Reinforced concrete columns are one of the fundamental components in residential structural design, serving as critical load-bearing elements that support the structure above.

These columns are made by casting concrete around a reinforcement usually made of steel bars or meshes, which work together to resist the forces that act upon the structure, such as compression and bending.

This article discusses the design requirements for rectangular reinforced concrete columns according to the requirements of Australian Standards AS 3600:2018. The article outlines a worked design example by comparing the manual hand calculation method to the use of structural analysis software ClearCalcs.

Design Considerations for Reinforced Concrete Columns to Australian Standards AS 3600

Australian Standard for Concrete Structures, AS 3600:2018, specifies the minimum requirements for the design and construction of concrete building structures, including columns. It also provides guidelines for load calculation and analysis, material properties and selection, compliance and quality assurance, and best practices and optimization techniques.

Understanding Structural Requirements

When designing reinforced concrete columns, it is important to consider the structural requirements and considerations, such as load types, design life, and safety factors. AS 3600 provides guidelines for the design of columns for strength and serviceability, including procedures for short and slender columns, requirements for construction joints, and tolerances for structures and members.

Material Properties and Selection

Designing reinforced concrete columns according to AS 3600 involves careful consideration of material properties and selection. These considerations ensure that the columns meet the structural and safety requirements.

Here are the key considerations for material properties and selection in reinforced concrete column design to AS 3600:

Concrete Strength:

• Selecting the appropriate concrete mix design with the required compressive strength to support the loads and environmental conditions.
• Ensure that the concrete strength complies with the specified requirements in AS 3600.

Reinforcement Bars:

• Choose the right type, size, and grade of reinforcement bars (rebar) based on design specifications.
• Ensure that the steel reinforcement complies with A S3600 standards in terms of yield strength and ductility.

Cover and Protection:

• Adequate concrete cover thickness over the reinforcement to protect it from corrosion, fire, and other environmental factors.
• AS 3600 provides guidelines on minimum cover requirements based on exposure conditions.

Durability:

• Consider the exposure conditions (e.g., aggressive environments, chemical exposure) and select concrete and reinforcement with appropriate durability characteristics.
• Ensure that AS 3600 requirements for durability are met to prevent deterioration over time.

Reinforcement Spacing and Configuration:

• Determine the spacing and configuration of reinforcement bars based on structural requirements and load-carrying capacity.
• Ensure that the design complies with AS 3600 provisions regarding minimum and maximum bar spacing and clear spacing requirements.

Loadings and Design Codes:

• Determine the axial and lateral loads, as well as the seismic and wind forces that the column will be subjected to.
• Design the column in accordance with AS 3600 to ensure it can withstand these loads safely.

Column Shape and Size:

• Determine the appropriate dimensions and shape of the column to meet architectural and structural requirements.
• AS 3600 provides guidelines for minimum and maximum dimensions and aspect ratios for columns.

Temperature and Fire Considerations:

• Account for the effects of elevated temperatures due to fire by incorporating fire-resistant materials or fireproofing.
• AS 3600 provides guidance on fire resistance design for concrete structures.

Construction and Workmanship:

• Ensure that the concrete mix is well-proportioned and placed properly.
• Implement proper quality control during construction to maintain the integrity of the column.

Inspection and Testing:

• Perform regular inspections and testing to verify the quality of materials and construction.
• Compliance with AS 3600’s inspection and testing requirements is essential to ensure the column’s integrity.

Sustainability:

• Consider sustainable construction practices and materials, such as using recycled aggregates or supplementary cementitious materials, to reduce environmental impact.

Serviceability Requirements:

• Ensure that the column design accounts for serviceability conditions, such as deflection limits and crack control, as specified in AS 3600.

Load Calculation and Analysis

Load calculation and analysis are critical steps in column design. To determine the load actions, the Australian Standard for Structural design actions, AS 1170, should be referred to determine the permanent and imposed actions.

AS 3600 then provides a guide to calculating the load capacities on columns, including axial force, bending moment, and shear force. The capacity reduction factors can be found in AS 3600:2018 Table 2.2.2 as per the below.

Section Capacity Line for a Column

The section capacity line for a column in AS 3600:2018 Section 10.6.2 denotes the strength interaction diagram (interaction curve) or failure envelope.

