Minimum Thickness of Concrete Slab, Beam, Column, Foundation

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.

Table of Contents

• 1. Minimum Thickness of Slabs
  • 1.1 Minimum thickness of One-way slab
  • 1.2 Minimum thickness of Ribbed slab
  • Slab thickness with embedded conduits and pipes
  • 1.3 Minimum thickness of Slab on the ground
  • 1.4 Minimum thickness of Diaphragms
  • 1.5 Minimum thickness of Two-way slab
  • 1.6 Minimum thickness of Drop panel
• 2. Minimum Thickness of Beams
• 3. Minimum Thickness of Columns
• 4. Minimum Thickness of Walls
  • 4.1 Bearing walls
  • 4.2 Exterior basement wall and Foundation wall
• 5. Minimum Thickness of Footings
  • Footing on soil
  • Footing on pile
  • Plain concrete structural footing
  • Raft foundation
• 6. Minimum Thickness of Other Concrete Members

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

• UBC recommended minimum thickness of slabs with embedded conduits and pipes to be 25mm greater than total overall depth of conduits or pipes.
• ACI 318-14 specify that, conduits and pipes shall not be larger in outside dimension than 1/3 the overall thickness of slab, wall, or beam in which they are embedded

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.

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

• ACI 318-14 provides suggested minimum thickness for non-prestressed beams, as provided in Table 2, unless deflections are calculated.
• Canadian Standard Association CSA provides similar table except for one end continuous which is l/18.

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.5: 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.6: 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

• UBC specified minimum thickness of exterior basement wall and foundation wall to be 191mm.
• The same value is recommended by International Residential Code (IRC 2009) for foundation wall.
  
  Fig.7: wall thickness

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.8: 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

Various Codes for Construction Material Testing

Various standard codes for testing of construction materials are specified by ASTM international, Eurocodes, IS Codes, BS Codes, etc. These codes specifies testing procedure and methods for concrete, masonry, aggregates, steel, soil and other building materials.

Codes and standards are living documents, continually revised and refreshed to address changing requirements and emerging technologies. Commonly, they are reviewed after certain years as part of a process of continual improvement. For example, ACI Code is being revised every three years.

This article presents most common and widely used codes used for testing of construction materials.

Table of Contents

• Various Codes for Construction Material Testing
• 1. ASTM International
• 2. Canadian Standard Association (CSA)
• 3. European Standard (EN Eurocodes)
• 4. British Standards (BS)
• 5. Indian Standards (IS)

Various Codes for Construction Material Testing

Standards and Codes which are widely used in the world or are references for other codes include:

1. ASTM International
2. Canadian Standard Association (CSA)
3. European Standard (EN Eurocodes)
4. British Standards (BS)
5. Indian Standards (IS)

1. ASTM International

It is the largest source of standards in the world for materials, goods, services and systems. ASTM internatinal also publishes documents on sampling and testing methods for health, safety and performance aspects of materials, effects of physical and biological agents and chemicals, and safety guidelines. Table 1 provides list of standards for testing various construction materials.

Table 1 ASTM international standards for testing different construction materials

Construction material	ASTM international standards
Soil	ASTM D2216, ASTM D4318, ASTM C136, ASTM D5268, ASTM D6913, ASTM D2974, ASTM D422, ASTM D2434, ASTM D1140, ASTM D7263, ASTM D5918, ASTM D698, ASTM D854, and ASTM D1557
Aggregate and Rock	ASTM C88, ASTM C295, ASTM C127, ASTM D5312, ASTM C128, ASTM D5313, ASTM C29, ASTM D4992, ASTM D5821, ASTM D4791, ASTM D3967, ASTM C131, ASTM D7012, and ASTM C535
Concrete and Masonry	ASTM C617, ASTM C1231, ASTM C39, ASTM C78
Asphalt	ASTM D6926, ASTM D2726, ASTM D6307, ASTM D5444, and ASTM D2041

2. Canadian Standard Association (CSA)

The CSA is a standard organization which develops standards in Canada. It publishes standards in print and electronic form and provides training and advisory services.

All materials used in construction of buildings and other structures in Canada is manufactured to meet the requirements of the CSA standards. Table 2 provides selected standards used for testing different construction materials.

Table 2 Selected CSA Standards for testing construction materials

Construction materials	CSA standards
Cement	CSA A3001, CSA A3002, CSA A3003, CSA A3004, and CSA A3005
Concrete	CSA A23.1, CSA A23.2, CSA A23.4, and CSA S806
Masonry	CSA A165.1, CSA A165.2, CSA A165.3, CSA A179, CSA A370, and CSA A371

3. European Standard (EN Eurocodes)

Test standards are part of the comprehensive system of European standards relating to construction. They are intended to be used for the determination of material and product properties required for the design of buildings and other civil engineering structures with the EN Eurocodes.

In particular, test standards related to the EN Eurocodes comprise testing for materials, e.g. concrete, masonry, timber and metallic materials, non-destructive test methods as well as fire tests. BS EN is abbreviation for official English version of European standards

Table 3 European Standards for testing different construction materials

Construction materials	European Standards (EN)
concrete	BS EN 12390-4:2000, BS EN 12390-5:2000, BS EN 12390-6:2000, BS EN 1170-4:1998
masonry	BS EN 1052-1:1999, BS EN 846-5:2000, BS EN 846-6:2000
timber	EN 594, EN 1075, EN 1380
metallic materials	BS EN 10002-1:2001
Plywood	BS EN 1072:1995
Stone	BS EN 12372:1999, BS EN 14617-15:2005, BS EN 14580, BS EN 14617-2:2004,
Aggregate	BS 812-2:1995

4. British Standards (BS)

British Standard publications, which are produced by the British Standard Institute, are technical specifications that give recommendations on a wide range of building and construction matters including materials, testing, health and safety, access and regulations and many more.

