help .error when I run.thank you

Question

http://www.mediafire.com/view/rh1m7zravmavcnk/tien.tcl

Diether · Answer

Hi Luan, 
 
This comment must definitely come too late for the objective project, but hopefully the following statements are helpful for the future and for other users viewing this question: 
 
The error message "Negative pivot" is a mathematical expression meaning that the system of equation cannot be solved as an element of the matrix becomes zero or negative, meaning in practice that the static system is not stable, but moveable. 
Most often the reason for this are missing constraints of the support springs in certain directions/rotations. It can also be that the activation of elements in the Schedule is not properly performed. 
 
Within the error message the program gives a hint of the node which is moveable and of the respective DOF (degree of freedom) which is not fixed: DOF 1 to DOF 6 means global direction/rotation Vx, Vy, Vz, Rx, Ry, Rz. However this reported position in many cases must not necessarily be the position where the DOF problem is induced, as the instability of the reported position can be a consequence of the instability of the actual position where the DOF needs to be fixed. 
 
(The subsequently displayed error “Load case xxx does not exist” is a bit misleading in this case as it is a consequence of the program’s not being able to calculate the load case due to the instable system. In this case it doesn’t actually mean that the load case has not been defined in the Load case definition (which is its usual meaning if not preceded by the negative pivot message)). 
 
As a help to easily find the problem, support conditions can also graphically be checked by selecting the option “Draw support conditions” in the View Options – as well as numerically in the respective menu or quickly by holding Ctrl + left clicking the spring element (which opens the element info). 
 
The activation of the current system after the calculation run can as well be checked by selecting the option “Draw active structure” in the View Options or for each stage step by step in the menu “Stage Simulation”.