If you have a flat slab with maximum span 7.5m, is it economical to design it as Flat Slab? And what is the minimum thickness for it?

Yes, it's economical to use 7.5 Span for flat slab as it varies from 5 to 20 meters. Thickness varies from 10 cm to 50 cm. For 7.5 m span, 25cm thickness may be suitable.

I think it`s ok.thickness divide the longer span / 32

minimum span 22 cm and it also economical

Its depend firstly for the type of slab , example  Fixed or simply supported or flat or one way or two way.

SO we need the other span length ?? if its two way span, and its depend on the building use ,AS we now the live load and dead load different between buildings.

The concrete design depend for many issues and calculations,Case by Case   

But if the question means7.5 *7.5 slab only we can design it as two way solid slab by thickens from up to cm as per design load   

 

Flat slab span is mostly ranging from 7-12m and still economical. For the minimum thickness you can refer to ACI 318M-11 table9.5(a) where for the flat slab you have an element with both ends continuous so 7500/28= 270mm. The punching forces are the most critical forces in flat slab but they can be avoided by multiple solutions such increasing slab thickness, columns' dimensions or concrete grade. Most project just add drop panels which has to satisfy ACI, span between two columns divided by3 is the minimum drop dimension. Additionally, to provide economical solution and ease the construction work you can just design flat slab with double headed studs that will resist the punching shear based on ACI 318M-11, 11.11.5. 

thanks for the invitation

I agree with the answer Engineer / Mohamed Massad

this is the maximum span what is the minimum to determine if this slab one way or two way and what is the condition of this slab if it sample support or continuous and determine if we can use solid or flat or hollow core slab as per the architectural we should see and study the geometry and determine what is the suitable system.

it is ( not ) economical the minimum thickness is cm

Yes it is, minimum slab thickness for up to 30mts is 85mm.