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

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

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

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

Types of Structures• Parking Garages

– California State University

– Moment resisting frame– Cast in place

• Ductile – lateral

– Precast • Brittle - gravity

Types of Structures

• Parking Garages

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

Types of Structures

• Office Buildings

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

Types of Structures

• Residential Housing

Typical Failure Modes

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

• Steel to Concrete Connections• Tilt-up Buildings

Typical Failure Modes

• Joints– Beam hinging– Corner joints– Roof joints

• Beam Alignment• Waffle Slab Failure

Column Failure

• Spalling• Vertical

Reinforcement Concentrated in Corners

• Inadequate Cover• Older Structures

– Non-ductile

Column Failure

• Tie Failure– Occurred in numerous buildings

Column Failure

• Tie Distribution

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

Column Failure

• Shear Failure

Column Failure

• Shear Failure

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

Column Failure

• Shear Failure

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

Column Failure

• Spiral Columns

Steel to Concrete Connections

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

Joint Failure

• Beam Hinging

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

Joint Failure

• Corner Joint

Joints

• Roof Joint

Beam Alignment Failure

• Inadequate Connection of Beam to Column

• Most Cases Experienced Concrete Spalling

Waffle Slab Failure

• Few Bars Passing Through Columns

• Punching Failure• Transfer of Moment

From Slab to Column

• No Secondary Resistance

• Progressive Failure

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

• 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

• 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

Conclusions

• Ductile Structures Fared Better• Parking Garages Suffered the Most

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

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

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