Nonlinear Seismic Isolators
By leveraging our expertise in seismic isolation, we have completely re-developed the modeling and analysis of seismic isolators since ProtaStructure 2024 & later. We have developed new tools to ensure the final precise design of seismically isolated buildings.
The design of earthquake-resistant structures requires expertise in advanced earthquake engineering. Seismic isolation is used to extend the period of the structure above the isolation layer, reducing earthquake forces, storey acceleration and relative displacements. Seismic isolators also introduce a certain level of damping to the structure. However, there are important points to consider in the construction and project phases of these structures.
Several key phases in the design of earthquake-resistant structures:
- Defining the structure's architecture and function: The building's function should be designed with the seismic isolators in mind during the architectural and mechanical design stages.
- Shaping the structural system: It is crucial to determine the placement of isolators and design the members that will safely transfer forces.
- Identifying site-specific earthquake hazards, determining appropriate ground motions, and target spectra.
- Modeling the structure and making preliminary designs to determine the types of isolator units. Linear elastic analysis may be sufficient at this stage, and the types of isolators are generally decided in collaboration with the manufacturers.
- Finalizing the design of the structure with the determined isolator types. Design, displacement, and period checks are performed using lower and upper bound values for isolators for different earthquake levels.
Seismic Isolator Library
The types of seismic isolators can now be defined with the help of the library located in the Modeling > Seismic Isolators ribbon toolbar.
In addition to the Linear Properties of the isolation units, the Nonlinear Properties can also be defined. A change made in the library is automatically reflected to all assigned types.
Nonlinear properties can be automatically calculated from parameters such as Axial Force, Friction Coefficient, and Net Pendulum Radius. for friction pendulum (FPS) isolator units.
Force-deformation relationship can also be manually defined by entering values such as Initial Stiffness, Post-Yield Stiffness, and Yield Force.
For Lead Rubber Bearings, nonlinear properties can be defined by entering values for Initial Stiffness, Post-Yield Stiffness, and Yield Force differently in both directions.
Assigning Seismic Isolators to Members
Assigning Seismic Isolators to Columns
To assign seismic isolators to columns:
1. Select a column on the screen and use the "Assign Isolator" command in the right-click menu.
2. Select one of the isolator types previously defined in the library from the "Seismic Isolator" list.
3. Enter the Position and Offset values and click the OK button.
4. An isolator will be inserted on the column's analytical frame at the specified position.
Assigning Seismic Isolators to Rectangular Walls and Core Walls
In previous versions of ProtaStructure before 2024, only one seismic isolator could be inserted to rectangular walls or core walls. If multiple isolators needed to be defined, their linear properties had to be combined to create a single lumped isolator by the user. However, in practice, there is a need to define multiple isolators for walls, and the approach of combining all isolator properties will not be valid (or sufficient) in non-linear analyses. This is because it will become impossible to determine the tensile forces that may occur when the isolators under the walls are lumped.
With ProtaStructure 2024 & later, you can individually define any number of isolators for wall panels. To do this:
1. Select a rectangular wall or a core wall on the screen and use the "Assign Isolator" command in the right-click menu.
2. On the isolator assignment window, a preview of the isolator placement will be shown.
3. You can place the isolators to correspond to the I and J insertion points of the wall panel. Alternatively, you can also ensure equal distribution across the section. In core walls, the wall legs will be automatically detected and listed. You can define isolators separately for each wall leg.
Analytical Modeling of Isolators Assigned to Shearwalls
In previous versions of ProtaStructure, shearwalls with seismic isolators had to be modeled with the "Mid-pier" option. With ProtaStructure 2024 & later, shearwalls with isolators can be modeled with the "Finite Element Shell" option.
The shell elements are divided by the isolator height to create an analytical model, and the specified number of isolator elements are analytically placed in between.
Nonlinear Time-History Analysis
Nonlinear characteristics of isolators will be automatically considered in a nonlinear time-history analysis performed in ProtaStructure. In such an analysis, all members except isolators are assumed to be linearly elastic, and no material plasticity is assigned.
1. To perform a time-history analysis, click the "Analysis > Time-History Analysis" button in the ribbon toolbar.
2. You can perform operations such as selecting and scaling ground motions before initiating the analysis.
Analysis Output
ProtaStructure will provide you with the detailed outputs for each isolator and the overall building after non-linear time-history analysis. You can use these outputs to generate your own custom design report for your isolated building.
Analysis Results
You can visualize the isolator response history for each ground motion on the charts provided at the Analysis Results tab.
1. You can obtain Displacement, Acceleration and Force Response History for X, Y, and Z directions. The requested results will be plotted on the same graph.
2. To refresh the plot according to the selections you have made, click “Generate Diagram” button.
3. Storey Accelerations and Relative Drift Ratios for the upper structure will also be plotted for all ground motions, their maximum and minimum responses.
Isolator Results Table
All isolator results will be tabulated in a detailed manner when you click “Isolator Results Table” tab. You can export this table to Excel and use it in your design reports.
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