G.8.2.3 Nonlinear Cable Members for Advanced Cable Analysis
Cable members used for a Advanced Cable Analysis are specified by using the MEMBER CABLE command.
When specifying cable members, either the initial tension in the cable or the unstressed length of the cable must first be provided. The initial tension is sufficient for keeping the cable in tension because the catenary theory behind the element formulation will prevent the cable from behaving in compression. However, a larger initial tension (or smaller unstressed length) increases the numerical stability
A nonlinear cable may have large deformation. The force equilibrium at cable ends is checked on every load step and on every equilibrium iteration.
- The nonlinear cable element provides stiffness and resistance forces to only three translational degrees (i.e., FX, FY, and FZ). It is not able to carry any moments. So, when two nonlinear cable elements meet at a common joint –and if there isn’t a support or a 3rd member connected to the joint– it is a point of potential instability. The program introduces a very weak spring to overcome this problem.
- Due to the reason as described in item 1, applying any moment to a cable element is not advisable.
- The cable is not able to carry any compression forces. So, when the load tends to cause compression in a cable –and if there are no other members connected to the cable and the cable can deform freely– there will be some numerical instability.
- Due to the instability problem explained in item 1, subdividing a cable member into several smaller cable elements should be done only when it is necessary. An example of one such necessary case may be when there is force load applied in the middle of the cable member. For this case, the cable member has to be broken so that the forces can be applied as joint loads at cable nodes.
- The increase of stiffness of the cable (by increasing the initial tension or decreasing unstressed length) is always beneficial for numerical stability. The iteration will stop only when the force equilibrium is reached.
- In advanced cable analysis, the self weight of a cable member is initially considered to obtain the initial configuration of cable members under self weight. Any additional weight required along with self weight can be included using FWY parameter in the MEMBER CABLE command.