FEA model for the whole object


Often the objects which we design require a more detailed analysis at the level of a specified fragment of the structure or element. For this purpose, often the whole object is modelled for the purpose of vertical element dimensioning, and horizontal elements such as floors are designed in a separate model, assuming their certain static scheme as faithfully as possible reflecting the global behaviour of these separated elements.

The problem begins to appear when the separated element must be loaded also with the remaining fragment of the structure, which we wanted to get rid of in order to reduce the large model. For the simplest example – I would like to analyse a complicated foundation slab – its separation from the model will not help me much, because loads which dimension it are transferred from the whole structure by means of columns or walls. The simple conclusion from this…I can isolate the slab as long as I load it according to the building scheme.

Advance Design allows you to exchange the support reactions of one model for loads generated in another model.

Fig. 1. Model of a simple residential and commercial building in Advance Design

At the moment I have a model of the entire building, which I can easily solve. However, I would like to divide the model e.g. into an underground and an aboveground part or into a foundation slab and the remaining part of the building. Maybe I need to analyse the foundation slab in detail and I need to reduce the size of the model to gain calculation time. Maybe I would like to divide the work into 2 workstations and leave the development of the ground slab to one of the co-workers and deal with the vertical elements of floor -1 or higher I am able to do this by creating, in a way, 2 independent models (e.g. of the said underground and aboveground part). The problem arises in the fact that the aboveground part will load the underground part, and I have just removed it from the model.

Fig. 2. Two independent FEA models

2 Foundation slab modelled on an elastic foundation, the part above the foundation slab supported by nominal rigid supports. At this point I can solve model one – i.e. the part above the foundation slab – without a problem.

Fig. 3. Reactions (vertical) from permanent loads

Saving reactions to a file and importing them in another task

Above are examples of support reactions from permanent loads. Of course, we can transfer all reactions (displacement/rotation) from all cases.

Please note that reactions are usually presented as an inverse vector, i.e. as a response of a support – here, however, our vertical reaction is directed downwards, as it is later to be a load on a foundation slab. The reverse of reactions can be reversed by changing the program settings in the results tab by switching off the option “Include reactions on supports”. On the BIM tab, the user can export the reactions to a text file and import them into the foundation slab model in the same way. The load cases and the position of forces in space are preserved.

Fig. 4. Reactions imported into the foundation slab model

Importantly, I can import reactions at any time, meaning potential changes to the output model are not threatening. I can also modify the geometry of the foundation slab freely – the loads are not associated with it, they are in a specific space in the model and load the element underneath them. The forces are in the same load cases as in the original model so the combinations do not change. I could, however, combine the loads differently because in a smaller, detailed foundation slab design I will be able to successfully prepare more combinations.

The only thing I would like to point out is that it is necessary to separate structural elements sensibly. Their work under loads may be influenced by the elements that we have removed. That is, in addition to transferring loads, they also stiffen the component under consideration and change its working character somewhat. It is relatively correct to separate the whole storey.

This method can also lead to a kind of phasing of the structure.

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