When we run the earthquake dynmaic spectrum the results shows positive displacements and forces. The results looks uni directional.
How to run the system for -ve directions.

Thanks,

In a "response spectrum analysis", sooner or later combinations are performed using SRSS. All results are positive amplitudes.

Thanks for your reply. Did not understand completely.The seismic displacements can be +ve and -ve since pipe will move in both direction, but how does caesar calculates operatic + seismic displacements?
Since operating displacements are either + or -ve, on top of that how Sesimic values are added to it? The resultant displacement should be + ve at some nodes and -ve at some nodes.

Ajay,

If you are aware of the details of what happens in a response spectrum analysis, kindly ignore my post. If not, pl. read more about it . Two sources are:

1) Dynamics of structures by Clough and Penzien.

2) Dynamics of structures: Theory and applications to earthquake engineering by Anil K Chopra.

3) Three dimensional static and dynamic analysis of structures by Wilson.( this book is downloadable if you search google).

Regards

SQRT[ (-value^2 + +value^2) ] = +answer

Ajay,
Earthquake dynmaic spectrum (Response Spectrum)results can be combined mostly either by CQC method or SRSS method. In Caesar you will get combination by SRSS method. The full form of SRSS is "Square root of sums of square".

For any parameter like displacement will be calculated based on number of mode shapes considered by you. for example if you have considered 10 number of mode shapes then all the maximum peaks in each modes will be combined by SRSS formula already mentioned by Mr.Richard. Since this formula is using square value of the respective parameter, so the results will be always positive irrespective of +ve or -ve sign.

Anidya Ji is already Mentioned the best reference for understanding the response spectrum method.