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Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, 2012 1 Collapse Assessment of Steel Braced.

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Presentation on theme: "Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, 2012 1 Collapse Assessment of Steel Braced."— Presentation transcript:

1 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Collapse Assessment of Steel Braced Frames In Seismic Regions Dimitrios G. Lignos, Ph.D. Assistant Professor, McGill University, Montreal, Canada Emre Karamanci, Graduate Student Researcher, McGill University, Montreal, Canada July 9 th -12 th, 2012

2 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Outline Motivation A Database for Modeling of Post-Buckling Behavior and Fracture of Steel Braces Calibration Studies Case #1: E-Defense Dynamic Testing Case #2: 2-Story Chevron Braced Frame Collapse Assessment Summary and Observations

3 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, In the context of Performance-Based Earthquake Engineering, collapse constitutes a limit state associated with complete loss of a building and its content. Understanding collapse is a fundamental objective in seismic safety since this failure mode is associated with loss of lives. Therefore, there is a need for reliable prediction of the various collapse mechanisms of buildings subjected to earthquakes. Dimitrios G. Lignos Quake Summit, San Francisco 2010Motivation

4 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, In the case of steel braced frames, one challenge for reliable collapse assessment is to accurately model the post-buckling behavior and fracture of steel braces as parts of a braced frame. 2.Another challenge is to consider other important deterioration modes associated with plastic hinging in steel components that are part of local story mechanisms that develop after the steel braces fracture This could be an issue for steel braced frames designed in moderate or high seismicity regions. 3.The emphasis is on a common collapse mode associated with sidesway instability in which P-Delta effects accelerated by cyclic deterioration in strength and stiffness of structural components fully offset the first order story shear resistance of a steel braced frame and dynamic instability occurs. Dimitrios G. Lignos Quake Summit, San Francisco 2010Motivation

5 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Steel Brace Model Gusset Plate flexibility and yield moment are modeled with the model proposed by Roeder et al. (2011) ? Model proposed by (Uriz et al. 2008) ε o indicates the strain amplitude at which one complete Cycle of a undamaged material causes fracture m material parameter that relates the sensitivity of a total strain amplitude of the material to the number of cycles to fracture

6 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, LHLH LBLB LBLB LHLH LHLH LBLB LHLH LBLB LHLH LBLB Steel Brace Database for Model Calibration Collected Data from 20 different experimental programs from the 1970s to date 143 Hollow Square Steel Sections 51 Pipes 50 W Shape braces 37 L Shape Braces Digitization of axial load axial displacement relationships (Calibrator JAVA software, Lignos and Krawinkler 2008)

7 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Steel Brace Database Slenderness Parameter Based on the local slenderness ratios (b/t), the majority of the braces are categorized as Class 1 based on CISC (2010) requirements (Same conclusions based on AISC 2010 Highly ductile braces)

8 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Calibration Process of the Brace Model Objective Function H Mesh Adaptive Search Algorithm (MADS, Abramson et al. 2009) Non-differentiable Optimization problem lacks of smoothness. MADS does not use information about the gradient of H to search for an optimal point compared to more traditional optimization algorithms. F exp : Experimentally measured axial force of the brace F simul : Simulated axial force of the brace δ i : Axial displacement of the brace at increment i

9 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Calibration Process of the Brace Model Based on a sensitivity study with a subset of 30 braces: Offset of 0.1% of the brace length is adequate Eight elements along the length of the steel brace Five integration points per element Section level: Stress strain relationship: Strain hardening of 0.1% Radius that defines Bauschinger effect R o =25 Based on the calibration study of the entire set of braces Exponent m =0.3 Strain amplitude ε o is a function of KL/r, b/t, f y

10 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Model Parameter Calibrations (Data from Tremblay et al. 2008) (Data from Uriz and Mahin 2008)

11 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, x: Lateral bracing Chevron CBF, 70%-scale HSS braces: b/t = 19.4, KL/r = 82.5 Validation with a Chevron CBF E-Defense (Okazaki, Lignos, Hikino and Kajiyara, 2012)

12 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Load Cells Connecting Beam Connecting Beam Test Bed Specimen Shake Table Direction of Shaking N E-Defense Chevron CBF: Test Setup (Okazaki, Lignos, Hikino and Kajiyara, 2012)

13 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, E-Defense Chevron CBF: Test Setup (Okazaki, Lignos, Hikino and Kajiyara, 2012) JR Takatori (1995 Kobe EQ) 10, 12, 14, 28, 42, 70% Damping h 0.03 inherent in test-bed system

