各位前辈好，我在用etabs做一个18层的分析时出现这个警告信息R I T Z A N A L Y S I S 11:46:13

NUMBER OF STIFFNESS DEGREES OF FREEDOM = 41088
NUMBER OF MASS DEGREES OF FREEDOM = 20544
NUMBER OF RITZ MODES SOUGHT = 18
NUMBER OF STARTING VECTORS = 91
VECTOR BLOCK SIZE = 91
FREQUENCY SHIFT (CENTER) (CYC/TIME) = .000000

* * * W A R N I N G * * *
FOR STARTING LOAD VECTOR: LOAD = DEAD
LOAD IS APPLIED TO ONE OR MORE MASSLESS DEGREES OF FREEDOM,
CORRESPONDING RITZ MODES MAY BE UNAVAILABLE OR INACCURATE FOR DYNAMICS


* * * W A R N I N G * * *
FOR STARTING LOAD VECTOR: LOAD = LIVE
LOAD IS APPLIED TO ONE OR MORE MASSLESS DEGREES OF FREEDOM,
CORRESPONDING RITZ MODES MAY BE UNAVAILABLE OR INACCURATE FOR DYNAMICS
振型数采用18个，初始向量有xyz方向的加速度和恒载以及活载，以前做了好多都没有出现类似的问题，这个是什么原因导致的，我真不知道，质量源定义也没有错误啊，请高手们帮帮我，这是哪儿的问题，应该怎么改？