which damages main structural members and results in residual drift SC steel MRF softens by gap opening and reduced contact area at joints Post-Tensioned Connection Welded Connection D H

9 Energy Dissipation from Energy Dissipation (ED) Elements
H Steel MRF Specimen PC2 L6x6x5/16, g/t = 4 Specimen PC4 L8x8x5/8, g/t = 4 Steel MRF subassemblies with post-tensioned connections with different size ED elements. By using larger angles the connection is capable of developing more energy dissipation and higher strength. The stiffness after decompression is increased with a larger angle.

10 Limited, Repairable Damage
Angle fracture Before testing @ 4% Drift After testing

11 Summary of SC Seismic-Resistant Structural System Behavior
Initial lateral stiffness is similar to that of conventional seismic-resistant systems. Lateral force-drift behavior softens due to gap opening at selected joints and without significant damage to main structural members. Lateral force-drift behavior softening due to gap opening controls force demands. Energy dissipation provided by energy dissipation (ED) elements, not from damage to main structural members.

12 NEESR-SG: Self-Centering Damage-Free Seismic-Resistant Steel Frame Systems
Project Scope. Project Goals. Status of Selected Research Tasks. Summary.

13 NEESR-SG: SC Steel Frame Systems Project Scope
Develop two SC steel frame systems: Moment-resisting frames (SC-MRFs). Concentrically-braced frames (SC-CBFs). Conduct large-scale experiments utilizing: NEES ES (RTMD facility) at Lehigh. non-NEES laboratory (Purdue). international collaborating laboratory (NCREE) Conduct analytical and design studies of prototype buildings. Develop design criteria and design procedures.

14 NEESR-SG: SC Steel Frame Systems Project Goals
Overall: self-centering steel systems that are constructible, economical, and structurally damage-free under design earthquake. Specific: Fundamental knowledge of seismic behavior of SC-MRF systems and SC-CBF systems. Integrated design, analysis, and experimental research using NEES facilities. Performance-based, reliability-based seismic design procedures.

15 NEESR-SG: Self-Centering Damage-Free Seismic-Resistant Steel Frame Systems
Project Scope. Project Goals. Status of Selected Research Tasks. Summary.

16 NEESR-SG: SC Steel Frame Systems Project Research Tasks
Develop reliability-based seismic design and assessment procedures. Develop SC-CBF systems. Further develop SC-MRF systems. Develop energy dissipation elements for SC-MRFs and SC-CBFs. Develop sensor networks for damage monitoring and integrity assessment. Design prototype buildings. Perform nonlinear analyses of prototype buildings. Conduct large-scale laboratory tests of SC-MRFs and SC-CBFs. Collaborate on 3-D large-scale laboratory tests on SC-MRF and SC-CBF systems.

17 Task 2. Develop SC-CBF Systems: SC-CBF System Concept
Rocking behavior of simple SC-CBF system.

18 More Complex SC-CBF Configurations Being Considered
base V P01 P01+P g P02 P02+P col roof

19 SC-CBF Design Criteria
Column Decompression PT Yielding Significant Yielding of Frame Members Failure of Frame Members DBE MCE Lateral Force Roof Drift IO CP LS Member yields PT steel yields Slide shows the overall performance goals for the system and where we expect the critical limit states to be reached. Under the design level earthquake we expect immediate occupancy. Column decompression and uplift is acceptable, but yielding of the post-tensioning is not. Beyond the DBE, PT yielding followed by significant yielding of the braves, beams, and columns (in that order) is acceptable. For the maximum considered earthquake the expected performance is still at the life safety level. Δgap

20 Current Work on SC-CBF Systems
Evaluate frame configurations. Evaluate effect of energy dissipation (ED) elements. Develop and evaluate performance-based design approach.

21 SC-CBF Configurations Studied
PT PT PT ED ED Frame A Frame B12 Frame B12ED

22 Dynamic Analysis Results (DBE)
1.4% 2.7% Roof drift: Effect of frame configuration. Effect of ED elements. 2.4% 2.7%

23 Pushover Analysis Results
PT Yield Decompression Roof Drift (%) Base Shear (V/Vdes)

24 Preliminary Results for SC-CBF
Dynamic analysis results indicate self-centering behavior is achieved under DBE. Frame A has lower drift capacity before PT yielding than Frame B: PT steel is at column lines rather than mid-bay. Frame A also has lower drift demand. Energy dissipation helps to reduce drift demand and improve response.

25 Task 3. Further Develop SC-MRF Systems: Current Work
Study of interaction between SC-MRFs and floor diaphragms by Princeton and Purdue. SC column base connections for SC-MRFs being studied by Purdue.

