8.2 Nonlinear static procedures (pushover analysis)

Nonlinear static procedures, like pushover analysis, help engineers evaluate how buildings perform during earthquakes. These methods go beyond elastic limits, showing how structures yield and fail under seismic loads. They're crucial for identifying weak points and understanding collapse mechanisms.

Pushover analysis involves creating a model, applying loads, and tracking structural response. It produces a capacity curve that shows how a building behaves as forces increase. While it has limitations, pushover analysis strikes a balance between simplicity and accuracy, making it a valuable tool in earthquake engineering.

Nonlinear Static Procedures: Fundamentals and Applications

Concept of pushover analysis

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• Analysis method evaluates seismic performance accounting for material and geometric nonlinearities
• Estimates structural capacity beyond elastic limit identifies weak points and failure mechanisms
• Determines sequence of yielding and failure in structure during seismic event
• Involves key components lateral load pattern incremental loading and capacity curve generation
• Provides insights into post-yield behavior and collapse mechanisms (soft story, shear failure)
• Commonly used in performance-based seismic design and assessment (ASCE 41, Eurocode 8)

Execution of pushover analysis

1. Create structural model accurately representing geometry and connectivity
2. Define material properties and nonlinear behavior including stress-strain relationships
3. Apply gravity loads to account for P-Delta effects
4. Select lateral load pattern based on expected seismic response (inverted triangular, uniform)
5. Incrementally increase lateral loads monitoring structural deformation
6. Monitor structural response tracking formation of plastic hinges and element failures

• Interpret results through capacity curve plotting base shear against roof displacement
• Identify performance points corresponding to different damage states (immediate occupancy, life safety)
• Analyze plastic hinge formation sequence to understand progressive collapse mechanisms
• Evaluate global and local deformation demands against acceptance criteria (FEMA 356, ASCE 41)

Limitations of nonlinear static procedures

• Assumes response dominated by fundamental mode may miss higher mode effects (tall buildings)
• Uses constant load pattern throughout analysis neglecting load redistribution
• Neglects dynamic effects like damping and inertial forces
• Potentially inaccurate for structures with significant torsional response (asymmetric buildings)
• Does not account for cyclic degradation of strength and stiffness
• Limited applicability to long-period structures sensitive to higher mode effects (bridges)
• May underestimate deformation demands in some cases (near-fault ground motions)

Pushover analysis vs other methods

• Linear static analysis simpler but less accurate for nonlinear behavior
• Linear dynamic analysis considers dynamic effects but limited to elastic response
• Nonlinear dynamic analysis most accurate but computationally intensive (time-history analysis)
• Pushover analysis balances simplicity and accuracy provides insight into post-yield behavior
• Advantages include computational efficiency and clear visualization of failure progression
• Disadvantages include simplified dynamic effects and potential underestimation of demands
• Often used as screening tool before more detailed nonlinear dynamic analysis (FEMA P-58)