L12=900;//leangth of section 1-2(outer),given in mm
L34=300;//leangth of section 3-4,given in mm
L13=600;//leangth of section 1-3,given in mm
t13=2;//thickness of section 1-3,given in mm
t12=1.5;//thickness of section 1-2,given in mm
T=10*10^6;//given in N.mm
A=20000;//given in N.mm^2
G=25000;//given in N.mm^2