1. Theoretical Background
Performance-Based Seismic Design (PBSD) defines the required performance of a structure based on its purpose and function, evaluating whether this performance can be sustained throughout its lifespan. This approach enables cost-effective designs aligned with the intended use and duration of the structure. It also establishes acceptable levels of damage and anticipated damage levels based on design objectives.
The primary process of PBSD involves setting performance objectives and capacities, followed by evaluating the damage levels of a structure under specific seismic magnitudes. This process leverages standards such as ATC-40 and FEMA-440 to define and assess damage levels based on seismic load intensities and the structure's purpose.
2. Methods for Seismic Performance Evaluation
(1) Pushover Analysis
Overview: Pushover analysis evaluates the deformation capacity of a structure using nonlinear static methods. It simulates how a structure redistributes dynamic loads among its components when certain parts yield or fail during an earthquake.
Process: Incrementally increasing loads are applied to identify weaknesses, which are addressed until the overall yielding pattern of the structure is determined.
Application: This method is widely used to assess the seismic resistance of existing structures and is also applied in designing new structures.
Steps:
Select load patterns
Apply vertical (V load) and horizontal loads (H load)
Compute the pushover curve
(2) Capacity Spectrum Method (CSM)
Overview: CSM predicts a structure's response by representing its capacity as a load-displacement curve and its demand as a response spectrum. The method compares nonlinear behavior to an equivalent linear response spectrum, with the intersection of the capacity curve and demand spectrum indicating the structural response.
Steps:
Develop a capacity curve
Convert to a Single Degree of Freedom (SDOF) system
Transform and compare with the demand spectrum
(3) Displacement Coefficient Method (DCM)
Overview: DCM, based on FEMA-356 and 440, estimates inelastic maximum displacement using elastic system displacement. It is simple, practical, and frequently applied during early design stages.
Steps:
Define target displacement
Develop bilinear curves and calculate displacements
Identify the performance point
3. Considerations in Seismic Performance Evaluation
(1) Ground Conditions and Loading
In soft soil conditions, DCM is not recommended due to limited reliability of modification factors, which may lead to inaccuracies.
(2) Reduction in Member Stiffness
Cracks in structural members can reduce initial stiffness. It is critical to incorporate these stiffness reductions into the analytical model.
(3) Safety Verification and Performance Level Compliance
Ensure that inter-story drift and member deformation remain within allowable limits at the target performance point. Verification according to standards like FEMA and Eurocode is essential.
(4) Effects of Higher Modes and Weak Stories
Neglecting the influence of higher modes in pushover analysis can lead to inaccuracies. Enhanced methods such as Modal Pushover Analysis (MPA) are recommended to address these effects.
4. Conclusion
The key methods for seismic performance evaluation include pushover analysis, the capacity spectrum method, and the displacement coefficient method. Each method follows specific procedures to evaluate deformation capacity and performance points. During evaluation, it is crucial to consider ground conditions, stiffness reduction, and safety verification to meet target performance objectives.
The application of PBSD contributes to achieving cost-effective designs and structural safety tailored to the intended use of the structure. Systematic evaluation is indispensable for its effective implementation.
For more detailed insights, refer to the video linked below.
