## Proportioning of flat slabs

**Proportioning of flat slabs**

**IS 456-2000 [Clause 31.2] gives the following guidelines for proportioning.**

**Drops**

1The drops when provided shall be rectangular in plan, and have a length in each direction not less than one third of the panel in that direction. For exterior panels, the width of drops at right angles to the non continuous edge and measured from the centre-line of the columns shall be equal to one half of the width of drop for interior panels

**Column head**

Where column heads are provided, that portion of the column head which lies within the largest right circular cone or pyramid entirely within the outlines of the column and the column head, shall be considered for design purpose as shown in Figs. 1.2 and 1.4.

**Thickness of Flat slab**

From the consideration of deflection control IS 456-2000 specifies minimum thickness in terms of span to effective depth ratio. For this purpose larger span is to be considered. If drop as specified in 1.2.1 is provided,

then the maximum value of ratio of larger span to thickness shall be

**= 40, if mild steel is used
= 32, if Fe 415 or Fe 500 steel is used
**

If drops are not provided or size of drops do not satisfy the specification 1.2.1, then the ratio shall not exceed 0.9 times the value specified above i.e.,

**= 40 0.9 = 36, if mild steel is used.
= 32 0.9 = 28.8, if HYSD bars are used
**

It is also specified that in no case, the thickness of flat slab shall be less than 125 mm.