Problem with using Abaqus in Earthquake analysis

Dear All,

 I have a problem simulating earthquake effect on a very simple structure. The structure is a 2 story steel plan frame structure. with lumped mass at its corners.

I am applying the Northbridge earthquake on it but I am getting very strange results.

 I am applying the Material density beside the lumped mass at corners.

 Both, eignvalues and frequencies and also responses are not correct.

 I am attaching the input file and also the acceleration time history for the earthquake. The input file extension was changed to txt so the server can accept it.

 Looking forward to hearing from you.

 Regards for all,

 Maher Elabd