# Modeling a Girder Grillage

### Technical Article

The structural analysis of a girder grillage is usually no big deal when using computer-aided calculation. There are different options to display and analyze a structure. They include, for example, the conventional way with performing structural analysis of components or modeling the entire structure.

Each option has certain advantages which have to be weighed up by the structural engineer.

#### Calculating as 3D Structure

Modeling in 3D provides flexibility. The acting loading can be defined directly to the supporting elements and a load distribution takes place automatically. The self-weight of the girders can be applied automatically here by means of their dimensions.

All this together offers the advantage that the loading as well as the structure are displayed graphically and can be checked. The structural engineer has a more detailed overview of the forces in the structure by applying the precise loading to the structure and carrying out exact modeling. A better design ratio and thus saving costs in the entire structure, but also in secondary components are possible.

Another big advantage of this calculation method is the handling of changes in the architecture. They can be taken over very easily to the structure which thus saves a lot of time.

#### Different Types of Modeling

The programs often offer different options to display the structure. It is, for example, possible to display the structure in one plane, the system axes. This prevents that additional internal forces are introduced to the structure due to eccentricities. In order to avoid that the individual components are displayed rigidly, it is necessary to arrange the hinges at the intersection points of the members.

In the present example, moment hinges (φy, φz) are arranged at the connections of the secondary beam to the main beam to prevent that the internal forces are transmitted for these forces. In the middle, where an effect of continuity of the secondary beams should occur, hinges with special degrees of freedom were arranged so that the internal force My can be transferred, but no torsion will be introduced into the main beam. These special hinges are called scissors hinges in RFEM/RSTAB.

The results display shows that only internal forces in Mdirection occur in the structure with the existing modeling. Furthermore, this structure can only be designed according to EC 3.

It may happen that the structure should be displayed with eccentricities. With the program, it is very easy to carry out this task. In RFEM/RSTAB, it is possible to arrange the offset automatically by means of the size of the section. This automatism offers the advantage that the size of the offset changes when the cross-section is modified.

When choosing this modeling, please pay attention to the arrangement of the hinges as well as to the degrees of freedom of the support conditions in order to not generate unintentionally additional forces in the structure.

#### Summary

A three-dimensional display of the structure simplifies the work of the structural engineer. They include, to name but a few, the automatic load distribution or the easier display of subsequent changes in the structure. However, special attention must be paid to the modeling to avoid unintentional effects.