How to automatically adapt a load in STATIK when we change the position of the bar or the knot on which it acts?

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The loads in STATIK are linear or point objects that are located anywhere in the model space. It is only when the program checks the loads that the link between the loads and the nodes or members occurs, depending on whether the loads act on one or the other.

When we move a member or a node the load does not follow it automatically, i.e. the load will be “floating” in space and will no longer be linked to the node or member.

The purpose of this article is to present a function that will help us to automatically adapt a load in STATIK when we move the knot or the bar on which it acts. This will not always be possible and will depend on the changes that have been made to the structure, but we will be able to use it in many cases (we will see the limitations later).

The function that allows us to adapt the loads is available in the dialog box that appears when we ask the program for the list of load hypotheses in the “Loads” tab:

Suppose we have a bar on which acts a linear load as shown in the following figure:

This bar is located on the axis y=0, z=0 and has a length of 10m, being its ends located at the coordinate points (0,0,0) and (10,0,0).

If we move the bar to any part of the space, we will see that the load will not move and will stay “floating” in its original position:

In this case, we have moved the bar vertically by 10m.

If we click on the button indicated above, we see that the following dialog box appears:

In this table we see that we have two blocks: the first one refers to the strategy to follow to move the load and the second one refers to which loads we want to move.

1.- Strategy.

a) Shortest distance to the bars/knots: with this strategy the program will detect the shortest distance between the elements and will move the loads following that direction.

b) Given displacement direction: if the displacement vector is known, we can indicate it and the load will be displaced following it.

2.- Parameters

The “Adapt loads to structure” button can be pressed with one or several load hypotheses selected, or with none, or with one or several loads selected within the active load hypothesis. Depending on the situation, we can ask it to adapt all loads of the selected hypotheses or we can select which loads we want to move within the active load hypothesis. Thus, we will have several options:

2.1- Adapt loads to the structure

a) All loads in the selected hypothesis: all loads of the selected hypothesis will be moved.

b) All selected loads: only the loads selected in the active load hypothesis will be shifted.

To be able to press “Adapt” in this option, we will have to have, logically, some load selected.

2.2- Maximum displacement distance

This parameter is key to indicate to the program the radius of action, i.e. the maximum distance to which the load can be displaced. If the bar is moved further than the distance indicated here, the load will not be moved.

In the case we have raised, the displacement vector of the bar is (0,0,10) and that vector also coincides with the smallest distance between the load and the bar, therefore, it will be equivalent to select either of the two displacement strategies:

With the first one:

With the second:

The situation will be different if the displacement of the bar does not occur in the line of action of the load and, therefore, the vector of minimum distance does not coincide with the displacement vector of the bar. In this case, the displacement strategy we select will be important. Let's look at it.

If we now move the bar with a displacement vector (3,4,5):

With the first strategy, the load is adapted as follows:

That is, it has not been adapted with the same length as it had. This is because the vector corresponding to the smallest distance does not coincide with the displacement vector of the bar.

With the second option, indicating the displacement vector:

The load is perfectly adapted to the new position of the member.

With the procedure presented in this article (which is analogous for point loads) we have a little help to adapt our loads to possible changes in the geometry of our structure without having to redraw all the loads.