Northridge Earthquake - Concrete Structures

1. Northridge Earthquake - Concrete Structures

2. Outline • Introduction • Types of Structures • Typical Failure Modes • Code Development • Conclusions

3. Types of Structures • Parking Garages • Large plan areas – number of lateral systems minimized • Not limited to a specific type of parking garage: • Precast • Steel components • Cast in place concrete – post tensioned • Hybrid systems

4. Types of Structures • Parking Garages • Ramps have the effect of shortening and stiffening adjacent columns • Precast elements often difficult to tie together • Performance far worse than other structures • 9 parking garages collapsed

5. Types of Structures • Parking Garages • California State University • Moment resisting frame • Cast in place • Ductile – lateral • Precast • Brittle - gravity

6. Types of Structures • Parking Garages

7. Types of Structures • Office Buildings • Fared better than parking garages • Shear walls performed reasonably well • Cracked but did not collapse • Most could use epoxy grouting • Non-ductile structures showed brittle failure in columns and piers

8. Types of Structures • Office Buildings

9. Types of Structures • Residential Housing • Apartments • Precast concrete used in basement parking experienced mixed results • Concrete and wood structures above did not fare well • Post-tensioned slab failure

10. Types of Structures • Residential Housing

11. Typical Failure Modes • Column Failure • Tie failure • Tie distribution • Shear failure • Spiral columns • Steel to Concrete Connections • Tilt-up Buildings

12. Typical Failure Modes • Joints • Beam hinging • Corner joints • Roof joints • Beam Alignment • Waffle Slab Failure

13. Column Failure • Spalling • Vertical Reinforcement Concentrated in Corners • Inadequate Cover • Older Structures • Non-ductile

14. Column Failure • Tie Failure • Occurred in numerous buildings

15. Column Failure • Tie Distribution

16. Column Failure • Shear Failure • Holiday Inn built in 1966 • Minor damage during 1971 earthquake • Red tagged, temporary shoring installed • Vertical column reinforcement between ties buckled – added confinement not provided by the concrete • Most severe damage between 4th and 5th floors

17. Column Failure • Shear Failure

18. Column Failure • Shear Failure

19. Column Failure • Shear Failure • Champaign Tower • 15 story building in Santa Monica • Non-ductile moment frames & shear walls • Column spans shortened by balconies • Experienced full length shear cracks • Typical short column behavior • Structure did not collapse

20. Column Failure • Shear Failure

21. Column Failure • Spiral Reinforced Columns • Spiral ties are more effective than rectangular ties • Need about 30% more link steel • Columns in following pictures do not have adequate confinement • Concrete outside of steel is lost

22. Column Failure • Spiral Columns

23. Steel to Concrete Connections

24. Tilt-up Building Failure • Commonly Used for Industry, Warehouses • 300 Structures Damaged • Poor Connection Between Roof & Tilt-up Panels Caused Failures • 1976 UBC – Minimum Tie Reinforcement • Post 1976 construction fared better • Passed Retrofit Ordinance after Northridge • 2,100 structures need to be retrofitted

25. Tilt-up Building Failure

26. Tilt-up Building Failure

27. Tilt-up Building Failure

28. Joint Failure • Beam Hinging

29. Joint Failure • Corner Joint • No transverse reinforcement • Insufficient anchorage for hooked bars • Widely spaced ties in members outside of the joint • No intermediate ties in column • Adequate confinement of concrete necessary

30. Joint Failure • Corner Joint

31. Joints • Roof Joint

32. Beam Alignment Failure • Inadequate Connection of Beam to Column • Most Cases Experienced Concrete Spalling

33. Beam Alignment Failure

34. Beam Alignment Failure

35. Waffle Slab Failure • Few Bars Passing Through Columns • Punching Failure • Transfer of Moment From Slab to Column • No Secondary Resistance • Progressive Failure

36. Code Development • Various Code Changes Over Last 40 Years • Varying Seismic Resistances • 1976 UBC Code Is First Code Similar to Current Codes • Separates “modern” and “older” buildings

37. Code Development • 1968 - Ductile Detailing in Frames • 1971 - Revised Detailing for Tilt-up Structures • 1976 - Design Forces Increased (Development of UBC) • 1988 - Improved Detailing of Shear Walls • 1994 - Shear Wall Design Provisions Introduced

38. Code Development Since Northridge • 1997 - New Requirements for Welded & Mechanical Splices for Precast Structures • 1997 - Provisions for Seismic Design of Precast Concrete Structures • 1997 - Requirements for Frame Members That Are Not Part of LFR System Must Be Detailed for Maximum Inelastic Response

39. Conclusions • Ductile Structures Fared Better • Parking Garages Suffered the Most Damage • Columns Lacked Confinement • Shear Failures Prevalent • Code Changes Seem to be Working