I have a structure that is X-braced with cables. I want to perform both types of analysis: Cable and Code Check for the columns and beams? I don't understand why, but I get slightly different results when I do just cable analysis vs. both (shown in attached file). I'm not sure which one is correct.
Question on behalf of Kim Nguyen
Cable Bracing Structure.std
As already mentioned, the difference is due to the method how the program handles advanced cable analysis. It is a purely non-linear process. Following considerations exist in Advanced Cable Analysis: 1. 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. Currently the program introduces a very weak spring to overcome this problem. 2. Due to the reason as described in Point No. 1, applying any moment to a cable element is not advisable. 3. The cable is not able to carry any compression forces. So, when the load tends to cause compression in a cable, if there are no other members connected to the cable and the cable can deform freely, there will be some numerical instability. 4. Due to the instability problem explained in Point No. 1, breaking of 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 5. 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. 6. In advanced cable analysis the self-weight of cable member is initially considered to get the initial configuration of cable members under self-weight. Precaution must be taken not to include self-weight of cable members again in the external applied load. If you want to cater the catenary effect of cables you should go for a non-liner advanced analysis. If you have a simple model, you can simply use linear cable analysis using cable members as tension only members.
Thank you!