A very usual computation that calculation engineers face on a daily basis is the computation of sections that evolve as the building process advances. These are section with various concreting phases, which can be prestressed or not, and where every one of the constituent elements of the definitive section comes in play in different moments.
When a problem is proposed, FAGUS can solve it in two different ways:
- Introducing various sections that correspond to each building phase and then carry out a particular analysis for every one of them.
- Carrying out a batch calculation.
To explain the calculation simply let’s propose the following case:
There is a precast and prestressed section to which a compression slab will be added on top. It is the typical case in which the simple section must be verified with regard to its Self-Weight and Prestressing and the composite section must be verified with regard to Dead Load and Live Load. The support section of the beam (negative moment) will be taken with the following phases:
- PHASE 1: Section without the compression slab and subjected to the Prestressing and Self-Weight load of the slab. The moment obtained from the global model of the structure is M1=-100.00 kNm.
- PHASE 2: Section with the compression slab and subjected to the Dead Loads. The moment obtained from the global model of the structure is M2=-50.00 kNm.
- PHASE 3: Section with the compression slab subjected to the Live Load. The moment obtained from the global model of the structure is M1=-150.00 kNm.
The complete section being studied is:
In it the two different concrete elements can be seen, the inverted T-section (C1) on the bottom and the compression slab (C2) on top.
The section with the dimensioned reinforcements is the following:
In it all the reinforcements that will exist in the final section can be seen.
According to classic theory, to carry out the section analysis in phases, each one of them will begin with the stress condition and residual strain from the previous phase. This is why it is interesting for these analyses to run a batch calculation with FAGUS.
If it is chosen to implement various sections equivalent to each one of the phases, the results obtained on the previous phase must be introduced manually so that FAGUS has that starting point. It must be taken into account that each section is analysed individually even if they are part of the same file (section library), which means that each section can be considered ‘an independent model’ in FAGUS.
With the batch calculation we only implement one complete section on which we will deactivate and activate the constituent elements, whether we want to consider them on the analysis of the different phases or not. The window of a typical batch calculation with the basic stress and strain outputs for this case would be the following:
In it the calculation sequence that the program will follow to obtain the results corresponding each of the phases can be seen rather intuitively. It is very important to observe the column ‘Step’ and check that the numbers are correlative and in ascending series: this means that all the steps are being carried out on the same section (but with some activated and deactivated parts). The results output would be the following:
In the results presentation a Phase 0 has been included in which the initial section has been analysed considering only the initial strain of the pretension (5.8 per thousand). This corresponds to step 2 of the calculation in series, that is, if we want to know the stress state of our section subjected simply to the prestressing, we must analyse the tensions given the stresses but entering 0 in all the fields and then recording all the results (printer icon).
The analysis in series tool allows the introduction of prestrains on any point using the icon shown in the image:
When a prestressing is introduced in the section (as in the example) it is not necessary to run this process because on the definition of the prestressing itself in the reinforcements tab that value can be entered and the program already considers it.
That is why there is a value of 5.8 per thousand (on yellow below) in all results in all phases.