Ultimate axial compression capacity (Y-axis): N, Ultimate moment capacity (X axis): M

Where M=N·e, e: eccentricity

Each point in the line represents a combination of bending moment and axial compression load, which the column can carry simultaneously.

• Point A (squash load): Only compressive axial load, no bending, e = 0
• Point B (both N and M): Zero stress in tension bars
• Point C (balanced failure state): Concrete crush (εcu) and steel yield (εsy) achieved simultaneously
• Point D (pure bending): No axial force
• Point E: (pure tension): Only tensile axial load, no bending, e = 0, not shown in Figure 10.6.2.1

Worked Design Example of Reinforced Concrete Columns by Hand Calculation

A 700 x 400 mm tied column is symmetrically reinforced, as shown in the figure below.

Establish an approximate interaction diagram for axial compression and bending about the strong axis.

The diagram is to be based on the following:

Take f’c = 25MPa, fsy = 400MPa, Es = 200GPa, dsc = dst = 62

For the worked example, the details of all coefficients can be found in AS 3600:2018 Section 1.7.

Point A: Pure axial compression

As per AS 3600:2018 Section 10.6.2.2:

$$N_{u_0}=0.85f_c’bD+A_{sc}f_{sy}+A_{st}f_{sy}$$

$\alpha_1$=$1.0-0.003f_c’$ with the limits 0.72 to 0.85

$$\phi N_{u_0}=0.65*((0.85*25*400*700)+(1810*400*10^{-3})+(1810*400*10^{-3}))= 4809kN$$

Point B: Zero stress in tension bars

Zero stress in tension bars means: the neutral axis passes through these bars, i.e., ku·d = d, or ku=1.0.

The decompression point is calculated taking the strain in the extreme compressive fibre equal to 0.003, the strain in the extreme tensile fibre equal to zero and using the rectangular stress block given in Clause 10.6.2.5.

$$N_u=C_c+C_s=0.85f_c’\gamma k_udb+A_{sc}f_{sy}$$

$$d=700-62=638mm$$

$$\frac {d-d_{sc}}{d} = \frac {\varepsilon _{sc}}{0.003}$$

$$\varepsilon _{sc} = \frac {638-62}{638} *0.003=0.0027>\varepsilon _{sy} = \frac {f_{sy}}{E_s}=0.002$$

Therefore the compression reinforcement bar yielded $\rightarrow  \sigma _{sc}=f_{sy}$

As per AS 3600:2018 Section 10.6.2.5, which details the transition from decompression point to bending strength.

$$\alpha _2=0.85-0.0015f_c’ \cdots \alpha _2\geq 0.67$$

$$\gamma =0.97-0.0025f_c’ \cdots \gamma \geq 0.67$$

$$\phi N_u=0.65*((0.81*25*0.91*1*638*400)+\frac {1810*400}{1000 (to convert to kN)}= 3527kN$$

Find out the position of applied axial compressive load by taking moment with respect to the position of tension bars.

$$N_u*h=C_c \left ( d- \frac {\gamma k_ud}{2} \right ) +C_s(d-d_{sc})$$

$$5427*h=4703(638-0.91*\frac {638}{2})+724(638-62)\rightarrow h=378mm$$

Next, we can find the eccentricity:

$$e=h-(\frac {D}{2}-d_{st})\rightarrow e=378-(700/2 -62)=90mm$$

The eccentricity can be used to find the moment capacity.

$$\phi M_u=\phi N_ue=0.65*5427*0.09=317kNm$$

Point C: Balanced failure condition

Under the balanced failure condition, concrete crushing and steel yielding is achieved simultaneously.

First, we will find out the position of the neutral axis (ku*d) where d = Depth of section – reinforcement cover = 700mm – 62mm = 638mm

$$\frac {k_ud}{d-k_ud} = \frac {0.003}{\varepsilon _{sy}} where \varepsilon _{sy} = \frac {f_{sy}}{E_s}=0.002, d=638mm \to  k_u=0.6$$

Then we will find out the strain and stress of the reinforcement in compression.

$$\frac {k_ud-d_{sc}}{k_ud}*\varepsilon _{cu} = \frac {383-623}{83}*0.003=0.0025 \gt \varepsilon _{sy}$$

Therefore the compression reinforcement bar has yielded $\longrightarrow \sigma _{sc}=f_{sy}$

