free vibration of the natural frequency

thanks for sharing the std and now my question why u consider y as -1 (because of self wt ) or either some other reason and my case i need to do lumped mass on top (250 ton) so kindly suggest for that 

2. mass participation is lesser than 80% in that std file

Yes. The -1 was to simulate the gravity.

Perhaps to avoid confusion, you can create 2 load cases. 1- for the static analysis per saying and 2- for the vibration analysis.

250 ton is equivalent to 250 000 kg * 9.81 m/s^2 = 2452 kN applied at all directions, see picture

Another option would be to create a "DUMMY" element at the top which would replace the forces. This element would have a MODIFIED density for example.

Assuming: 1 meter long dummy element and the same cross section as the tower (7.5 m diameter or Area = 44.178m^2 ), the dummy element density = 250 000 kg / ( 44.178m^2 * 1 m) =  5659 kg / m^3 or in terms of force density = 5659 kg / m^3 * gravity = 55.494 kN/m^3

Cantilever Tower Frequency dummy.STD

***ERROR*** MATERIAL TYPE HAS NOT BEEN PROVIDED YET.
STRENGTH COMMAND MUST FOLLOW TYPE COMMAND IN MATERIAL DEFINITION BLOCK. it showing those error and i removed  RY 1 RY 1 even though it wont go  i m using staad connect edition 

and 2 case u said i need to apply all three global direction of 2500kn as a nodal load and then x=1 and y=1 and z=1 and nodal analysis bcz above picture wasnt clearly visible and thnx for ur quick response

For some reason the material property is missing 

if you want to use the command file editor .. just add the missing lines for the dummy material:

ISOTROPIC DUMMY
E 2.1718e+07
POISSON 0.3
DENSITY 55.494
TYPE CONCRETE
STRENGTH FCU 27578.9

or use the file attached.

0714.Cantilever Tower Frequency dummy.STD

MODE FREQUENCY(CYCLES/SEC) PERIOD(SEC) ACCURACY

1 0.300 3.33521 5.005E-16
2 0.300 3.33521 1.251E-16
3 1.837 0.54450 6.403E-16
4 1.837 0.54450 4.239E-12
5 4.997 0.20013 2.307E-16
6 4.997 0.20013 4.802E-10

MODE FREQUENCY(CYCLES/SEC) PERIOD(SEC) ACCURACY

1 0.288 3.47082 0.000E+00
2 0.288 3.47082 2.710E-16
3 1.772 0.56444 2.294E-16
4 1.772 0.56444 7.146E-13
5 4.842 0.20651 1.106E-11
6 4.842 0.20651 1.300E-08

see here so much variation of 2. dummy beam ,1. nodal force in all direction 

Please, check the your model

both methodologies are giving me identical results.

yes marcelo u were right thank you so much for doing spoon-feeding to me you're best explain person keep it up and i have another issue for prestressing on column can u plz explain send some model like this plz 

yes marcelo u were right thank you so much for doing spoon-feeding to me you're best explain person keep it up and i have another issue for prestressing on column can u plz explain send some model like this plz