Deactivation of Elements

Question:

Hi all,

we have been looking into a construction sequence model and noticed some strange behaviour.
1) Deactivating an element also deactivates the boundary conditions (supports/springs) attributed to the node of the element, essentially removing one of our support when this is not the intention. We are looking to cut out a temporary prop, but the pile below remains providing support (see CS: 3 in attached model where the supports are missing).
2) The solution for CS: 3 is not correct. The reactions remain from the previous stage, while the forces in the members have been recalculated for the new structural system:
 

Can you please have a look and comment? This seems like a fundamental error in the CS solver for deactivating elements if there is no explanation for it.

Please note that this is quite urgent for us as we have a deadline next Monday and currently our model is giving incorrect results.

Thanks and regards,



Answer:

Hi,

I got a reply from the developer.
After all, this issue resulted from the fact that node no. 2 was not included in the structure group 1. Once node 2 is included in the structure group 1, the boundary conditions of node 2 are not deactivated at stage 3 and the reactions are as shown below.

In your original model, untitled.mgb, node 2 is not assigned to any structure group. In this case, the program follows the rule below. 
At stage 1, element 16 is activated and the associated node 2 is automatically activated.
At stage 2, element 1 and 2 are activated and the associated node 2 is automatically activated one more time.
At stage 3, element 16 is deactivated and the associated node 2 is automatically deactivated. Note that the program decides that node 2 was activated at stage 2 rather than stage 1.

At stage 3, Boundary Group 2 of node 2 is activated at stage 1, while node 2 is activated at stage 2. Thus, the program decides that at stage 3, Boundary Group 2 is not assigned. This is why the point spring support is not displayed at stage 3.

On the other hand, Boundary Group 2 is never deactivated since it is activated at stage 1. This is why we got reaction at node 2.

In conclusion, it is recommend to activate/deactivate nodes instead of elements when boundary group which is assigned to the nodes is activated/deactivated. Although the current status might not be perfect. we would like to keep it because a change about this rule can cause another confusion among other users.

Regards,
DK

Creation date: 8/10/2018 2:33 AM (dklee@midasit.com)      Updated: 8/10/2018 12:05 PM (dklee@midasit.com)
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