that's the analysis of the beam .... there is no torsion at all on the beam even i put dead load and live load on just on side ( i expected some torsion when i do that but for my surprise there was no torsion. i don't really know why)
may be it's just a plastic settlement crack as the guys said...
http://files.engineering.com/getfile.aspx?folder=a675807c-3d00-4ac1-99bc-0e383d487ab8&file=Capture.PNG
i assume that slab reinforcement is as shown in this pic ....i think it's sufficient to prevent...
sorry eng. Releky i hadn't seen the dimension when i wrote my last reply ...i think with these dimensions 30 cm for this main beam and 10 cms for the slab ...may be u have to worry ....i think that this main beam doesn't have an adequate depth especially that there is no internal column and...
that static system tells that it's a solid slab ....if so, that may be the cause because the rigidity of the solid slab much smaller than the beam .but if it's a flat slab it will be hard to occure . or may be it's more than one cause ..i really started to believe that it could be a pouring...
this photo displays what happens because of rusting of steel. may be it looked like your beam on it's earlier stages .at this photo we can notice that the upper part of the damaged area is the most damaged part . which can tell that may be it begins with a crack at this part ..i can't see ur...
but i think if it was a problem of an inadequate vibration, there would be no way for this crack to appear again after plastering and painting . may be it's because of the rusting of longitudinal bars. i think there r some marks of old wetness on the surface