26 Interaction of SC-MRFs and Floor Diaphragms (Princeton)
2Dgap qr Dgap Collector Beams Approach 1. Transmit inertial forces from floor diaphragm without excessive restraint of connection regions using flexible collectors.

27 Interaction of SC-MRFs and Floor Diaphragms (Purdue)
Approach 2. Transmit inertial forces from floor diaphragm within one (composite) bay for each frame.

28 SC Column Base Connections for SC-MRFs (Purdue)
Post-Tensioned Bars Reinforcing Plate Energy Dissipation Plate Slotted Keeper Angle Beam at Grade

29 Moment-Rotation Response at Column Base
Identifying appropriate level of column base moment capacity and connection details, leading to laboratory experiments.

30 Task 4. Develop Energy Dissipation Elements for SC-MRFs
SC systems have no significant energy dissipation from main structural elements: Behavior of energy dissipation elements determined SC system energy dissipation. Energy dissipation elements may be damaged during earthquake and replaced. For SC-MRFs, energy dissipation elements are located at beam-column connections.

31 Quantifying Energy Dissipation
Define relative hysteretic ED ratio bE bE : Relative ED capacity bE = x 100(%) Area of yellow Area of blue For SC systems: 0 ≤ bE ≤ 50% Target value: bE = 25% Hysteresis Loop

32 ED Element Assessment Consider several ED elements:
Metallic yielding, friction, viscoelastic, elastomeric, and viscous fluid. Evaluation criteria: Behavior, force capacity versus size, constructability, and life-cycle maintenance. Friction ED elements selected for further study.

33 Bottom Flange Friction Device
Friction bolts Friction PL Column angle Col. angle bolts Slotted plate W36x300

34 BFFD Moment Contribution
BFFD contribution to connection moment capacity COR+ COR- MFf+ = Ff ∙r+ MFf- = Ff ∙r - MFf = Ff∙r |MFf+ | > |MFf- |

35 Test Setup Beam Bracing Bracing PT Strands Column 3/5 Scale BFFD
W21x111 Beam Bracing Bracing PT Strands Column 3/5 Scale Test setup elevation BFFD Test setup

36 Test Matrix Test No. Loading Protocol qr,max (rads) Experimental
Parameter 1 CS 0.035 Reduced Friction Force 2 0.030 Design Friction Force 3 Fillet Weld Repair 4 EQ 0.025 Response to EQ Loading 5 0.065 Effect of Bolt Bearing 6 Assess Column L Flex., CJP 7 Effect of Bolt Bearing, CJP CS: Cyclic Symmetric EQ: Chi-Chi MCE Level Earthquake Response

37 Test 2: Design Friction Force
Beginning of Test 2 qr = rads qr = rads

38 Test 2: Response -1.0 -0.5 0.0 0.5 1.0 -0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04 Rotation, q r (rads) Normalized Moment, M/M p,n theoretical decompression stiffness reduction imminent gap opening M + -

39 Test 2: Comparison with Simplified Model
Md + M+Ff 2M-Ff 2M+Ff 1 Axial stiffness of PT strands & beam Md + M-Ff

40 Results for ED Elements for SC-MRFs
Friction ED element: Reliable with repeatable and predictable behavior. Large force capacity in modest size. BFFD: Provides needed energy dissipation for SC-MRF connections. When anticipated connection rotation demand is exceeded, friction bolts can be designed to fail in shear without damage to other components.

41 Task 8. Conduct Large-Scale Laboratory Tests
Two specimens, one SC-MRF and one SC-CBF, tested at Lehigh NEES ES (RTMD facility). 2/3-scale 4 story frame. Utilize hybrid test method (pseudo dynamic with analytical and laboratory substructures). Utilize real-time hybrid test method, if energy dissipation elements are rate-sensitive.

42 9. Collaborate on 3-D Large-Scale Laboratory Tests
Large-scale 3-D SC steel frame system tests at NCREE in Taiwan under direction of Dr. K.C. Tsai. Interaction of SC frame systems with floor diaphragms and gravity frames will be studied. 3-D tests are part of Taiwan program on SC systems. Project team is collaborating with Taiwan researchers: US-Taiwan Workshop on Self-Centering Structural Systems, June 6-7, 2005, at NCREE. 2nd workshop planned for October 2006 at NCREE.

43 Summary Two types of SC steel frame systems are being developed:
Moment-resisting frames (SC-MRFs). Concentrically-braced frames (SC-CBFs). Research plan includes 9 major tasks: Significant work completed on 7 tasks. Numerous conference publications available from current project. Large-scale experiments utilizing NEES ES at Lehigh are being conducted. Ongoing collaboration with NCREE in Taiwan.

44 Self-Centering Steel Frame Systems
Thank you.

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