Wave spreading application

Hi,

I request some details on the wave spreading computation in the fatigue FTOPT2 card.

If I define a single heading wave (probability of occurrence 1.0 or 100%) at say 0 degrees and also specify a wave spreading, is the wave energy distributed from 270 degrees to 90 degrees as per the defined wave spreading? I would appreciate a write up/calc. note illustrating the application of wave spreading for such a wave definition that is coded in SACS. I could not find information in the SACS manual to answer this question.

Thanks and Regards

Sanjay

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  • Sanjay,

    That is not how the wave spreading works in SACS. When you select the wave spreading option, SACS will take the load cases from +- 90 degrees from the fatigue load case direction and factor those load cases using cos^n(theta) so that total equivalent energy of the factored load cases is equal to the one-dimensional spectrum. For instance, see a copy of the output from the fatigue listing file for a structure with 8 load case directions with all of the factored load cases below. You need analyze each direction in order to generate the transfer function for each load case, otherwise SACS would not be able to determine the response of the structure from each direction.

    Regards,

    Geoff

                       ***** WAVE SPREADING FACTORS *****

                         **** CONTRIBUTING LOAD CASE ****

       LOAD  DIRECT      LOAD  DIRECT  DIRECT   FACTOR

       CASE              CASE           DIFF

              (DEG)             (DEG)   (DEG)

         1     0.00        7   270.00  -90.00   0.0125

                           8   315.00  -45.00   0.2500

                           1     0.00    0.00   0.4751

                           2    45.00   45.00   0.2500

                           3    90.00   90.00   0.0125

         2    45.00        8   315.00  -90.00   0.0125

                           1     0.00  -45.00   0.2500

                           2    45.00    0.00   0.4751

                           3    90.00   45.00   0.2500

                           4   135.00   90.00   0.0125

         3    90.00        1     0.00  -90.00   0.0125

                           2    45.00  -45.00   0.2500

                           3    90.00    0.00   0.4751

                           4   135.00   45.00   0.2500

                           5   180.00   90.00   0.0125

         4   135.00        2    45.00  -90.00   0.0125

                           3    90.00  -45.00   0.2500

                           4   135.00    0.00   0.4751

                           5   180.00   45.00   0.2500

                           6   225.00   90.00   0.0125

         5   180.00        3    90.00  -90.00   0.0125

                           4   135.00  -45.00   0.2500

                           5   180.00    0.00   0.4751

                           6   225.00   45.00   0.2500

                           7   270.00   90.00   0.0125

         6   225.00        4   135.00  -90.00   0.0125

                           5   180.00  -45.00   0.2500

                           6   225.00    0.00   0.4751

                           7   270.00   45.00   0.2500

                           8   315.00   90.00   0.0125

         7   270.00        5   180.00  -90.00   0.0125

                           6   225.00  -45.00   0.2500

                           7   270.00    0.00   0.4751

                           8   315.00   45.00   0.2500

                           1     0.00   90.00   0.0125

         8   315.00        6   225.00  -90.00   0.0125

                           7   270.00  -45.00   0.2500

                           8   315.00    0.00   0.4751

                           1     0.00   45.00   0.2500

                           2    45.00   90.00   0.0125

  • Thank you for your response. I seek a couple of clarifications
    a) In the example I had previously posed, if waves in other directions (i.e. other than 0 degrees) are not specified, will SACS pretty much disregard the directionality? Or will SACS use the 0.4751 (say) factor from the table that you presented?
    b) Alternatively, perhaps SACS requires the following in fatigue analysis for correct wave directionality
    (i) Transfer function is generated for all 8 headings (0-360) - wave response module
    (ii) Define directionality in the fatigue input file for say 0 degree (50% probab) and 45 degree (50% probability) headings only and SACS will pick the appropriate transfer function based on the cos spreading for the other directions to calculation damage...

    Regards

    Sanjay
  • Sanjay,

    a) SACS will not factor the load if there are not any other valid loads (+/- 90 degrees from direction) so the factor in your example would be 1.0. You can easily generate a model to test this.

    b) You are correct that you need to supply an adequate number of loads to account for the response of the structure to different wave directions. You are also correct in that can define the probability of occurrence for each wave direction via the fraction of design life input in the fatigue input file for each fatigue case. The fatigue analysis will then use the responses from factored load cases to account for spreading with the fraction of design life to determine the fatigue damage.

    Regards,

    Geoff

  • Dear Geoff McDonald,

    Could you please explain detail of difference between 02 options: WSTF - Transfer function averaging and API - Response function averaging (API Energy approach) in wave spreading type of fatigue input?

    Thanks!

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