They are essential reference for architects, developers, building owners, site managers, building contractors, structural engineers and materials specifiers. Table 3 provide a selected list of British Standards for testing construction materials.

Table 4 Selected British Standards for testing construction materials

Construction materials	British Standards
Concrete	BS 1881-119:1983, BS 1881-121:1983, BS 1881-127:1990
resin and polymer/cement compositions	BS 6319-10:1987, BS 6319-11:1993, BS 6319-2:1983, BS 6319-3:1990, BS 6319-6:1984, BS 6319-7:198,
Mortar	BS EN 1015-11:1999, BS EN 1015-12:2000,

5. Indian Standards (IS)

Indian Standards are large number of documents that published by Bureau of Indian Standard (BIS) and involves almost all aspects of civil engineering.

The BIS has developed a set of guidelines in order to guarantee the quality standards of the deferent types of construction materials. These are classified into two broad categories namely; Building Materials including Paints and Civil Engineering Design and Construction.

These standards contain guidelines for testing cement, concrete, concrete admixtures, additives, soil, rock, steel, and aluminum.  Table 5 provide list of Indian standards used for testing various construction materials.

Table 5 Indian Standards for testing construction materials

Construction material	Indian standards
Cement	IS 650 – 1991, IS 14032 – 1988,
Coarse / Fine Aggregate	IS 2386 (Part I To VIII) 1963
Bricks	IS 3495 (Parts I TO iv) 1976
soil	IS: 2720 (Part. XIII) 1986,  IS:2720 (Part.30) 1980

Types of Stirrups in Column

Different types of stirrups are designed for columns based on their varying cross-sections, the number of longitudinal reinforcement bars and the load carrying capacity. Stirrups in column construction are commonly known as vertical ties or transverse reinforcement.

There are different types of stirrups or ties used in column construction:

1. Helical Reinforcement
2. Lateral Ties

A brief description of different types of stirrups are stated below.

Table of Contents

• 1. Helical Reinforcement
• 2. Lateral Ties
  • • 1. Lateral Tie Configuration for 4- Bars
    • 2. Lateral Tie Configuration for 6- Bars
    • 3. Lateral Tie Configuration for 8- Bars
    • 4. Lateral Tie Configuration for 10- Bars
    • 5. Lateral Ties for different column cross sections

1. Helical Reinforcement

The helical reinforcement differs from the lateral ties as the latter one possess spacing between individual ties. In helical reinforcement ,instead of spacing, the measured value is pitch as shown in the figure-1 below.

Fig.1. Helical Reinforcement and Lateral Ties in Column

Compared with lateral ties, the helical reinforcement provides more ductility and flexibility to the column constructed. They also provide more efficiency to support the longitudinal reinforcement. The buckling resistance of the column structure is improved by the use of helical reinforcement.

The helical reinforcement can also be used as spiral reinforcement. Helical bars are mainly recommended for seismic design. Under the action of seismic loads, the concrete that is attached to the helical reinforcement is initially peeled off. This helps to provide a warning sign about the structural condition of the column.

The helical reinforcement is a good choice in terms of uniformly distributing loads when compared to the normal rings (lateral ties).

2. Lateral Ties

The lateral ties are transverse reinforcement which forms an individual ring with a fixed spacing between each the links (figure-1(c)). The lateral stirrups provided can be two-legged stirrups, four-legged stirrups or six-legged stirrups etc depending on the column cross-section and the number of vertical or longitudinal reinforcement bars employed.

Here different Lateral tie configuration for different number of vertical reinforcement bars are explained. The configurations are based on the ACI 315-99 recommendations.

1. Lateral Tie Configuration for 4- Bars

The figure-1 below shows the tie configuration for 4 numbers of vertical column bars. This is a typical type of configuration employed for simple column design. This configuration can be called as 2 legged stirrups column type.

Fig.2: Lateral Tie Configuration for 4- Bars

2. Lateral Tie Configuration for 6- Bars

Fig.3: Lateral Tie Configuration for 6- Bars

The first arrangement in figure-3 above is followed when the spacing of vertical bars are less than 150mm ( Less than 6”). When The spacing is greater than 150mm, the second arrangement is followed where crossties are employed.

3. Lateral Tie Configuration for 8- Bars

Fig.4: Lateral Tie Configuration for 8 Bars

The first arrangement above shows the typical 8 number vertical reinforcement arrangement. Here the spacing will be less than 6”. When the spacing is greater than 150mm two crossties are used as shown in the second arrangement (figure-4).

The third arrangement is called bundled bars arrangement. Here two bars are bundled at the corners.No cross ties are hence employed. The maximum number of bars that can be bundled are 4.

4. Lateral Tie Configuration for 10- Bars

Fig.4: Lateral Tie Configuration for 8 Bars

In this case, it is necessary to have cross-ties other than the square ties. This can also be arranged in bundled bars as shown in the second arrangement( figure-4). Here bundle of 2 bars is placed at four corners and two remaining bars are supported with the help of cross ties.

5. Lateral Ties for different column cross sections

Fig.5.Lateral Ties for different column cross sections

Among the arrangements shown in figure-5, the arrangement for 16  bars makes use of diamond ties. Diamond ties are very difficult to accurately fabricate, so it is avoided. It is also difficult to place and align them properly. This tie arrangement is not recommended by ACI 315 due to the difficulties associated with it. But some country standards employ this arrangement for simple column design.

Fig.6.Lateral Tie Arrangement for 16 bars with cross ties.

A 16 bars column arrangement as per ACI 315 is shown above. Here bundled bars can. For 16 bars, 4 bundled bars can be provided at each corner.