How to model the reinforced concrete flat-slab structure using the EQUIVALENT FRAME method?

14 January 2017

This post describes the main assumption of modelling a two-dimensional equivalent frame for the reinforced concrete flat-slab structure. It shows the modern implementation of calculation algorithms for the equivalent frame method using software for structural analysis. In my video tutorial, I summarise any comments and instructions by showing the particular stages of modelling the flat-slab structure step by step.

What is so specific about modelling the equivaleny frame?

The equivalent frame method assumes that the structural system works within the elastic and isotropic range of materials (this is the material has the same properties in all directions). The key assumption of this method consists in grouping in mind the analysed reinforced concrete flat-slab structure into frames crossing in two mutually perpendicular directions. The structure should be divided into four frames in total, two at each directions, where one frame is always the centre line and one is the end line.

In the next step, rigidity of columns and sections from the equivalent frame should be calculated. Cross-section of frame members has a rectangular shape with the height equal to the slab thickness, and the width corresponding to the axial spacing between columns (in perpendicular direction to the slab section axis). Section rigidity increases in the zone of joining flat floor slab with the column. It should be calculated from the proper equation (equation 6.4 p. 243 [2]). However, the column cross-section in the equivalent frame model is assumed in accordance with its geometry, and in the zone of column passing through the slab line, its rigidity is assumed to correspond to infinite rigidity. From technical point of view, the effect similar to column infinite rigidity in the node can be achieved by increasing nearly three times the height of column section, while the width value is constant.

Example of modelling the equivalent frame

This video tutorial shows the methodology of modelling one of four equivalent frames for reinforced concrete flat-slab structure. In the process of modelling I defined not only the main structure axes, but also auxiliary axes to determine sizes of flat floor slabs and columns. In that way, it is easier to enter different cross-section values for column and slab section lengths. The equivalent frame model was developed using Autodesk Robot Structural Analysis 2016 software.