请各位帮我看看这个模型运行出现负的周期是什么原因，如何解决，谢谢！
* * * W A R N I N G * * *
ZERO OR NEGATIVE MODAL-MASS VALUES WERE DETECTED DURING ITERATION,
THE RESULTING EIGEN-MODES MAY NOT BE VALID - CHECK THEM CAREFULLY !!

THE STRUCTURE IS UNSTABLE OR ILL-CONDITIONED !!
CHECK THE STRUCTURE CAREFULLY FOR:
- INADEQUATE SUPPORT CONDITIONS, OR
- ONE OR MORE INTERNAL MECHANISMS, OR
- ZERO OR NEGATIVE STIFFNESS PROPERTIES, OR
- EXTREMELY LARGE STIFFNESS PROPERTIES, OR
- BUCKLING DUE TO P-DELTA OR GEOMETRIC NONLINEARITY, OR
- A FREQUENCY SHIFT (IF ANY) ONTO A NATURAL FREQUENCY

NUMBER OF EIGEN MODES FOUND               =           3
NUMBER OF ITERATIONS PERFORMED            =           7
NUMBER OF STIFFNESS SHIFTS                =           0

没有啊，我这边运行是正常的。好像周期是0.38

没问题，我用sap2000 14打开的。还进行了杆件设计选型。