kill(all);

the structure nodes are numbered counter-clockwisely, starting from the externally loaded point "1".

Mf_12 :    0*(1-xi) + F*a*xi$
Mf_23 :    0*(1-xi) +   0*xi$
Mf_34 : -F*a*(1-xi) +   0*xi$

Mt_12 :   0$
Mt_23 : F*a$
Mt_34 :   0$

U: integrate( (Mf_12^2/2/E/J + Mt_12^2/2/G/Jp) * a , xi, 0,1 )
  +integrate( (Mf_23^2/2/E/J + Mt_23^2/2/G/Jp) * b , xi, 0,1 )
  +integrate( (Mf_34^2/2/E/J + Mt_34^2/2/G/Jp) * a , xi, 0,1 );

delta : diff(U,F);

dim:[a=800,b=1000,c=600,F=1000,E=210000,G=210000/2/(1+3/10),J=(40^4-36^4)*%pi/64,Jp=(40^4-36^4)*%pi/32];

ev(delta,dim,numer);