Dynamic analysis in structural engineering is used to evaluate how structures respond to time-varying loads. This analysis is crucial for understanding how structures behave under dynamic forces like earthquakes, wind, human-induced vibrations, or moving machinery. It’s essential for ensuring the safety and reliability of structures, particularly in areas prone to seismic activity or structures sensitive to dynamic loading.
Modal analysis
Modal analysis identifies the natural frequencies at which a structure vibrates when disturbed. You can then determine the specific patterns or mode shapes that the structure undergoes at each natural frequency. Modal analysis looks at the interaction of stiffness and mass in the structural response, but we've taken this one step further. In Oasys GSA, you can tailor your modal analysis to provide you with the information that is most relevant to your project requirements.
Modal analysis vs Ritz analysis
Traditionally modal analysis requires calculating a large number of modes to accurately capture the dynamic characteristics of a structure. This is especially true when there is a significant difference between the horizontal and vertical stiffnesses of the structure.
One way that has been used to address this issue is to use Ritz analysis which provides approximations to the eigenvalues. Oasys have enhanced the modal analysis capabilities in GSA to ensure the results you obtain are appropriate for the subsequent response analysis. The modal analysis options in GSA mean that you find only the eigenvectors that capture the most important aspects of the behaviour of your structure. This approach not only improves accuracy but also significantly reduces memory usage and computational time compared to existing methods. With a clearer understanding of your models, you have greater confidence in your designs.
Response analysis
Dynamic response analysis predicts how structures will react to various external excitations. Some of these, such as seismic analysis, will assess the ability of the structural design to withstand earthquake excitation. Others, such as footfall analysis will be more concerned with the comfort of the inhabitants. In GSA these response analyses build on the results of the modal analysis. These help engineers identify potential weaknesses and ensure that structures perform well under various conditions.
Footfall
Footfall induced vibration analysis is used to calculate response factors, for a structure to human footfall loads (excitations). It's important to understand how your structure will respond to use. Will the structure be most sensitive to each footfall (transient response) or will the walking across a floor allow the response to build up (resonant response)? Will these human-induced vibrations disturb the occupants or even hinder their work? GSA provides you with the answers you need. View the list of case studies to see footfall analysis in action.
Watch this webinar for more information about the vibration analysis options in GSA.
Response spectrum
Response spectrum analysis (RSA) is a statistical approach used to estimate the peak response of structures subjected to seismic activity. An earthquake can excite a structure in all directions: north-south, east-west and vertical. The structure itself responds differently in the different modes with sway, twisting and bouncing modes. The response spectrum (usually a code-defined spectrum) along with the participation factors (filtering on direction) allows each modal response to be scaled for each of the directions independently. These scaled modal responses are then combined to estimate the seismic response in each of the different directions. The response in the different directions can then be combined either with a linear or SRSS combination to give an upper bound on the response of the structure.
A further refinement is that a code defined accidental torsion case can be created, so that the final seismic response is a combination of the RSA and the accidental torsion responses.
Along with these detailed results GSA also calculates key overall results such as base shears and storey displacements and forces, and interstorey drift.
Harmonic
Harmonic analysis calculates how elastic structures respond to sinusoidally varying loads such as rotating machinery. These can produce excitations that cause vibrations within the surrounding structure. Using harmonic analysis in GSA, you can determine the maximum response across a range of frequencies, ensuring that structures do not experience excessive vibrations. This is crucial for identifying potential resonance, fatigue, and other harmful effects of forced vibrations.
Linear time-history
Linear time history analysis is used to calculate the transient linear structure responses to dynamic loads or base acceleration using modal superposition. This approach is based on modal results and involves applying a specific time-varying load or series of loads to a structural model and observing its response at each time step. In GSA, you can take your model from a geometrical space into a modal space, solve it in modal space and then transfer it back to the geometric space.
You can find out more about Oasys’ structural dynamic analysis capabilities on the GSA documentation site.