# truss problems warning RCOND

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Bilal Ates on 30 Jan 2021
Commented: Bilal Ates on 2 Feb 2021
a=cos(pi/6);
b=cos(pi/3);
% Fab Fag Fbc Fbg Fcd Fcf Fcg Fde Fdf Ffe Fgf Ax Ay Ey
T=[ b 1 0 0 0 0 0 0 0 0 0 1 0 0 ;% Ax
a 0 0 0 0 0 0 0 0 0 0 0 1 0 ;% Ay
-b 0 1 b 0 0 0 0 0 0 0 0 0 0 ;% Bx
-a 0 0 a 0 0 0 0 0 0 0 0 0 0 ;% By
0 0 -1 0 1 b b 0 0 0 0 0 0 0 ;% Cx
0 0 0 0 0 a a 0 0 0 0 0 0 0 ;% Cy
0 0 0 0 -1 0 0 b b 0 0 0 0 0 ;% Dx
0 0 0 0 0 0 0 a a 0 0 0 0 0 ;% Dy
0 0 0 0 0 0 0 -b 0 -1 0 0 0 0 ;% Ex
0 0 0 0 0 0 0 -a 0 0 0 0 0 1 ;% Ey
0 0 0 0 0 -b 0 0 -b 1 -1 0 0 0 ;% Fx
0 0 0 0 0 -a 0 0 -a 0 0 0 0 0 ;% Fy
0 -1 0 -b 0 0 -b 0 0 0 1 0 0 0 ;% Gx
0 0 0 -a 0 0 -a 0 0 0 0 0 0 0 ];% Gy
F=[ 0; 0;0;0;0; 0;0; 0;0; 0;0;-8;0;-6 ]; % Applied forces, as shown on the truss
M=inv(T)*(-F)
Warning: Matrix is close to singular or badly scaled. Results
may be inaccurate. RCOND = 3.454464e-19. ???
what should I do
Bilal Ates on 30 Jan 2021
This is the question I want to solve
When I applied the same operation on a different question, it found the correct values. but I did not understand how to apply the method you showed above

Andreas Apostolatos on 31 Jan 2021
Hello,
As John mentioned before, your stiffness matrix 'T' is most likely incorrectly computed.
Using the latter system that you posted, I was able to compute a stiffness matrix with the following sparsity pattern,
Visualizing the sparsity pattern of your stiffness matrix 'T', one can see that in addition to the previous remarks, this stiffness matrix it is even nonsymmetric:
adding to the accurate comment of John that its rank deficiency is way too large. Such systems are typically symmetric since the underlying weak forms are symmetric, unless you add constraints by means of Lagrange Multipliers or similar, which I do not believe is herein the case.
In any case, I was able to solve your system and obtain the following deformation with some custom values for 'E' and 'A', see the following screenshot:
Therefore, I would advise you to review your computation of matrix 'T' because it is most likely incorrect.
I hope this information helps.
Kind Regards,
Andreas
Bilal Ates on 2 Feb 2021
Thank you all for your help, I ended the mission. I wish you good work and success @Andreas Apostolatos @John D'Errico

R2019a

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