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Assume I have a cantilever beam fixed at one end. I can conduct a static analysis in FEM for a case where I apply a load on the other end, condition being the load is consistently applied on it and time is not an issue here. On the other hand, I apply the same load at the same location on the beam but not consistently, but instead only for a few miliseconds, for example. So my question is that during these miliseconds i.e. during which I apply the load, should the results of Displacements and Stresses within my model by any different to if I apply the same load at the same location consistently, without a duration of time. If not, then why do we even need to conduct a Transient Analysis when we already have the availability of a much simpler module i.e. Static Analysis?

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  • $\begingroup$ Is that like calculus where only a small change is assumed? $\endgroup$
    – Solar Mike
    Oct 28, 2021 at 19:32

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I am not aware there is such an analysis method called "transient analysis" for civil/structural engineering, I think you mean the analysis of structure with the "transient loads.

By definition, the transient load is temporary and brief in nature. In structural engineering, except for the dead weight, almost everything else can be called a transient load, for example - live load, wind load, seismic load, moving load, etc. However, due to the uncertain natures of the transient loads (its intensities, durations, and potential of reoccurrences), historically, they have been evaluated using statistics means and formatted to fit the equivalent static analytical methods, or converted conservatively to the equivalent static loads by the code authorities for simplicity in general design tasks.

The static analytical methods and the equivalent static loads have been proven to yield satisfactory results prior to the advent of PCs, which have led to the fast advancement in structural engineering (capability) and material science. As the structures get more complicated, and the interests in saving by fully utilizing the strength of the materials, for certain cases, the use of the "equivalent static methods and equivalent static loads" are sometimes deemed no longer appropriate, thus the calls for the use of the "real loads" instead. The most noticeable cases are the time-history analysis of seismic resistant structures, and utilizing wind tunnel data for wind analysis. Without using the "real (transient) loads", many structures would not have been built.

Does the duration of load affect the performance of a structure? No, and yes. No, unless stressed to yield, a 1 kip force placed on the tip of a cantilever beam for 5 minutes and 5 hours or 5 days will yield the same amount of deflection. However, yes, the duration of the sustained load is the main concern of the long-term deflection due to creep.

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  • $\begingroup$ Yes, exactly, that is my question. Why would transient analysis needs to be conducted in the first place when we already have the static analysis available? I mean we can just take the max value of the force during the transient phase and check out if the structure is failing or not, and make some necessary design modifications if needed. What else do we need to check out which makes the use transient analysis using transient loads to become compulsory, instead opting out for static analysis using static loads only? $\endgroup$ Oct 29, 2021 at 7:20
  • $\begingroup$ @RameezUlHaq because you can load a human thigh bone slowly and it is strong, but hit it sharply with a hammer and it breaks easily… $\endgroup$
    – Solar Mike
    Oct 29, 2021 at 13:05

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