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Structural considerations for so cal seisimicity
- 1. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 1 Structural Considerations for SoCal Seismicity Ashwani Dhalwala, M.S.,S.E. California Engineering Group (CEG) Sources of Uncertainity • Seismic Input-PGA,Amplification,Range • Structural Model- Geometry, Soil, Stress/Strain Relationships, Buckling • Dynamic Model-Mass,Damping,Integration Operators, Nonlinear Degradation • Structural Response- Large Displacements, Connection Response
- 2. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 2 California Fault Zones San Andreas Fault
- 3. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 3 FEMA 355D Research • Numerous connection configurations tested and recommended • High notch toughness electrodes specified • Flaws introduced by backup bars eliminated • Resulted in improve connection performance FEMA 355D Research • Resulted in AISC Seismic Supplement No. 1 Welded Unreinforced Flange – Welded Web (WUF-W) and Bolted Flange Plate (BFP) Moment Connections • Adoption of RBS Moment Connection
- 4. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 4 Unresolved Issues • Reduced performance due to Size Effects • Effect of panel zone yielding • Brittle behavior due to local triaxiality • Triaxiality partially reduced by modifying the weld access hole • Rational continuity plate design • Lateral torsional buckling Unresolved Issues • Cover plated connection • Strain rate effects • Effects due to vertical accelerations • Weld residual stresses • Column/Beam moment capacity ratio
- 5. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 5 Unresolved Issues • Unresolved issues apply to all steel connections – Braced Frames, EBFs, Steel Shear Walls, Base Plates Suggested Cover Plate Design Full Scale Tests Circa 1998 Non Linear Continuum Mechanics Analysis by CEG 1999 Plastic Range in Red Proposed and used in design by CEG Simulation
- 6. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 6 CBC 2013 • Use of updated USGS maps • High Peak Ground Accelerations representing SoCal Seismicity (SCS) more accurately NF and FF Structural Response Ref: Mateescu et. Al.
- 7. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 7 NF & FF Ground Motions Ref: Mateescu et. Al. Brittle cracking in steel Column Fracture in 11 Story Building Northridge Earthquake Courtesy P.Maranian
- 8. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 8 Brittle cracking in steel • Observed during the Northridge Earthquake • Is brittle cracking a near field phenomena? Brittle cracking in steel Ref: Fracture Mechanics Texts
- 9. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 9 What happens to a material with a small crack? Yields then work hardens, absorb energy and redistribute stress. In other words, crack makes no significant difference! Get high stress around crack, crack propogates and get sudden failure. Stress around crack is high due to Kt , but nominal stress is much lower than material yield strength! What happens when you nick a brittle material?? Ref: Fracture Mechanics Texts Brittle cracking in steel • Brittle Fractures are initiated at a the atomic scale due to severing of atomic bonds – Trans-granular Fracture • Ductile Fractures – Inter-granular Fracture • Determine which one governs • Function of Connection Size, Geometry • Function of Applied Loads
- 10. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 10 A plastic zone forms at the crack tip where the stress would otherwise exceed the yield strength σy. Ductile Fracture: Stages of ductile fracture: b. Plastic def’m when stress exceeds yield. c. Weaken and fail locally due to inclusions which act as stress concentrations – this creates tiny voids. Voids continue to grow and coalesce to form larger voids. d. Remaining area gets smaller increasing stress until tensile strength is exceeded then fracture.Ref: Fracture Mechanics Texts Brittle cracking in steel • Most fracture analysis is performed using continuum mechanics principles and relates to stress fields at the crack tip where resistance of the material to crack extension “ fracture energy G” is formulated using material properties E and “v” and the Rayleigh wave speed Cr (speed of sound on a free surface) • Precisely when new cracks emerge cannot be predicted – source of uncertainty
- 11. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 11 Control of Brittle Fracture • AISC Seismic Supplement No. 1 • WUF-W Connection • Currently permitted for SMFs Control of Brittle Fracture Two main causes 1. Plane Strain Conditions 2. Ductile to Brittle Transition Temperature
- 12. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 12 Control of Brittle Fracture Plane Strain Conditions caused by 1. Triaxiality from external applied loads 2. Thickness effects 3. High Strain Rates Control of Brittle Fracture Triaxiality from external applied loads: • Reduces shear deformation • Restricts yielding of material • Material is 100C% brittle if: • Triaxiality >= Fy
- 13. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 13 Brittle cracking in steel Triaxiality Triaxiality and Strain Rates Triaxiality = (sigma1+sigma2+sigma3)/3 Seppala, Belak, Rudd – Lawrence Livermore Labs Control of Brittle Fracture Ductile to Brittle Transition Temperature
- 14. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 14 Control of Brittle Fracture Ductile to Brittle Transition Temperature • Occurs in Bcc Materials such as steel • Restricts yielding of material • Material is 100C% brittle if: • Triaxiality >= Fy Control of Brittle Fracture Ductile to Brittle Transition Temperature • Occurs in Bcc Materials such as steel • Lower shelf – cleavage failure • Higher shelf – void coalescence • Intermediate – mixture of the two above
- 15. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 15 Control of Brittle Fracture Ductile to Brittle Transition Temperature • Transition at higher temperature results in early brittle failure. Control of Brittle Fracture Ductile to Brittle Transition Temperature Transition temperature increased by: • High stresses • High strain rates • Thick material • Weld residual stresses and fast cooling rates • Hydrogen entrapment (embrittlement)
- 16. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 16 Control of Brittle Fracture WUF-W Connection Solution #1 Maintain plane stress conditions by controlling thickness of material and beam depth to span ratio Max material thickness for plane stress conditions: t = 400 K^2 * (1-v^2)/E((Fy+Fu)/2) This calculates to 0.645” for 50 ksi steel Control of Brittle Fracture This, for all practical purposes requires beam flange thickness of 11/16” or less. Limit maximum beam depth to 18”
- 17. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 17 Control of Brittle Fracture WUF-W Connection Solution No. 2 Between 11/16” and 1” flange thickness, Compute triaxiality at the beam column connection without vertical acceleration component. Col/Beam moment capacity = 1.1 Add column tension force due to vertical acceleration + overturning forces + moment magnification. Control of Brittle Fracture Compute new triaxiality Increase the column moment capacity by the ratio of : New triaxiality/Original triaxiality.
- 18. Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG ceginc@gmail.com March 17, 2014 Seismic Risk Reduction in Steel Structures Steel Committee 18 The End • Comments??????