Next, we can calculate the axial capacity under the balanced failure condition.

$$N_{ub}=C_c+C_s-T_s$$

$$\phi N_{ub}=\phi((0.81f_c'\gamma k_udb)+(A_{sc}f_{sy})+(A_{st}f_{sy})$$

$$\phi N_{ub}=0.65* \frac {(0.81*25*0.91*1*638*400)+(1810*400)-(1810*400)}{1000(to convert to kN)}=1835kN$$

Next we can find out the position of applied axial compressive load by taking moment with respect to the position of tension bars.

$$N_{ub}*h=C_c(\frac {d-\gamma k_ud}{2})+C_s(d-d_{sc})$$

By substituting in the terms used in the previous equations we find h=611mm. We can then find the eccentricity of the axial load.

$$e=h-(\frac {D}{2}-d_{st})\to e=611-(700/2 -62)=323mm$$

The eccentricity can be used to find the moment capacity.

$$\phi Mu=\phi N_ue=0.65*2823*0.323=593kNm$$

Point D: Pure Moment Capacity

When there is no axial capacity (Nu = 0), the moment capacity (Mu0) can be calculated by finding the bending capacity as an under-reinforced beam section similar to the ClearCalcs article Beam Design by AS 3600:2018.

We can simplify the procedure further by ignoring the compressive reinforcement (Asc) which provides relatively moment capacity. In doing so we can assess the beam as a singly reinforced beam section.

We will first find the location of the neutral axis by using force equilibrium of the concrete compression and the tensile reinforcement.

$$\alpha _2f_c'b\gamma k_ud=A_{st}f_{sy}$$

By transposing the above equation, we can find

$$k_ud=\frac {1810*400}{0.81*25*0.91*400}=98mm$$

Then we can find the pure moment capacity as per the below.

$$M_{uo}=A_{st}f_{sy}(d-\frac {\gamma k_ud}{2})$$

$$\phi M_{u_0}=0.8*(1810*400*(638-\frac {0.91*98}{2}))*10^{-6}(\text{to convert to }kNm)=344kNm$$

Point E: Pure Tensile Capacity

The tensile capacity is only attributable to the reinforcement and is simply calculated below.

$$N_{u,ot}=A_{sc}f_{sy}+A_{st}f_{sy}$$

$$\phi N_{u,ot}=0.85*((1810*400)+(1810*400))*10^{-3}(\text{to convert to }kNm)$$

$$\phi N_{u,ot}=1231kN$$

Interaction Diagram

Next, we can assess the strength interaction diagram which will show us a failure envelope for the section we have been assessing.

The section can withstand any forces within the envelope. By ensuring the selected column and applied loads lie inside but near the failure envelope, we can produce the most structurally efficient design, reducing material costs.

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Worked Design Example of Reinforced Concrete Columns Using ClearCalcs

Let’s continue with our previous example.

A 700 x 400 mm tied column is symmetrically reinforced as shown in figure below. Establish an approximate interaction diagram for axial compression and bending about the strong axis. The diagram is to be based on the following:

Take f’c = 25MPa, fsy = 400MPa, Es = 200GPa, dsc = dst = 62

Let’s assume we have done the structural load calculations and determined a dead load of 300kN axially and a moment of 50kNm as well as a live load of 500kN axially with a moment of 60kNm.

First, we input the loads and key properties into the ClearCalcs Rectangular Concrete Column Calculator to AS 3600:2018 as per the below.

We can then choose different longitudinal reinforcement and fitment reinforcement arrangements to come up with the most structurally efficient column to reduce the amount of steel required to be purchased by the builder, saving money on the project.

Using the traffic light checks, we can get instantaneous feedback on the utilisation of the column under the specified design loads.

We can continue reducing the column dimensions and/or reinforcement used until we achieve a governing utilisation of ~80%.

The best part about ClearCalcs is the instant feedback that the software provides, rather than having to calculate the section capacity line from scratch, as in the case of hand calculations.

Best Practices for Column Design

Minimum Bending Moment

As per AS 3600:2018 Section 10.1.2 at any cross-section of a column, the design bending moment about each principal axis shall be taken to be not less than $N^*$ times $0.05D$, where $D$ is the overall depth of the column in the plane of the bending moment.

