There are 290 small equation solver pivot terms (e.g., at the ROTZ      

 degree of freedom of node 337).  Please check for an insufficiently     

 constrained model.  If you wish to bypass this error message without    

 resolving the cause of the small pivots, use the PIVCHECK,OFF command.  

如上，模型為雙層殼接觸，頂層殼施加半徑預應力，觀察底層殼滑移與彎曲情況，建立四分之一殼模型，約束為關于兩條直角邊對稱，中心點處無移動，為何會報錯說節點337處ROTZ自由度問題?。▓D中紅線的點），求大神解答，不勝感激。