Options in RFEM for Considering Construction Stages

Technical Article

The current state of the development of finite element analysis software and computer technology allows for the calculation of more complex structure. More and more frequently, the FE calculation is performed on the entire model. In this context, certain practical construction problems may appear. One of these problems is the consideration of a construction process in the model.

Problem Background

Generally, buildings are not constructed in one go, but rather in stages. From a structural point of view, the individual structural parts are combined into a building during the construction. At the latest upon completion of the construction process, the structure designed for the final state is available. Before reaching the final state, the structure goes through the various construction stages.

When adding the structural parts to the structure during construction, their self‑weight is active. The effective self‑weight has to be derived either from the structure itself or from the auxiliary structures, and implemented in the building ground. This also applies to the loads limited in time, which may occur due to the construction method or environmental conditions during the construction. Which structural parts are effective and in which structural system is also dependent on the construction methods and the construction process. Depending on whether the structure or the individual parts are used for load application during the construction, the structural parts of a building are already subjected to loading before the final stage, which results in deformations.

Options in RFEM

RFEM provides two basic options for displaying or considering the construction stages in building construction. First, there is the RF‑STAGES add‑on module. Second, RFEM provides the option to deactivate the individual structural elements in load cases and load combinations.

The RF-STAGES add‑on module analyses the structure in the individual construction stages using a nonlinear FEM calculation. This method allows you to consider nonlinearities and redistributions due to removal of the structural elements. An equation system is created, which is then modified and calculated in the individual stages, depending on the settings made. This is an iterative calculation. The equilibrium conditions must be met for each stage.

$$\lbrack\mathrm K(\mathrm u)\rbrack\;\cdot\;\left\{\mathrm u\right\}\;=\;\left\{\mathrm F\right\}$$
is the stiffness matrix
{u}  is the deformation vector
{F}  is the force vector

In RFEM, you can deactivate the individual structural elements (members, surfaces, solids, nodal supports, line supports, surface supports) for a load case or load combination. This option allows you to display simple construction stages, for example, a linear building system. For each construction stage to be considered, you have to create a separate dead load case or a load case with permanent loads. In RFEM, you can select the ‘Deactivate’ check box in the calculation parameters of the load case. In the last tab, the individual structural parts can be deactivated, which will not be effective at the time of the construction stage.

Figure 01 - Calculation Parameters of Load Case

Figure 02 - Deactivating Structural Parts

The results of the individual stages can be superimposed later in a result combination. In the case of this approach, please note that this is a linear superposition of the individual stages’ results, which are calculated on an undeformed system.

Figure 03 - Result Combination

Comparison of Both Options in Example

Now, here is the comparison of both methods shown in a general example of the building construction. The example is a four‑storey reinforced concrete structure, which is designed in four stages. The individual floors are built one after the other and no structural part is removed during the construction progress.

Figure 04 - RF-STAGES: Deformations at Final Stage

Figure 05 - RFEM: Deformations at Final Stage

Figure 06 - RFEM: Model Without Construction Stages

Result
max uZ [mm] Model
7.06 Model in RF-STAGES
7.06 Model RFEM with construction stages
7.89 Model RFEM without construction stage

Summary

The example shows that it is possible to display simple stages of building construction in RFEM by deactivating structural elements. In spite of basically different methods, the same deformations of the structure result in RFEM and RF‑STAGES. Furthermore, the example shows that the consideration of construction stages for a four‑storey building has an evident effect on the deformations due to dead and fixed loads. By considering the construction stages, the resulting deformations are about ten percent lower than the deformations resulting from the calculation on the entire model.

Reference

[1]   Barth, C. & Rustler, W. (2013). Finite Elemente in der Baustatik-Praxis (1st ed.). Berlin: Beuth.
[1]   Langhammer, A. (2013). Berechnung und Modellierung von Systemänderungen im Zuge von Bauzuständen im Rahmen von Finite-Elemente-Lösungen (Dissertation). Dresden: Hochschule für Technik und Wirtschaft.

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