MAXIMUM SIZE OF CONCRETE SLAB, BEAM & COLUMN | CALCULATION -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

## Jun 27, 2021

MAXIMUM SIZE OF CONCRETE SLAB, BEAM & COLUMN | CALCULATION

MAXIMUM SPAN/LENGTH OF THE BEAM

MAXIMUM DEPTH OF THE BEAM

The maximum span & depth of a beam (cantilever / simply supported / continuous)  are decided by deflection limits

These deflection limits are defined in terms of L/d ratio, where ‘L’ is the effective span & ‘d’ is the effective depth of the beam member.

According To Is Code On Clause 23.2.1 of IS: 456 (2000) pg. no. 37 The same provisions are stipulated for beam spans of up to 10 m.

Cantilever; Span/d <= 7

Simply supported; Span/d <= 20

Continuous; Span/d <=26

Greater than 10m

Simply supported < = 20x10/span

Continuous < = 26x10/span

Thus, depending on the length provided, the minimum allowable depth of the beam member can be calculated, and likewise the maximum allowable span can be calculated using the L/d ratio for a given depth of the beam member.

Note:

Effective Span: Usually the c/c span of the beam member is taken into account for further reference. See clause 22.2 of IS:456 (2000) pg. 34

Effective depth: d = D - (cover)- (dia of bottom bar/2)

D = effective depth

D = overall depth

MAXIMUM THICKNESS OF THE SLAB

Standards such as ACI 318 specify the minimum thickness of the slab based on the length of the slab.

·       Simply Supported Slab = L / 20

·       One End Continuous Slab = L / 24

·       Both End Continuous Slab = L / 28

·       Cantiliver = L / 10

However, in other criteria, they do not directly specify the minimum slab thickness.

 MINIMUM THICKNESS OF THE SLAB AS PER IS 456:2000 Simply supports slabs spanning in one direction L/30 Simply supports slabs spanning in Two directions (Two-way slab) L/35 Continuous slabs spanning in one direction L/35 Continuous slabs spanning in Two direction L/40 Cantilever slabs L/12

MAXIMUM ALLOWABLE HEIGHT OF A COLUMN WITHOUT ANY BRACE OR TIE

Columns can build as long column fails by buckling.

So if the slenderness ratio of the column is greater than 12, that the column is classified as a long column.

Slenderness ratio=L(eff)/R(min)

L (efff) has to be determined as per IS 456:2000

R (min) = ( I(min) /Area of cross-section )1/2

But simply it can be understood as if column size is of 250X 250 mm then as per slenderness ratio limit if 12

Therefore,

Slenderness ratio = Length /shorter dimension

i.e., 12= length/250

i.e., Length= 12 x 250=3000 mm

So if you make a column of size 250mm x 250mm it should not exceeds the height of 3m. if you need to build beyond it. It needs tie beams

MAXIMUM COLUMN SIZES FOR DIFFERENT STORIES

 S.NO NO. OF STORY SIZE 1 1-2 230mm x 230mm 9” x 9” 2 1-3 300mm x 300mm 12”x12” 3 1-10 750mm x 750mm 30” x 30” 4 1-20 1-7 800mm x 800mm 32” x 32” 8-15 600mm x 600mm 24” x 24” 16-20 450mm x450mm 18” x 18” 5 1-30 1-10 850mm x 850mm 34” x 34” 11-20 650mm x 650mm 26” x 26” 20-30 475mm x475mm 19” x 19”

NOTE: These are the standard RCC column sizes, but it will increase or decrease with respect to the load.

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