Responce Spectrum Analysis and its results 1

[Alirezai]

hi,
when we use the Responce Spectrum analysis, because of the mode combination methods (CQC, SRSS) all the results such as forces and moments have the same sign and all are positive. my question is how we can find out which one is positive and which is negative? which one is tension force and which is compression force? . . . There are some procedures that we can use to find out this , but none of them is practical to use in softwares. I need a guide that can be used in structural design softwares such as SAP & ETABS and use analysis results directly to design and Iterate to achieve convergance in final design in shortest possible time.
Thanks alot.

 

[Qshake]

The response spectrum will always give a "maximum" for output and nothing more can be read into the analysis or the results. This is mainly because whatever the individual contributions of the modes are combined as you indicated by CQC Complete Quadratic Combination or by SRSS Square Root Sum of the Squares. Thus a modal response that was actually negative or perhaps by someone's definition causing sidesway to the left - is no longer applicable. What is important is the "magnitude". Also it is important to note that for EQ the direction may be right or left or whatever direction was specified or defined by the engineer.

The beauty of the RSA is the simplicity and the high degree of certainity associated with the "maximum" value. The downside is giving up the notion of what a structure's response is every fraction of a second. Which, for the most part, unnecessary. However, to achieve the time-history analysis of a structure you have to use that method of analysis (time history). The problem with the time history is that it is only good for a single location, single event, and is very sensitive to the soil-site investigation and scaling of known accelograms.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Qshake]

One more thing I'd like to add is that most structural analysis programs will allow you to access the modal values. However, you must understand that indvidually they don't mean that much and its not until you've combined them that the results of a RSA are useful.

If you desire the response of a structure every second for a forty second or more period then you will have to use the time history analysis. And as I've stated above the input is far more complicated.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[trainguy]

Do a code specified linear static analysis to get a feel for the behaviour before launching into the more complex one.

tg