5.2 Site response analysis

Site response analysis estimates ground motion at specific locations, considering local soil conditions and seismic wave effects. It's crucial for accurate seismic hazard assessment and designing earthquake-resistant structures, influenced by soil properties, layering, and bedrock characteristics.

The process involves modeling one-dimensional wave propagation through soil layers, using either equivalent linear or nonlinear methods. Results include amplification factors, response spectra, and time histories, which inform engineers about site-specific seismic behavior and potential risks.

Site Response Analysis Fundamentals

Concept of site response analysis

• Process estimates ground motion characteristics at specific sites considers local soil conditions and seismic wave effects
• Predicts site-specific ground motions crucial for accurate seismic hazard assessment informs earthquake-resistant structure design
• Influenced by soil properties (stiffness, density, damping), layering, stratigraphy, bedrock depth and characteristics
• Applied in seismic microzonation studies, building code requirements, performance-based earthquake engineering (PBEE)

One-dimensional wave propagation modeling

• Seismic waves (P-waves, S-waves, Rayleigh waves) propagate through earth materials
• Wave equation: $\frac{\partial^2u}{\partial t^2} = v^2 \frac{\partial^2u}{\partial z^2}$ describes motion in elastic medium
• Assumes horizontal soil layers and vertically propagating shear waves
• Transfer function relates input and output motions frequency-dependent amplification
• Soil column discretized into layers assigned material properties
• Boundary conditions: free surface at top, rigid or elastic bedrock at base

Equivalent linear vs nonlinear methods

• Equivalent linear iteratively approximates nonlinear behavior using strain-compatible soil properties (SHAKE program)
• Nonlinear performs time-domain analysis with constitutive soil models (DEEPSOIL, OpenSees)
• Input motion selection uses recorded or synthetic time histories spectral matching techniques
• Soil properties characterized by shear modulus reduction and damping ratio curves
• Groundwater effects considered in analysis

Interpretation of analysis results

• Amplification factors frequency-dependent show PGA amplification
• Response spectra display acceleration, velocity, displacement vs period compare surface and bedrock
• Time histories output acceleration, velocity, displacement series peak values and duration effects
• Strain profiles show maximum shear strain distribution with depth identify critical layers
• Stress-strain relationships display hysteresis loops for nonlinear analysis assess energy dissipation

Limitations of site response analysis

• 1D analysis neglects basin effects and topography assumes vertical wave propagation
• Soil property uncertainties from field and laboratory measurements affect results
• Input motion variability sensitive to selection and scaling
• Nonlinear behavior challenges equivalent linear approximations complex constitutive models
• Model validation limited by scarcity of strong motion data difficulty predicting nonlinear soil behavior
• Spatial variability lateral soil heterogeneity 3D effects in complex geology not captured