I am currently using abaqus 6.21 to analyse the dynamic response of vibration through the soil....I tried using direct integration and modal analysis...i understand that direct is for non linear system and modal is for linear system....but the results i get for direct don't seems logical but seems more logical for the modal case....what i figured out was that the damping value used is causing this....but i can't be sure of it...can someone tell me which method is more correct and what value to use for the damping...i tried rayleigh's damping for direct by finding the alpha and beta value by solving simultaneous eqn using the first two modes of the system.... and i input 5% damping for modal...can someone tell me how to get a correct damping value for direct method?...and if i need to use modal...is 5% the correct value to be used?