How to create a shock response spectrum from time history data with APDL commands

dloomandlooman Forum Coordinator

In the input below the POST26 RESP command is used to compute the shock response spectrum for a 20g triangular acceleration of 10 milli-sec duration. The time-history includes time after the shock load for the lower frequencies to reach their peak displacement value. The input assumes that a modal analysis has been done and expanded.

! After a modal analysis...


amplitude=20                  ! pulse amplitude (20 g)

width=0.01                    ! pulse width (10 milli-secs)



acel(1,0)=0.0, 0.5*width, width, 5.0*width

acel(1,1)=0.0, amplitude, 0.0, 0.0



/axlab,y,ACEL (g)

*vplot,acel(1,0),acel(1,1)    ! verify acceleration input

The table array, acel, can now be used as input to the RESP command in the time-history post-processor.


store,alloc,10        ! allocate the number of frequencies to be solved for

vput,acel(1,0),1,0    ! variable 1 is time values

vput,acel(1,1),2,0    ! variable 2 is acceleration values

data,5,1,10,1,Frequncy ! variable 5 is frequency list (I love this old command!) 


10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0


! Use parameters for RESP command input to make more readable

Spec=6            ! variable to contain spectrum output

Freq=5            ! variable with frequencies to be solved for

Inp_Acel=2        ! variable with acceleration input

Out_Type=3        ! create acceleration spectrum

Damp=0.05         ! damping (ratio to critical) to be used

Tinc=0.0005       ! integration time step, 20 pts per cycle at 100 hz

Inp_Type=1        ! acceleration input, new at 14.0




/title,Response Spectrum for %amplitude%g, %width*1000% m-sec shock load (5% Damping)

/axlab,x,Frequency (Hz)

/axlab,y,Peak Acceleration (g)

xvar,Freq         ! x axis is frequency

plvar,Spec        ! plot acceleration spectrum



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