FE Slab Analysis

The FE Slab design utilises the results of a finite element analysis to provide automation of the slab design process. Thus the first step of the process is to carry out the FE analysis of the structure. For details on creating and analysing a FE model, see the Masterframe FE Analysis section.

As part of the FE Slab design, the FE properties have been updated. Therefore, it is necessary to ensure that the FE materials are set as new Codified Material in the Material/Thickness tab of the FE module. This area layout is shown below.

The FE Module allows for the the selection of Concrete or Steel from the Material Type drop down menu. When set to concrete, a range of concrete strengths are indicated, while selecting steel allows for the selection of standard steel grades. In the case of a slab design, The Material Type should be set to Concrete and an appropriate concrete strength selected.

Mesh setup

To improve the accuracy and resolution of the results in the FE Slab design module, it may be necessary to review the FE mesh after an initial model analysis and modify the meshing parameters. In general, a better resolution of the results occurs with a reduced mesh size. However, a more dense mesh, while reducing the area affected by singularities, will lead to increased peak values and these can affect the determination of the reinforcement in the peak zones which occurs over the head of columns. While peak smoothing can reduce to a degree the effect of this, it can also be helpful to employ stiff regions at the column/wall and slab interfaces.

When reducing mesh sizes and/or using the column/wall stiff regions, the modification of the mesh size to increase the node density results in a longer analysis time. Thus a balance may need to be struck between refining the mesh size and the required increase in the time required to complete the analysis. For larger models, it may be necessary to refine the mesh for the slab design and then suspend all serviceability load cases, thus reducing the number of load cases to be considered while using a reduced mesh size.

Model Setup

The design of punching shear depends on the slab thickness (effective depth) and the footprint of the column, for the internal columns. However, for columns on the edge of a slab, the position of the column relative to the slab edge also influences the length of the shear perimeter. In a general FE analysis, it is often advisable to avoid placing column elements a short distance inboard of a slab edge, since this leads to a significant reduction in the mesh size local to the column which leads to an increase in the complexity of the mesh geometry and a longer analysis time. However, in certain circumstances it may be necessary to model the columns with the column centroid (represented by the 1D line element in Masterframe) in it's true position relative to the edge or boundary of the FE surface to then better represent the shear perimeter associated with the column.

Where is it required to create joints in slabs, these are modelled using FE surface edge releases. To enable the correct punching shear perimeters to be determined in the vicinity of the joint/edge release along the slab boundary, it may be necessary to position the slab edge in the true position relative to the column centrelines to accurately determine the shear stresses in the slab as well as the correct length of the punching shear perimeters.