 # force time history ZPA option

There are seven steps outlined under the Time History Analysis heading in the "Missing Mass and ZPA Correction" help topic for applying the ZPA option to force time history analysis. The discussion uses terminology that isn't defined in the help file. I would appreciate an explanation or example to clarify the seven-step process - in particular, the first four steps are unclear, leading to questions such as the following:

• Is the TIH load sum discussed in Step 1 algebraically adding the forces from all three directions at each node for each time step? If not, what is the method?
• Steps 2 - 4 refer to "discretized" and "undiscretized" loads. What is meant by these terms?
• Is Step 2 the absolute maximum of the result of Step 1 over all time points?
• Is Step 4 the envelope of the results of Steps 2 and 3?
• Hi TIm,

It appears you are using an older version of AutoPIPE?  You have a question about TIH here but in the latest release, Version 12.04.00.13, this is called the THP for TIme History Profile.  I would recommend updating AutoPIPE.  Please see the responses to your questions below.

• Is the TIH load sum discussed in Step 1 algebraically adding the forces from all three directions at each node for each time step? If not, what is the method?

Here’s an example of the THP file.  The loads are being summed from here, where there is no specified direction.  The direction is later applied in the THL file. • Steps 2 - 4 refer to "discretized" and "undiscretized" loads. What is meant by these terms?

What this approach is trying to do is identify a value of load that can be used for ZPA check. For time history you can do this two ways.

1. Discretized - Consider each time step one by one and figure out the combined maximum load and apply it.
2. Undiscretized - Ignore the time step and take the maximum for each individual load and use that load value.
• Is Step 2 the absolute maximum of the result of Step 1 over all time points?

Yes, Steps 1 and 2 are both considering the time steps, so they are looking at the discretized loads.

• Is Step 4 the envelope of the results of Steps 2 and 3?

Yes.

Thanks,

Jessica Alegre   • • • Hi Jessica,

Thanks for replying. Yes, I am using an older version of AutoPIPE (v10) but we are in the process of upgrading.

I still need some clarification on the ZPA process that you described:

With respect to the first bullet point in our discussion, what loads are being summed? The help file says ZPA Step 1 is "Sum all TIH loads associated with a node by sampling at the analysis time step." Two of the possibilities I have at the moment are that

• it sums the all force values in the node's TIH file (which doesn't make sense to me), or
• it calculates a static response of the whole structure at each time step to find which time step is "worst" (except ZPA static analysis doesn't occur til Step 6).

If I could understand the above, it would help me with the whole process. I have been trying to make sure we have a good knowledge of what the software is doing when we run an analysis, so thank you for your help.