advanced analysis methods

[Saedhalteh]

Can someone help me with a simple clarification or resources to understand the Steady State, Floor Spectrum, time history analysis ?? and when it is required to use these types of analysis .

 

[FEA way]

These are all linear dynamics analysis procedures that can be found in FEA software and are usually solved using modal superposition method. I will refer to them from this point of view.

Steady state (harmonic response) analysis is used to obtain structural response in frequency domain to harmonic excitation. Loads are sinusoidally varying and you have to specify the frequency range for which the response will be found.

Floor spectrum is usually called response spectrum. This type of analysis is done to evaluate the peak response of a structure subjected to so called base motion (time-varying motion of structure's base for example in case of earthquake). To use this type of analysis you have to specify a spectrum (such data is usually experimentally obtained for seismic events or motion of the vehicle on road).

Time history (transient) analysis will let you calculate structural response in time domain. Load is also defined as a function of time.

 

[JoshPlumSE]

When you do you one versus the other.

Steady State Analysis: Typically when you have excitation at a known frequency and you want to see how the structure responds. This could be due to dynamic equipment loading, walking excitation or such.

Time History Analysis: This is often done in similar cases to Steady State Analysis, but is a little more accurate (IMHO). Unfortunately, it can also be a lot more analysis intensive. I like it for start-up or run down analysis for equipment. Say the equipment takes a certain amount of time to achieve it operating frequency, during this time it will pass through some modes of the support structure. How bad with the response be? Will the frequency increase quickly enough that you don't get damage from resonance? Same sort of thing for run down analysis. The equipment shuts down and will pass through the support structures modal frequencies before it comes to a stop.

Response Spectrum: If you have a time history input motion, then you can convert this into a response spectra and get essentially the same results for your structure. Normally, this is done not with a true time history, but with a code assigned seismic design spectra.