5.16.3. Pushover Analysis

The non-linear static analysis method.

Pushover analysis is a method used in structural engineering to evaluate how a building will behave under seismic forces. It is considered a non-linear static analysis method because, instead of applying dynamic forces that vary with time, a static load is applied incrementally until the structure reaches a target displacement. This allows engineers to understand the structure's performance and identify weaknesses in its design.

Imagine pushing a swing. You apply a force steadily until it reaches the maximum height. Similarly, in pushover analysis, engineers apply increasing forces to simulate the effects of an earthquake, observing how far the structure can withstand pressure before it fails.

Load applied incrementally until target displacement (SDOF-based).

In pushover analysis, the load is applied incrementally, meaning that instead of applying the full earthquake loading all at once, the force is increased gradually in steps. This step-wise approach allows engineers to monitor the structure's response at various stages of loading. The process continues until the structure reaches a predetermined target displacement, which is a critical point defined based on the expected performance during an earthquake.

Think about filling a balloon with air. You add a little air over time and watch how it expands. At some point, if you continue to add air (load), the balloon will either pop or stretch to its limit. Pushover analysis works similarly — not only do engineers observe how much the structure 'stretches' (displacement) as they apply load, but they also see how it can handle that load incrementally.

Converts actual structure to a capacity curve of an equivalent SDOF system.

The result of the pushover analysis is a capacity curve, which represents the relationship between the lateral force applied and the corresponding displacement of the structure. This capacity curve can be viewed as a graphical representation that indicates how much force the structure can resist before it starts to fail. By comparing this curve to the demands predicted during seismic events, engineers can assess whether the structure meets safety requirements. The equivalent SDOF (Single Degree of Freedom) system helps simplify the analysis by reducing the complex behavior of the structure into a single value that characterizes its response.

Consider a game of Jenga. When you first pull out blocks (applying a load), the tower remains stable, and you can do this a number of times before it starts to lean (displacing) and eventually falls. The capacity curve is like recording how many blocks you can remove versus how unstable the tower becomes: this data helps you understand when it will fail. Similarly, in engineering, the capacity curve provides information on how much loading a structure can handle before it is at risk of failure – crucial for safety in earthquakes.

--