我遇到一个问题想请教下，我给梁加均布荷载，用sfbeam命令，可以加载上，但计算时老是弹出如下的警告
*** WARNING *** CP = 8.219 TIME= 15:07:08
Out of the 16945 defined elements, only 100 elements are selected.

*** WARNING *** CP = 8.234 TIME= 15:07:08
Coupled node set 49 has unused node 100621.
*** WARNING *** SUPPRESSED MESSAGE CP = 8.562 TIME= 15:07:10
Small negative equation solver pivot term encountered at UY DOF of node
1324. Check for an insufficiently constrained model.

*** ERROR *** SUPPRESSED MESSAGE CP = 8.562 TIME= 15:07:10
The value of UY at node 1218 is 1.828820271E+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.

但是直接用GUI加载就能计算，只是什么原因呢？ 命令流部分如下：
esel,none
esel,s,,,1286,1294
esel,a,,,2697,2705
esel,a,,,3050,3058
esel,a,,,1639,1647
sfbeam,all,1,pres,250000,,

第一个警告不知道怎么回事
第二个出现的节点是建支座时耦合的节点
第三个也不知道是怎么回事
第四个我试了下，它提示的节点刚好是加均布荷载时荷载端对应的节点

请各位大牛帮帮我~