A question that we have repeatedly received and that has been the subject of interesting discussions is how FAGUS calculated the value of the effective height d depending on which behavioural models are chosen for concrete on the ‘Analysis parameters’ box.
It is known that the effective height d is defined in the Eurocode as the distance from the most compressed fibre to the centre of gravity of the reinforcements subjected to tension.
The computation of this parameter is direct when there is a regular cross section in which the centre of gravity of the reinforcements in tension is exactly known, however, it is not as obvious if there is an irregular cross section.
The cases that the Eurocode shows are very simple and, in them, determining the value of the useful height of the cross sections is straightforward. FAGUS can work with cross sections with any geometry and, hence, it cannot work with geometrical data but rather with centres of gravity of elements in tension and in compression. This is why the definition of the ‘design parameters’ is so important, because depending on the behavioural model chosen for tensions on the concrete and also on the complexity of the cross section, the parameter d will be exactly the one defined by the Eurocode or it will be obtained as a function of the position of the centre of gravity of the zone in tension.
If the 2/1 behavioural models are chosen as indicated in the following figure:
We see a parabolic-rectangular model for the compressive behaviour (model 2) and assuming that concrete admits stresses equal to its fct for the tensile behaviour (model 1), the centre of gravity of the passive reinforcements will no longer be used to obtain the useful height but instead, all the elements in tension (including the concrete) take part in its calculation.
At this point, it is important to highlight the fact that the program can consider that the concrete works in tension and, thus, cracking analysis can be carried out with that hypothesis. The suitability of this will always be up to the calculist. From CUBUS we always recommend to make a cracking calculation without considering the tensile resistance of concrete (model 0).