Pushover analysis is highly recommended for dissipative structures, because one of the results is the level of ductility for the given structure. Using the proper ductility level can reduce design earthquake loads significantly.
Pushover analysis can only provide valid and realistic results if the structural model captures the nonlinearities in the structure.
This is a difficult task, especially for reinforced concrete structures. A common approach to this problem is to develop models with concentrated plasticity: the plastic deformation of each element is concentrated into a discrete number of points (so-called plastic hinges) along the element. The current common practice is to have a hinge at the proximity of each end of the element. Several types of plastic hinges are being researched, but the most widespread ones are flexural hinges. Flexural hinges consider the nonlinearity in the bending moment – element rotation relationship. Each hinge is characterized by a moment-rotation diagram that describes the bending capacity of the element at different levels of rotation.
The following description shows how to create pushover load cases and set their properties before performing a nonlinear static analysis.
Calculate vibration mode shapes and frequencies
When running the vibration analysis be sure to use the convert loads to masses option with the appropriate load case if there are loads defined that need to be considered static. Check the table of seismic equivalence coefficients in the Table Browser. Vibration results will appear only if the Vibration tab is selected.
Create a new pushover load case
Pushover load cases can be created, renamed and deleted in the Load Cases Load Groups dialog window. The initial configuration of four load cases is created by clicking on the Pushover Load button.
Setting pushover load parameters
After creating the load cases the parameters for the loads can be set up by clicking on the Pushover Analysis button in the toolbar of the Loads tab.
Run a Nonlinear Static Analysis
After the pushover loads are set up, the analysis can be performed by clicking at the Nonlinear Static Analysis button in the Static tab and selecting Pushover Solution Control. The parametric load case shall be one of the Pushover cases while the constant load case shall be the Seismic Load Combination.
The control node shall be one of the roof nodes of the structure. If performing pushover for a 3D model, pay attention to selecting the direction that matches the pushover load direction. The maximal displacement depends on the structural system, but a good rule of thumb is to push the structure until approximately 3-5% roof drift is reached (i.e. 3-5% of the total height of the structure).
Pay attention to applying a large enough number of increments to be able to capture the formation of plastic hinges and to avoid convergence difficulties. Following the geometric nonlinearity of the elements is strongly recommended for pushover analysis to capture the P-Delta effect.
Analysis results include graphical representation of hinge status at each step of the analysis. The red hinges are in an inelastic state, meaning that the internal forces in the given element have surpassed its yielding moment limit and this affects the stiffness of the element and through that the stiffness of the entire structure. A properly designed ductile moment resisting frame will have plastic hinge formation in its beams first and after the beams the columns at the base will yield which eventually leads to the collapse of the structure.
It is important to avoid plastic hinge formation in the other columns of the structure and this often leads to the omission of plastic hinges from the columns except for the base column to speed up the analysis. If this practice is followed, it is important to check that the bending moments in the columns are below the yield moment to make sure that the columns are indeed in elastic state.