Nonlinear Pushover Analysis in STATPRO

Aug 12, 2024

Notes on Nonlinear Pushover Analysis in STATPRO

Introduction

  • Overview of session on using nonlinear pushover analysis in STATPRO.
  • Structure of session: theoretical background, methods, and application.

Pushover Analysis Overview

  • Definition: Performance-based analysis method to achieve a performance objective in structures.
  • Focus on limiting damage conditions and life safety.
  • Performance Objective: Desired level of seismic performance; specified maximum allowable structural/non-structural damage.

Methods of Pushover Analysis

  1. Capacity Spectrum Method

    • Develop demand and capacity spectra; find intersection points.
    • Evaluate performance of structural components.

  2. Displacement Coefficient Method

    • Used in STATPRO.
    • Main goal: Find target displacement (Δt).
    • Reference: FEMA 356-2000, Section 3.3.

Force-Controlled vs. Deformation-Controlled Actions

  • Force-Controlled Actions:
    • Elements not allowed to exceed elastic limits (brittle/non-ductile).
    • Type 3 curve: significant degradation after limited deformation.
  • Deformation-Controlled Actions:
    • Elements permitted to exceed elastic limits in ductile manner.
    • Type 1 curve: deformation beyond yield point is critical.

Nonlinearities in Pushover Analysis

  • Consider geometric and material nonlinearities.
  • Includes hinge formation for bending moments in beams.

Lateral Load Distribution Methods

  • Methods:
    1. Uniform distribution.
    2. Inverse triangle distribution (based on fundamental mode shape).
    3. Proportional to mass at each level.

Steps in Nonlinear Pushover Analysis in STATPRO

  1. Define Frame Type: Moment frame or braced frame (default is moment frame).
  2. Provide Gravity Loading Information: Dead loads and live loads.
  3. Mathematically Model: Nonlinear load deformation characteristics of components.
  4. Classify Elements: Primary (resist collapse) vs. secondary (limited impact on reliability).
  5. Define Pushover Hinges: Acceptance criteria and properties based on FEMA 356.
  6. Perform Pushover Analysis: Monitor conditions until limits are reached.

Acceptance Criteria for Performance Levels

  • Immediate Occupancy (IO): Limited post-surfact structural damage.
  • Life Safety (LS): Significant damage with margin against collapse.
  • Collapse Prevention (CP): Near total or partial collapse.

Analysis Execution

  • Pushover analysis performed until:
    • Cumulative base shear <= defined base shear.
    • Displacement at control joint exceeds specified value.
    • Structure becomes unstable.

Result Generation

  • Demand spectrum generated per FEMA 356 guidelines.
  • Member sectional forces checked against capacity for hinge formation.
  • Hinge unloading and load redistribution described.

Frame Types and Input Parameters

  • Types:
    • Concentric brace frames and moment frames.
    • User can specify effective length factors and non-linearity effects.
  • Convergence Check: Optional parameters for nonlinearity convergence.

Modeling Rules and Limitations

  • Only steel structures considered.
  • Hinge development options available.
  • Control node selection critical (usually at roof center of mass).
  • Regular buildings without adverse distortion recommended.

Application Example Overview

  • Example walkthrough in STATPRO including input specifications and result evaluations.
  • Focus on defining loading patterns, specifying hinge properties, and analyzing results post-analysis.