Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics...

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Villanova UniversityDept. of Civil & Environmental Engineering

CEE 8414 – Structural DynamicsNorthridge Earthquake

Northridge Earthquake - Concrete Structures

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

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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

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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

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Types of Structures• Parking Garages

– California State University

– Moment resisting frame– Cast in place

• Ductile – lateral

– Precast • Brittle - gravity

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Types of Structures

• Parking Garages

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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

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Types of Structures

• Office Buildings

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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

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Types of Structures

• Residential Housing

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Typical Failure Modes

• Column Failure– Tie failure– Tie distribution– Shear failure– Spiral columns

• Steel to Concrete Connections• Tilt-up Buildings

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Typical Failure Modes

• Joints– Beam hinging– Corner joints– Roof joints

• Beam Alignment• Waffle Slab Failure

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Column Failure

• Spalling• Vertical

Reinforcement Concentrated in Corners

• Inadequate Cover• Older Structures

– Non-ductile

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Column Failure

• Tie Failure– Occurred in numerous buildings

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Column Failure

• Tie Distribution

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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

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Column Failure

• Shear Failure

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Column Failure

• Shear Failure

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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

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Column Failure

• Shear Failure

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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

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Column Failure

• Spiral Columns

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Steel to Concrete Connections

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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

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Joint Failure

• Beam Hinging

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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

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Joint Failure

• Corner Joint

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Joints

• Roof Joint

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Beam Alignment Failure

• Inadequate Connection of Beam to Column

• Most Cases Experienced Concrete Spalling

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Waffle Slab Failure

• Few Bars Passing Through Columns

• Punching Failure• Transfer of Moment

From Slab to Column

• No Secondary Resistance

• Progressive Failure

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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

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• 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

Code Development

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• 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

Code Development Since Northridge

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Conclusions

• Ductile Structures Fared Better• Parking Garages Suffered the Most

Damage• Columns Lacked Confinement • Shear Failures Prevalent• Code Changes Seem to be Working

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Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics...

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