- Model correctly in STATIK the support of a board on its bottom face when the sections have axis point on the top face.

Find out more about CUBUS programs by accessing our website:

www.cubus-software.com

The purpose of this article is to explain how the supports of a bridge deck on its underside can be modeled in STATIK.

The introduction of an axis point in a section to be used in a STATIK model means that the axis of member introduction passes through that point, i.e. the member axis no longer passes through the cdg of the section. In other words, introducing an axis point in a section causes the section to move in its plane until it is positioned so that the axis of the member passes through that point. We enter the axis of the member and then choose which point represents that axis: by default it is the CDG but if there is an axis point, it is the axis point.

The bars or structure lines that constitute the STATIK models connect at nodes. These nodes can also connect to supports, which will define the boundary conditions of our structure.

Let us suppose that we want to model a bridge whose deck is made up of box-section segments that will be supported at its base.

In this case, we have a segmental section supported at its base on the piers and abutments and the loads will act on the top face of the section. This must be modeled correctly so that the behavior is as close as possible to reality. If we think, for example, of a braking load, which acts on the upper face as a horizontal load, it will cause the center of gravity of the section to have the corresponding axial force and the moment caused by that axial force due to the eccentricity between the load and the center of gravity.

In STATIK, the loads are introduced on the axis of the members. Since in this case we want the loads to be introduced on the top face of the board, we will have to define an axis point on that face so that our axis of introduction of the structure line of the board passes through there.

Now, we have to connect the structure line with the rest of the elements. In this case, we want to support the deck on the piles and the supports on the abutments.

We see that there is a vertical distance between the axis of introduction of the board structure line and the support points.

To model this correctly, we have the eccentricities tool between a point on the structure line and a point external to it. In our example, we will have to do this operation 4 times (once for each support), being two of the external points the two supports of the abutments and the other two the top points of the piles.

We will follow the following steps, for example, for the connection in a stirrup:

1.- We introduce a node that has the support conditions we need:

2.- Select the node of the structure line that has to connect with the support, press the right mouse button and go to “Properties”:

3.- We introduce the eccentricities that this node has with respect to the support in global coordinates. In this case, there is a distance of 4.41m in vertical, which supposes an eccentricity of -4.41m in Z:

The same procedure should be carried out on the piles and on the other abutment.

Finally, it should be noted that with this procedure, we avoid using auxiliary elements, such as infinitely rigid bars, which do nothing but dirty the model and make it heavier.