What role does the Scale Factor play in defining the Load Pattern in the Pushover Load Case? How does it relate to the collapse mechanism?
When defining a Pushover Load Case, the Scale Factor is used to determine the size of the initial load.
Pushover analysis methods include Load Control and Displacement Control.
The Load Control method defines the expected collapse load (Qud) of the structure and performs the analysis by gradually increasing the load up to the expected collapse load.
Displacement Control is a method that presets a target displacement that can occur in the structure and performs the analysis by increasing the displacement until the target displacement is achieved.In both of the above methods, the structure must be subjected to some form of load until it collapses, so it is necessary to determine this load.
This load is called Load Case(Qud) (expected collapse load) in the load control method and Load Pattern in the displacement control method.
[Image 1] Pushover Load Case / Load Control [Image 2] Pushover Load Case / Displacement Control
In the load control method, the load with the scale factor becomes the predicted collapse load.
The scale factor for obtaining the expected collapse load in the load control method is first determined by performing a static analysis to check the elastic displacement of the building when applying a specific load (usually a static earthquake load), determining the limit of the inelastic displacement for building collapse, and specifying the scale factor as the magnification of the load to be increased.
However, since the building behaves nonlinearly as it collapses, it is not possible to obtain the scale factor exactly based on the results of the linear analysis, and the scale factor is determined by repeatedly increasing and decreasing from random values around the collapse point.
The reason why the type of load is needed in the displacement control method is that when the target displacement is set as "a load should be applied until a point is displaced in a certain direction by how much", the load must be reapplied in some form to deform the structure. Therefore, the type of applied load (Load Pattern) is needed.
For example, if the target displacement in the X direction of a point on the top floor is set to 30cm, the load reflecting the scale factor defined in the Load Pattern is applied to the structure, and the load is gradually increased until the target displacement is reached.
Therefore, in the displacement control method, the shape of the load is meaningful, but the Scale Factor does not have much meaning.
However, the Scale Factor is used to determine the size of the initial load. For example, if the Scale Factor is defined as 1, the load needs to be increased by 5 times to reach the target displacement, while if the Scale Factor is defined as 2, the load only needs to be increased by 2.5 times to reach the target displacement.
Here, how many times the load needs to be increased is automatically calculated by the program during the analysis.
This value is represented by the Load Parameter and is displayed in the Message Window.
[Image 3] Pushover Analysis Message