Question
The analysis is performed without any problems in the post-construction stage, but a singular error occurs in the construction stage analysis.
Answer
This error arrives when the boundary or boundary nodes are not activated in the construction stage. Check whether the support nodes were assigned to any structure group and that the structure group is activated in the correct construction stage.
One of the most common mistakes that users make in construction stage analysis modeling is inactivating boundary conditions during the construction stage.
1. Inactivating the Rigid Link of the pylon in the construction stage analysis
The above model indicates a construction stage where the pier, girder, and pylon of a cable-stayed bridge are activated. What needs to be carefully examined here are the node points marked in blue at the upper part of the pylon.
A rigid Link is a boundary condition used to model the structure with rigid body motion, and it is one of the parts of the structure. However, in some cases, Rigid Link is not activated for the pylon, as cables have not been installed. Therefore, some users choose to activate the Rigid Link later along with the cables, rather than activating it simultaneously with the tower structure.
In this case, the displacement of the master nodes and slave nodes in the structure becomes unrelated until the Rigid Link is activated. This means that the master node will have displacement, while the slave nodes will remain in their defined position without any constraints.
That would result in inappropriate modeling, as the slave node of Rigid Link would remain in its original position while the master node of Rigid Link would have already had displacement when the cables were installed.
2. Inactivation of boundary conditions in construction stage analysis
The first model in the above image represents the boundary conditions in the Base stage, while the model below it represents the boundary conditions during the construction phase. For the model that considers construction stage analysis, it is important to check if there are boundary conditions defined for each construction stage to ensure that the structure does not become unstable. Make sure that boundary conditions are properly activated or inactivated in the construction stage analysis.