The Constructor

Minimum Thickness of Concrete Slab, Beam, Column, Foundation

Madeh Izat Hamakareem
Minimum Thickness of Structural Concrete Members

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Minimum thickness of concrete slab, beam, column, foundation and other structural members is selected to meet the design requirements as per standard codes. Minimum thickness of concrete structural elements based on ACI 318-14, IRC 2009, IS 456 2000, and UBC 1997 is presented. Design process includes proper assumption of structural element sizes and then check the suggested dimensions to make sure that it meets the requirements of the design. If proper structural dimension is not assumed, then the design would be time consuming and required considerable effort since substantial trials will be involved till satisfactory dimensions are specified. That is why majority of codes provide minimum dimensions and specifically thicknesses for almost all structural elements.

1. Minimum Thickness of Slabs

1.1 Minimum thickness of One-way slab

ACI 318-14 provides suggested minimum thickness for one-way solid slab, as provided in Table 1, unless deflections are calculated. Table 1 minimum thickness of one-way solid slab unless deflections are calculated
Minimum thickness, h
Simply supported One end continuous Both end continuous Cantilever
Members not supporting or attached to partitions or other construction likely to be damaged by large deflections
l/20 l/24 l/28 l/10
Notes: Values given shall be used directly for members with normal weight concrete and Grade 420 reinforcement. For other conditions, the values shall be modified as follows: a) For lightweight concrete having equilibrium density (wc) in the range of 1440 to 1840 kg/m3, the values shall be multiplied by (1.65 – 0.0003wc) but not less than 1.09. b) For fy other than 420 MPa, the values shall be multiplied by (0.4 + fy/700).

Fig.1: one way slab thickness

1.2 Minimum thickness of Ribbed slab

ACI 318-14 recommend the same value of non-prestressed beams as provided in Table 2. Unified Building Code (UBC) specified minimum thickness of ribbed slab to be 1/12 distance between ribs or 51mm.

Fig.2: Ribbed one way slab

Slab thickness with embedded conduits and pipes

1.3 Minimum thickness of Slab on the ground

UBC recommends minimum thickness of Concrete floor slabs supported directly on the ground to be 89mm, whereas BCGBC4010A - Apply structural principles to residential low-rise constructions determined minimum thickness to be 100mm.

Fig.3: slab on ground

1.4 Minimum thickness of Diaphragms

UBC recommend concrete slab and composite topping slab serving as structural diaphragm used to transmit earthquake forces to be 50mm.

1.5 Minimum thickness of Two-way slab

ACI 318-14 provided recommendations to determine minimum thickness of slabs (including slabs with beams, flat slabs, flat plates) that can be found here.

Fig.4: Two way slab

1.6 Minimum thickness of Drop panel

sometimes drop panels used at top of columns to improve shear strength of slabs. The minimum thickness of drop panels shall be quarter of slab thickness beyond the drop.

2. Minimum Thickness of Beams

Table 2 minimum thickness of non-prestressed beams unless deflections are calculated
Minimum thickness, h
Simply supported One end continuous Both end continuous Cantilever
Members not supporting or attached to partitions or other construction likely to be damaged by large deflections
l/16 l/18.5 l/21 l/8
Notes: Values given shall be used directly for members with normal weight concrete and Grade 420 reinforcement. For other conditions, the values shall be modified as follows: a) For lightweight concrete having equilibrium density (wc) in the range of 1440 to 1840 kg/m3, the values shall be multiplied by (1.65 – 0.0003wc) but not less than 1.09. b) For fy other than 420 MPa, the values shall be multiplied by (0.4 + fy/700). The depth of beam can also be estimated based on span/depth ratio. IS 456 2000 provides span to depth ratio to control deflection of beam as provided in Table 3. Table 3: Span to depth ratio based on the span and type of beams, IS 456 2000
Beam span Beam type Span/depth ratio
Up to 10m Simply supported 20
Cantilever 7
Continuous 26
Greater than 10m Simply supported 20*10/span
Cantilever -
Continuous 26*10/span

Fig.4: Reinforced concrete beam thickness, h

3. Minimum Thickness of Columns

Dimensions of columns is based on the requirements of the design and several shapes can be selected for the columns such as square, rectangular, trapezoidal, cylinders, and others.

Fig.5: Column dimensions

4. Minimum Thickness of Walls

4.1 Bearing walls

UBC recommends minimum thickness of bearing wall to 1/25 supported height or length whichever is shorter or not less than 102mm.

4.2 Exterior basement wall and Foundation wall

5. Minimum Thickness of Footings

Footing on soil

The minimum depth for footing on soil is advised to be 150mm.

Footing on pile

The minimum depth for footing on pile is recommended to be 300mm

Plain concrete structural footing

The minimum thickness for plain concrete structural footing is suggested by ACI 318-14 and set as 200mm, and the same value is suggested by UBC. It shall be known that plain structural footing is not suitable to be used for the top of piles.

Raft foundation

The minimum thickness of raft foundation is 300mm.

Fig.7: Footing thickness

6. Minimum Thickness of Other Concrete Members

Table 4: minimum thickness for other structural elements
Structural Element Thickness, mm
Pile cap 600mm
Levelling concrete below liquid retaining structures 100mm
Levelling concrete for other RCC foundations 75mm
Underground pit / reservoir (below ground water table) walls and slab 200mm
Underground pit (above water table) walls and slab 200mm
Parapet wall 100mm
Chajja 100mm
Cable / pipe trench walls and base slab 125mm
Precast trench cover 125mm
Insert plate 12mm
Corner angle 6mm
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