ANSYS两段命令流在分析上有啥区别

最近两天试着用ANSYS建一个瞬变模型，想看看程序有什么反应，于是按上图中（a）编段命令流。

第一步：我是用BEAM188单元是三维单元，认为建立模型时应该将平面外约束都给约束上，于是有下面的命令流：

FINISH$ /CLEAR$ /PREP7  
ET, 1, BEAM188$ TYPE, 1        
C30=1$ MP, EX, C30, 33000000000     
MP, PRXY, C30, 0.3$ MP, DENS, C30, 2600 
Z1010=1    
SECTYPE, Z1010, BEAM, RECT$ SECDATA, 0.01, 0.01
/VIEW, 1, 1, 1, 1$ /VUP, 1, Z
N, 1, 0, 0, 0$ N, 2, 3, 0, 0$ N, 3, 6, 0, 0$ N, 4, 9, 0, 0    
MAT, C30$ SECNUM, Z1010                                
EN, 1, 1, 2$ EN, 2, 2, 3$ EN, 3, 3, 4     
D, 1, UX, 0, 0, 0, 0, UY, UZ, ROTX, ROTY
D, 2, UZ
D, 3, UZ
D, 4, UX, 0, 0, 0, 0, UY, UZ, ROTX, ROTY
NSLE, , , 2$ ESLN, S$ ENDRELEASE, , -1, ROTZ
NSLE, , , 3$ ESLN, S$ ENDRELEASE, , -1, ROTZ
F, 2, FY, 1             
FINISH                                                 
/SOLU$ SOLVE$ FINISH

运行后出现一个警告和一个错误提示：

 *** WARNING ***                         CP =       2.328   TIME= 22:27:18
 No shape testing has been performed on 2 of the 3 selected elements.   
 To perform shape testing on these elements and see any warning         
 messages about the others, please issue the CHECK command.             

 *** ERROR ***                           CP =       2.703   TIME= 22:27:19
 The value of UY at node 2 is 6.355886571E+13.  It is greater than the  
 current limit of 1000000.  This generally indicates rigid body motion  
 as a result of an unconstrained model.  Verify that your model is      
 properly constrained.  

错误信息应该是几何可变体系造成的，这是我期望的结果，但警告信息是什么意思呢？

第二步：我又琢磨，是不是没有平外约束也可以进行判断，于是把上面的边界约束改了一下，其它部分没有改变，下划线部分改成如下形式：

FINISH$ /CLEAR$ /PREP7  
ET, 1, BEAM188$ TYPE, 1        
C30=1$ MP, EX, C30, 33000000000     
MP, PRXY, C30, 0.3$ MP, DENS, C30, 2600 
Z1010=1    
SECTYPE, Z1010, BEAM, RECT$ SECDATA, 0.01, 0.01
/VIEW, 1, 1, 1, 1$ /VUP, 1, Z
N, 1, 0, 0, 0$ N, 2, 3, 0, 0$ N, 3, 6, 0, 0$ N, 4, 9, 0, 0    
MAT, C30$ SECNUM, Z1010                                
EN, 1, 1, 2$ EN, 2, 2, 3$ EN, 3, 3, 4     
D, 1, UX, 0, 0, 0, 0, UY
D, 4, UX, 0, 0, 0, 0, UY
NSLE, , , 2$ ESLN, S$ ENDRELEASE, , -1, ROTZ
NSLE, , , 3$ ESLN, S$ ENDRELEASE, , -1, ROTZ
F, 2, FY, 1             
FINISH                                                 
/SOLU$ SOLVE$ FINISH

运行后出现两个警告和一个错误提示，多了一个警告信息：

 *** WARNING ***                         CP =       3.906   TIME= 22:31:35
 No shape testing has been performed on 2 of the 3 selected elements.   
 To perform shape testing on these elements and see any warning         
 messages about the others, please issue the CHECK command.             

 *** WARNING ***                         CP =       4.344   TIME= 22:31:37
 Small equation solver pivot term encountered at ROTX DOF of node 5.    
 ANSYS automatically constrained this DOF.  Check for an insufficiently 
 constrained model.                                                     

 *** ERROR ***                           CP =       4.344   TIME= 22:31:37
 The value of UY at node 2 is 1.475167508E+14.  It is greater than the  
 current limit of 1000000.  This generally indicates rigid body motion  
 as a result of an unconstrained model.  Verify that your model is      
 properly constrained.

第三步：于是我将下划线部分改成：

D, 1, UX, 0, 0, 0, 0, UY, ROTX 
D, 4, UX, 0, 0, 0, 0, UY, ROTX

运行后出现一个警告和一个错误提示，跟第一步结果类似。

问题

第一个：为什么要加ROTX这个约束？

第二个：perform shape testing 该如何修改？