Time History Analysis Results - Member forces

Question

Hello, 
 Can someone give some clarification about the issue below? 
 In Section 5.31.4 of the STAAD.Pro Technical Reference Manual , in the Notes it says that for a Time History Analysis 
 " b - Results are the individual maximums over the time period. Thus, derived quantities such as section forces and stresses, plate surface stresses and principal stresses should not be used ". 
 Does this mean that the results for member forces in the postprocessing mode are onerous? 
 
 Could you respond ASAP to this query please? 
 
 Kind Regards, 
 
 Skerdi

Sye · Accepted Answer

Yes, as you mentioned in point 1, one should use 2 combinations to capture the worst effect. 
 
Regarding your point 2, first of all I would like to mention that it is possible to capture not only displacements but also member forces and support reactions at each instant of time. You can choose the SAVE option when defining the time history definition and that would save the member forces and reactions in a separate file by the name “model_name.frc” within the same folder where the model is. You can always browse that file to find out forces at any instant and as you said, these are not maximums but real time data. You can assess the worst effect. However this would be quite a task for a reasonably sized model and so personally, I would prefer to go with option 1.

Sye · Answer

Results like nodal displacements, support reactions, beam end forces are all correct in a time history analysis. However you need to remember that like the other items in a time history analysis, beam end forces are also individual maximums and may not be occurring at the same instant of time. Since intermediate section forces are derived based on the beam end forces, so it does not make sense to derive these based on quantities that do not coexist. Similar logic applies for derived stresses too. The note explains the same.

Sye · Answer

The forces that you get in the frc file at each instant of time, are the corresponding values at the start/end nodes of the member at that instant. The individual maximum value out of all time instants is reported for each node in the post processing table in STAAD.Pro. For determining this maximum value, all +ve and all –ve values in the frc file, for that node, are considered and the one which is highest in magnitude ( positive or negative ) is reported in the postprocessing table. The member force values reported at the nodes in the postprocessing table are therefore not corresponding values but individual maximums. In other words, what you get in the postprocessing mode at the start or end nodes is the highest magnitude of response ( positive or negative ). That is why, when you do a static + time history and a static – time history, you are covered for the worst case scenario. 
 
As long as you are interested in the member start/end forces or reactions, you are fine with what STAAD is reporting in the tables or output. All you need to do is create two combinations ( static+/- time history). On the other hand, if you would like to get results at intermediate sections then you would need to get the values at the ends at each instant from the frc file and then find out the intermediate section forces manually. Subsequently you will need to add the static effect to it. 
 
Same logic applies for the .tim file as well. As long as you are trying to find out max nodal displacements, you will get it directly from the post processing tables. If you would like to get relative displacements/rotations between the start and end node then you would have to know the corresponding values at the nodes at each instant and would need to refer to the .tim file.

Sye · Answer

Results at the start and end nodes from the time history load case, represent the maximum values. The stresses are calculated at the start/end sections based on these forces and these are correct. Also as I mentioned before, when you do a static + time history and a static – time history, you are covered for the worst case scenario. Section forces however should not be considered. 
 
Code check results per AISC 360-05 would definitely be meaningful provided that for combinations involving time history cases, only the start/end sections are considered for design. Using the SELECT command is fine too.

Sye · Answer

You can have multiple time history functions. The displacements that you get in the tim file would combine the effects of all the time history loads. You should also get the max displacement value, that you see in the post processing mode, corresponding to one of the time instants in the tim file. This is what you can do to cross check. Go to the post processing mode Dynamics > Time-Disp page and look at the displacement vs time plot for a node. Note down the max value and also the corresponding time ( say t1 sec). Now open the .tim file and look for the displacement data corresponding to that same node and corresponding to the time instant of t1. The values should match. 
 
Yes please choose the SAVE option for each although even if you use the SAVE for one, it works for the others too but as a matter of good practice, I would suggest using the SAVE for each. 
 
As mentioned earlier, the results are the combined effect of all the functions and not for individual ones.