DLF/ Spectrum generator

Hi,
Can some one tell me when we use DLF/Spectrum generator and what exactly does it do.
Do we need to use this option in time history analysis .
i have also attached the figure showing this button in Caesar.

The DLF/Spectrum Generator takes a time-history pulse or wave form and converts it from the "time domain" into a DLF in the "frequency domain". There is an article about this in the November 1994 newsletter (available on this site).

Hi Richard ,
Thanks for providing me the vital resources I have gone through this news letter and new letter related to time history analysis.
To better undertand DLF / Specrtum Generator.
1.Consider a verical straight pipe 6 m.Anchor at node 10 and and guide at another end at node 30.(6m from anchor at node).Create an intermediate node 20 at 3m.
2.At intermedaite node 20 we have a dyanmic load of 30 kN on - z direction.
3.Now go to time history module enter the force time profile directly without using DLF/Spectrum generator.
see the the results and deflection at node 20 in Z direction comes say =10 mm.
Now if in the above problem with all parametrs same ,if i enter the force time profile using DLF/ spectrum generator. The displcement at node 20= 20 mm( almost double).
My question is
1. why in the simple force time profile (without using DLF spectrum generator)i could not get 20 mm displacment at node 20.Does simple time history(without using DLF spectrum generator) give results without considering DLF factor.
2. How do we relate these two discrepancies in result? OR

3.We always have to use DLF spectrum generator

Hi Richard,
If DLF/Spectrum Generator takes a time-history pulse or wave form and converts it from the "time domain" into a DLF in the "frequency domain" then why do i get different displacemnts in above example.

The time history analysis is exact (assuming your input is correct). The Response Spectrum method is statistical in nature. You will not obtain the same results between the two methods.

Another way to say what Rich said...
The force response spectrum typically combines modal response using square root sum of the squares while time history combines mode algebraically. Depending on modal signs, time history results could be greater or less than the response spectrum results.

Hi Dave and Richard,
Thanks masters of Caesar for making this clear and i concude from this discussion that ,i should not use DLF /Specrtum generator while doing Time history analysis.
I should also do some homework on vibrations and please suggest some simple book on this topic.

Regards