General Mechanical

General Mechanical

What’s better, a Shock Response Spectrum or a Transient?

    • Dave Looman
      Ansys Employee
      In doing tech. support I am sometimes asked, Should I do a shock response spectrum?  The temptation is to answer No.  This is based in part on the assumption that the caller doesn't have a response spectrum and would be unlikely to want to create one. A response spectrum is step two of a two step process in which step one is a transient analysis. Typically a dynamics specialist creates the response spectrum for the analysts. nThis reasoning is only partly correct. Creating a shock response spectrum for a transient loading such as a 20g half sine pulse is actually relative simple to do.  As opposed to a seismic (earthquake) response spectrum which requires a long time-history to be solved, a shock response spectrum could be based on a handbook of dynamic load factors for the shape of the shock loading.   Damping isn’t too significant in shock response, so it could be conservatively ignored. nIf you don’t have a handbook solution, the APDL RESP command can be used to create a response spectrum for any arbitrary transient acceleration. So you actually can create your own shock response spectrum.nSome newer spectrum methods such as the Rosenbluth method (ROSE command) make the spectrum combination more accurate and conservative. The Rosenbluth method is the same as the double sum method (DSUM command) except that the sign of cross terms is preserved.  The td value input with the parameter "width" is the duration of the excitation. In conjunction with damping, shorter excitation duration increases the coupling between closely spaced modes. There is also a method to sum high frequency modes more directly (more conservative) in accordance with NRC Regulatory Guide 1.92, Rev 2. July 2006. High frequency modes tend to be more in phase so a more direct summation is appropriate. The high frequency range is referred to as the "rigid response" and is specified with the RIGRESP command. The mass not represented by the extracted modes ("missing mass") is accounted for with the MMASS command.nPossible advantages of doing a shock response spectrum over doing a transient analysis are the ease of identifying maximum result values, a smaller results file and a shorter solution.   Procedurally however, a full transient is much simpler and can include nonlinearities. It may require a 100 solutions though and could require a few hours to run. Then the 100 sets of results will need to be sifted through to find the maximum results. nA second alternative is a mode-superposition transient. This option provides a fast solution (expansion pass can be time-consuming), but still has the disadvantage of providing a lot of data to sort through and isn’t as simple a procedure as a full transient. Like a response spectrum, a mode-superposition transient requires a linear model.nSo going with a response spectrum has some advantages over a transient, even if you have to create it yourself!  There are a couple of disadvantages to be aware of though. After the response spectrum mode combination, the quantities on the results file are all positive. That makes it possibly incorrect to sum nodal forces, average element results at a node, transform results into a coordinate system or compute derived quantities such as equivalent stress, SEQV.  (In APDL special coding is used when FSUM is requested that performs the FSUM on each mode and then repeats the combination on the modal FSUM values. Also, SUMTYPE,PRIN can be issued before the combination to obtain conservative derived stresses like SEQV)nDespite the advantages of a shock response spectrum, response spectrum provides a greater benefit when used to represent a long transient like an earthquake which can be very time-consuming to run as a transient.n
    • Keyur Kanade
      Ansys Employee
      Thanks!n
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