- Design Considerations
- Ground material properties
- Moving loads
- Blasting loads
- Cyclic loads
GTS NX will enable you to fully benefit from the use of 2D and 3D models for seismic analysis. One of the key advantages of using 2D and 3D models for seismic analysis is the ability to consider project complexities such as soil anisotropy, irregular soil stratigraphy, surface waves, irregular topography, and soil-structure interaction. Another key advantage of using 2D and 3D models is that seismically induced permanent displacements can be estimated.
GTS NX is capable of performing response spectrum analysis for conventional projects as well as time history analysis for high-end seismic design calculations. For response spectrum analysis GTS NX uses the natural frequency to calculate peak values of seismically induced displacements, ground velocities and ground accelerations.
For time history analysis you have the option of using the modal superposition method to estimate the displacement of structures from a linear superposition of modal displacements. You can also use the direct integration method to integrate the dynamic equilibrium equation over the given time steps. Lateral soil springs can be modeled for deep foundation seismic analysis. These soil springs make it possible to account for non-linear behavior of the soil during seismic loading.
GTS NX is fully capable of performing nonlinear dynamic analysis, which utilizes detailed ground motion records along with structural models to calculate highly accurate results. Dynamic loads for blasting, trains and earthquakes can be efficiently created with the generator functions. An extensive database of the most widely used seismic time history records is available as well. These options effectively streamline the process so that you can obtain the desired results with a much lower investment of time and effort than is typically associated with nonlinear dynamic analysis.
Eigenvalue analysis is used to analyze the inherent dynamic properties of the ground/structure, and this can be used to obtain the natural mode(mode shape), natural period (natural frequency), modal participation factor etc. of the ground/structure. These properties are determined by the mass and stiffness of the structure. In other words, if a structure is determined, the natural frequency and vibration mode (natural mode) are also determined and the number of properties are the same as the degree of freedom of the structure. For real cases, the structure does not vibrate at a single mode shape and multiple modes overlap to display a complex vibration shape.
Here, the Mass participation factor is a mass percentage factor that represents how much of the structure participates in the vibration for each vibration mode when the structure is vibrated at a complex vibration mode. For example, if the first mode mass participation factor is 60%, 60% of the total mass of the structure participates in the first mode. Hence, the a mode with a high mass participation factor is considered in the earthquake wave for analysis.
For general structure, considering only vibration modes with a mass participation factor sum of around 90% is still regarded as a sufficiently accurate analysis. However, the ground material properties are relatively smaller that structural properties and it is hard to have a mass participation factor of 90% in Eigenvalue analysis. The period is also relatively smaller and no specific standard exists.
Natural periods are defined as the time taken for a structure to vibrate from its natural vibration state to the particular mode shape using a natural value that 1:1 corresponds to the natural mode.
[Natural mode shapes]
[Natural mode shapes]
The General seismic design criteria requires that each mode’s effective model mass included in the analysis should retain more than 90% of the total mass. This is to include most of the major modes that influence the result.