WATER TANK TOP SLAB BEHAVIOUR

Question

Hi , 
 Please help me to sort out this issue. 
 Actually, I am modeling an underground rectangular water tank of 13m x 10m with top slab on it . 
 Top slab is supported on side walls and intermediate beams in between the slab. 
 After assigning all the loads, analysis done. Top slab is behaving as a whole without considering the intermediate beams .(i.e. slab is spanning for whole length ) 
 Hence the slab thickness arrived was very huge. 
 Please help me to sort out this issue. 
 
 regards 
 Varunkumar P

Modestas · Answer

Probably the slab and intermediate beams do not shear common nodes (slab and beams are not meshed). If you could attach your model, we could check it.

Gumpmaster · Answer

Your beams are modeled so that the centroid of the beam is coincident with the centroid of the slab. This will reduce the stiffness of the beams. You can offset the beams in the negative Y direction to achieve the proper stiffness. Ensure that the offset and beam depth don't double count the concrete that's shared between the beam and the slab. The relatively stiffer beams will attract more load and reduce the moment in the slab. 
 The 0.2m top slab thickness is actually on the thin side for such a slab. The US codes would recommend 50mm concrete cover over the reinforcing on a water tank.