1. General
In FEM-Design, we can run a cracked section analysis. In a very simplified description, it will calculate the deformation of a concrete element considering reduction of stiffness due to possible cracking. It can affect bars and plates.
2. When to use it
Usage depends on the expected result. On a single element it will affect the stiffness and thus the deformation. It may also influence the internal forces and stresses and redistribute these within the element when cracks appear.
In a building model, it also affects the overall distribution of the forces – since it changes the stiffness of the objects. This way it is suitable for getting more realistic force distribution and thus more realistic internal forces of elements (and more realistic design as well).
In a very big model, this is not always feasible, since the cracked section analysis takes a long time. Moreover, the analysis is iterative and thus non-linear. This also means that it must be run for every load combination the user is interested in and cannot be used with load cases nor with load groups.
More in depth description on how it works, can be read on the FEM-Design WIKI page: https://wiki.fem-design.strusoft.com/xwiki/bin/view/Manuals/Theory%20Manual/Non-linear%20calculations/#HCrackanalysisinFEM-DesignPlate It is also good to take a look at the Cracked section analysis manual in the WIKI: https://wiki.fem-design.strusoft.com/xwiki/bin/view/Manuals/User%20Manual/Analysis/#HCracked-SectionAnalysis
3. How to use it
To run the cracked section analysis, at least one element (or the elements that the user is interested in) must have reinforcement inside. It is not necessary to have the analysis, nor the actual design done. User can just manually draw the reinforcement and then do the analysis with the cracked section selected.
If the element has no reinforcement, it will not be affected by the cracked section option.
To run the analysis with the cracked section option, user must turn on the “Cr.” option for each load combination they are interested in. It can be set from the “Setup by load combinations…” option in the “Calculations” menu.
Figure 1. How to turn on the cracked section analysis
It works with SLS and ULS combinations. It has nothing to do with the design calculation called “crack width” (that crack width design calculation can only be run for quasi-permanent combinations, but here we have no limitation).
Note: Plastic analysis and cracked section analysis cannot be run at the same time. The plastic analysis (“PL”) option is turned off automatically when “Cr” option is selected.
4. Very important additional notes
As mentioned earlier, the results of the cracked section analysis appear in deformations (for single elements). For example, here is a single beam and plate. The internal forces of both elements are the same, but it can be seen how the cracking option has changed the deformation:
Figure 2. Deflections of beam and plate without and with the cracked section analysis turned on
It is very important to understand that stiffness from added reinforcement is considered when “Cr” is selected. This means that if we add much reinforcement, the stiffness of the whole element can increase despite the cracks.
This is also true if the load is not big enough to cause the cracking. Then again, the added reinforcement will increase the stiffness. This is exactly what happened in the plate here, where deformation has been reduced despite the “Cr” option being selected (the plate does not crack, but we add reinforcement to make it stiffer).
Here is an example of the beam that was used in the previous examples. Here it is heavily reinforced. The deformation is decreased from 4,84313mm to 1,82589mm despite the cracking.
Figure 3. Heavily reinforced beam having less deformations with "Cr" than without it