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What is the function of the option 'rib' that you find in the properties menu of the results?

If you have modeled a slab with ribs, an option rib will appear in the properties window of the result. This option can be ticked.  This is the case for the results of both the slab and the beam (rib).

The meaning of this is the following. Ticking on the option rib means that you want to show the results for the entire T-section. When deselecting the option rib, the results are displayed separately for the part of the beam (rib) that hangs under the slab and for the slab itself.

In Scia Engineer a concrete slab of 9 x 6 m with a thickness of 200 mm is modelled as an example with ribs on the mutual headway of 1m with a section of 200 x 400 mm.  The effective width that is given for the rib is 0.5m. The load consists of a floor load of 5kN/m² and self weight (results shown for ULS comb).

The moment My for the beam separately (option rib ticked off) and for the entire T-section (option rib ticked on) is displayed below (respectively left and right).

The results for the moment mx in the slab are shown below. If the option rib is ticked off (bottom right), the moment mx will be equal to zero over the effective width because it is calculated when considering the beam as a T-section. This way the moment will not be taken into account twice.