Wave response Program

Upon reading the manual of Wave Response, it suggests that it calculates equivalent static load in order to solve directly (section 1.2). How is this type of analysis different from a normal static conventional analysis ? 

1) Stiffness of the structure remains same as that of a standard Stiffness Analysis (static) ?

2) DAF for the structure automatically gets involved in this equivalent static Load ?  

3) PILSUP command requires input of  Load cases to calculate pile stiffness in Global X or Y Direction. Stiffness of Pile is intrinsic and dependent upon Soil Properties & Pile Geometry. How do load cases determine the stiffness ? Should the load cases entered be the environmental condition or Dead Load ? 

Parents
  • 1) The stiffness of the structure does not change.

    2) The equivalent static load will include dynamic response of the structure.

    3) The soil stiffness is non-linear and is dependent upon the load on the piles. The load used to linearize the piles should be close to the load on the piles experienced during the dynamic response analysis. The environmental load should be a good load case to linearize your piles.

    Regards,

    Geoff

  • Follow-up question on the same context.

    DAF calculation based on Wave response suggests to divide Dynamic base Shear by Static Base Shear (Section 2.4.3, Wave respone manual). Shouldnt it be Dynamic response by Static Response ? 

    DAF determination using Wave response again continues to be an approximate way of doing a dynamic analysis. The reason being, it suggests that DAF has to be factored results or loading obtained from purely static analysis. It is again a form of quasi static loading method (Approximate)- Ideal for cases when loading frequency is far lesser than natural frequency of Structure.

    What about the structures in which natural frequency and loading frequency are comparable. In those cases, this method will be inaccurate. Kindly give your outlook regarding the issue.

  • I think it is better to back to basic in the book.

    Equivalent static force is one of method to recover the member stresses from dynamic analysis. 

    The equivalent static generated by SACS contains both 'static and inertia'. I understand that SACS generated equivalent static from the fundamental below (I cut from Anil K. Chopra's book).

    Then, it is considered as dynamic load (static+inertia). Btw, you shall put 'US or ES' in col 19-20 of WROPT. 

    The member forces and stresses obtain from solving structural stiffness with equivalent static shall be considered as 'dynamic member forces or dynamic member stresses'.

    This is 'exact dynamic solution'. It is not 'approximate solution' as you mention.

    You should forget about section 2.4.3 of SACS. This section try to guide how we can determine the 'approximate DAF' by ignoring phase lag between static and dynamic.

    However, in reality, DAF is not constant even you have only one excitation force i.e sine curve force. The DAF is vary with time, however, at certain time, the DAF will be maximum.

    Fex, you have excitation force with period of 15sec 

    Max. static force = 5 kn occur at time = 2sec (At 2 sec, dynamic force = 7kn)

    Max. dynamic force = 12 kn occur at time = 8sec (At 8sec, static force = 8kn)

    Approximate DAF is Max.static / Max. dynamic = 12/5 = 2.4, however, you will notice that max. dynamic and max. static not occur in the same time. Then, actual DAF should be smaller as below.

    At 2 sec, DAF = 7/5 = 1.4

    At 8 sec, DAF = 12/8 = 1.5

    Btw, if you use approx. DAF = 2.4 in your calculation by multiplying on your static response, it will conservative at all. This is the procedure that you mentioned. 

    However, SACS wave response did not use this approximate method.

    SACS calculated dynamic load which is called 'Equivalent static load' at all time point (you need to put ALL in col.15-18 of WROPT).

    Then, this dynamic load is already taken into account the phase lag between static and dynamic load. In the other word, SACS used DAF = 1.4 at 2sec and 1.5 at 8sec rather than used DAF=2.4 for all time points.

    Geoff, please help to correct me, if I understand something wrong.

    Regards,

    Kasiphon

    Answer Verified By: Geoff McDonald 

  • In brevity are you intending that, when wave response module is to be used for determining dynamic stress, it is a precisely perfect dynamic analysis. If the USER is intending to get DAF, it rather gives respective Dynamic & Static Shear Loads which the user can divide to get DAF based on his level of conservativeness ? 

Reply Children
No Data