Thank you so much for your reply and your advice. I will try the multi-body part and will see if it works. On the other hand, it seems that the actual structure is really unstable. I got many different results every time I solved and they are really sensitive to the value of prestressed and the external force that I put on. It makes me so confused and I have many doubts about the modelling especially the joint.
That lead me to the second question. Since I use a spherical joint (DOF=3 RX RY RZ) to connect two different types of element BEAM188 (DOF=6) and LINK180 (DOF=3), the question is : Does it cause any conflicts when we connect the two elements with the different DOF ?