14 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, East Brace 42% 70% Response of Braces: 70% JR Takatori (Okazaki, Lignos, Hikino and Kajiyara, 2012)

15 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Global Response: 70% JR Takatori (Okazaki, Lignos, Hikino and Kajiyara, 2012)

16 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, x9.5 A500 Gr. B) 152x9.5 A500 Gr. B) 152x9.5 A500 Gr. B) 152x9.5 A500 Gr. B) Column W10x45 Beam W24x117 2,743 6,096 Reaction Beam PL 22 (A572 Gr.50) PL 22 (A572 Gr.50) Lateral support Case Study #2: 2-Story Chevron Braced Frame (Uriz and Mahin, 2008)

17 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Rigid offset Steel beam & column spring (Bilinear Modified IMK Model) Shear connection spring (Pinching Modified IMK Model) Gusset plate spring (Menegotto-Pinto model) Case Study #2: 2-Story Chevron Braced Frame

18 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Rigid offset Steel beam & column spring (Bilinear Mod. IMK Model) Shear connection spring (Pinching Mod. IMK Model) Gusset plate spring (Menegotto-Pinto model) Case Study #2: 2-Story Chevron Braced Frame Liu and Astaneh (2004)

19 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Rigid offset Steel beam & column spring (Bilinear Mod. IMK Model) Shear connection spring (Pinching Mod. IMK Model) Gusset plate spring (Menegotto-Pinto model) Case Study #2: 2-Story Chevron Braced Frame (Ibarra et al. 2005, Lignos and Krawinkler 2011)

20 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Case Study #2: 2-Story Chevron Braced Frame (Lignos and Krawinkler 2011)

21 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Case Study #2: Loading Protocol (Uriz and Mahin, 2008)

22 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Case Study #2: Quasi-Static Analysis-Global Response

23 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Case Study #2: Incremental Dynamic Analysis Collapse Capacities seem a bit high Indicates that a closer look of the individual responses in terms of base shear hysteretic response is needed and not just story drift ratios. Based on 2% Rayleigh Damping (damping matrix proportional to initial stiffness)

24 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Validation of Simulated Collapse-Small Scale Tests (Lignos, Krawinkler & Whittaker 2007) (NEESCollapse) Collapse

25 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Validation of Simulated Collapse-Full Scale Tests (Suita et al. 2008) (Lignos, Hikino, Matsuoka, Nakashima 2012) Collapse

26 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Canoga Park Record: Story Drift Ratio Histories SF=2.0

27 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Dynamic Analysis: Base Shear-SDR1 Due to Artificial Damping Artificial damping is generated in the lower modes with the effective damping increasing to several hundred percent. Following the change in state of steel braces after fracture occurs, large viscous damping forces are generated. This forces are the product of the post-event deformational velocities multiplied by the initial stiffness and by the stiffness proportional coefficient.

28 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, IDA Curves: Damping Based on Current Stiffness Based on 2% Rayleigh Damping (damping matrix proportional to current stiffness)

29 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Dynamic Analysis: Story Drift Ratios (SF=2.0)

30 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Dynamic Analysis: Brace Response

31 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Base Shear-First Story Collapse Intensity Collapse Fracture of East Brace Fracture of West Brace

32 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, First Story Column Collapse Intensity 152x9.5 A500 Gr. B) 152x9.5 A500 Gr. B) 152x9.5 A500 Gr. B) 152x9.5 A500 Gr. B) PL 22 (A572 Gr.50) PL 22 (A572 Gr.50) Lateral support

33 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Summary and Observations 1. Modeling of Post-Buckling Behavior and Fracture Initiation of Steel Braces is Critical for Evaluation of Seismic Redundancy of Steel Braced Frames. Proposed steel brace fracture modeling for different types of steel braces is based on calibration studies from 295 tests. 2. For collapse simulations of sidesway instability, modeling of component deterioration of other structural components is also critical (Beams and Columns) 3. Non-simulated collapse criteria could be dangerous. Story drift in conjunction with base shear of the system needs to be considered. 4. Modeling of damping can substantially overestimate the collapse capacity of steel braced frames For Rayleigh Damping, damping matrix proportional to current stiffness should be considered.

34 Collapse Assessment of Steel Braced Frames in Seismic Regions, Dimitrios G. Lignos, QuakeSummit, Boston, July, Acknowledgments Dr. Uriz and Prof. Steve Mahin (University of California, Berkeley) for sharing the digitized data of individual steel brace components and systems that tested over the past few years. Professor Benjamin Fell (Sacramento State) for sharing the digitized data of steel brace components that he tested 4 years ago at Berkeley.


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