We have different types of seismic analysis in Advance Design. In this short blog, we will show you how to use seismic analysis based on response spectrum method. The method is a linear-dynamic analysis which considers the contribution of each natural mode of vibration to calculate maximum seismic response of a structure based on the defined design spectrum. This method is based on mode shapes and natural periods of the structure; therefore, it captures the seismic response more accurately compared to the static equivalent method.
The procedure is very easy in Advance Design which has three parts: The Modal analysis to compute the modes of vibration, the Seismic load family to define the design response spectrum, and the load cases that represent seismic directions.
The calibration option can be activated to automatically scale the results based on the minimum number of shear forces required by the selected building code. Several standards are supported, including ASCE 7, NBCC, and Eurocode with different national appendixes.
1- Load family definition
The first step is to add a new seismic family in your model using the right click menu in the project browser. There can only be one seismic load family, but you can define as many seismic directions as needed by adding new load cases. The name of the family will reflect the code selection the software configuration.
The property list for the load family allows you to configure global parameters that affects the entire structure such as the location, site class and importance factor. They will be used to create and adjust the acceleration spectra. When using NBCC code, the region can be selected from a dialog box ( ) and the spectral accelerations are automatically assigned based on Annex C.
If calibration is used on the model, it is important to correctly specify the height, the number of stories correctly and the presence of irregularities. These parameters will be used to calculate the minimum base shear during the analysis and scale the results accordingly.
2- Seismic directions
Each seismic load case represents a seismic direction. The main parameters to define are the type of seismic load resisting system and the orientation of the seismic direction. The RdRo factor is selected automatically based on the type of structure but it can also be chosen manually.
3- Modal analysis
When creating a new seismic load family, the modal analysis will be automatically generated in the project browser. It can also be created manually with a right-click on "settings" and selecting "Modal analysis" from the contextual menu.
Types of modal analysis
There are two types of modal analysis available in Advance design: Eigen Vectors (default) and Load Dependent Ritz Vectors. The main advantage of using the new ritz vector method is that it manages to compute only the vibration modes that are significantly excited by the loading and can provide higher participation ratio with a smaller number of modes. It also allows Advance Design to automatically determines the number of modes computed by selecting a target participation ratio.
The seismic mass of the model can be calculated from point masses and self-weight of the structure, or from a load combination. When using the load combination option, the dialog box ( ) is available to define the combination of existing load cases.
If you need to include accidental torsion, the percentage of torsion is defined in the modal analysis for both horizontal directions and Advance Design will automatically calculate the width of the building and the amount of torsion to add.
After the modal analysis is done, a new category of results will be available in the FEM Results tab to display the mode shape in 3D. You can also use the Animation function to watch the movement of the modes.
Additional results are also present in the report generator. The "Seismic Settings" table contains a good summary of all the parameters that were selected and provides more details on how the internal force was calculated. It allows you to quickly verify values such as the total mass calculated, the participation ratios, predominant modes and the total shear force at the base of the structure.
This table also provides important information regarding the calibration (V, VD, chosen V). All of the internal forces displayed in the 3D view and tables are scaled to match the "Chosen V" and there is no need for manual scaling of the results.
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