Longitudinal Reinforcement for Columns

As per AS 3600:2018 Section 10.7.1 the cross-sectional area of the longitudinal reinforcement in a column shall:
(a) be not less than $0.01A_g$ except that, in a column that has a larger area than that required for strength, a reduced value of $A_{sc}$ may be used if $A_{sc}f_{sy}$ > $0.15N^*$; and
(b) not exceed $0.04A_g$ unless the amount and disposition of the reinforcement will not prevent the proper placing and compaction of the concrete at splices and at junctions of the members.

Groups of parallel longitudinal bars, that are bundled to act as a unit, shall have not more than four bars in any one bundle and shall be tied together in contact.

Lateral Restraint to Longitudinal Reinforcement

Appropriate lateral restraint for longitudinal reinforcement shall be provided as per the below example. Further detail on the angles and locations of the fitment hooks can be found in AS 3600:2018 Section 10.7.4.2.

Transverse Reinforcement for Columns (Fitments & Helices)

The diameter and spacing of fitments and helices shall be as per AS 3600 Section 10.7.4.3.

The spacing of fitments, or the pitch of a helix, shall not exceed the smaller of b and 15db for single bars; or 0.5b and 7.5db for bundled bars as per AS 3600 Section 10.7.4.3, where b is the smaller cross-sectional dimension of the column an db is the diameter of the smallest bar in the column.

Detailing of fitments and helices shall be as per AS 3600:2018 Section 10.7.4.4:

1. A rectangular fitment shall be spliced by welding, or by fixing two 135° fitment hooks around a bar or a bundle at a fitment corner. Internal fitments may be spliced by lapping within the column core.
2. A circular shaped fitment shall be spliced either by welding, or by overlapping and fixing two 135° fitment hooks around adjacent longitudinal bars or bundles.
3. A helical reinforcement shall be anchored at its end by one and one half extra turns of the helix. It may be spliced within its length either by welding or mechanical means.
4. Where hooks or cogs are specified in combination with bundled bars, the internal diameter of the bend shall be increased readily accommodate the bundle.

Crack Control

For the control of flexural cracking in a column, the requirements of AS 3600:2018 Clause 8.6 shall be satisfied which is the same as the crack control for concrete beams design.

Case Studies

Three hypothetical examples of reinforced concrete column designs to AS 3600 to illustrate how the standard might be applied in practice.

Case Study 1: Residential Building

Project Description: A structural engineer is designing reinforced concrete columns for a 5-story residential building in a suburban area. The columns need to support the vertical and lateral loads of the structure.

Design Process:

• Determining Loads: The engineer calculates the dead loads, live loads, wind loads, and seismic loads that the columns will experience.
• Material Selection: The engineer selects concrete with the appropriate compressive strength and reinforcement bars based on design requirements.
• Column Dimensions: Based on the loads, the engineer determines the required dimensions and aspect ratios of the columns.
• Cover and Durability: The project location is in a mild environment, so the engineer selects a concrete mix with the required durability for the specified exposure class.
• Reinforcement Spacing: The spacing and configuration of the reinforcement bars are designed according to AS 3600 requirements.
• Serviceability and Deflection: The engineer ensures that the column design meets the specified deflection limits to maintain serviceability.
• Quality Control: During construction, regular inspections are conducted to verify that the materials and workmanship adhere to AS 3600 standards.

Case Study 2: Commercial High-Rise Building

Project Description: An architectural firm is designing the structural system for a 20-story commercial high-rise building in a city center. The reinforced concrete columns are a crucial part of the design.

Design Process:

• Loads and Codes: The project’s structural engineer determines the vertical, lateral, and seismic loads and references AS 3600 for code compliance.
• High-Strength Concrete: To support the building’s height and loads, the engineer specifies a high-strength concrete mix.
• Column Shape: Considering architectural aesthetics, the columns are designed with a unique shape, adhering to AS 3600’s guidelines on column dimensions and aspect ratios.
• Fire Resistance: The columns are designed to meet AS 3600’s fire resistance requirements using fireproofing materials.
• Sustainability: Supplementary cementitious materials are used to reduce the project’s environmental impact.
• Testing and Inspection: Quality control during construction involves rigorous inspections and testing to ensure compliance with AS 3600 and the project’s specifications.

Case Study 3: Bridge Construction

Project Description: An engineering firm is designing a bridge with reinforced concrete columns to span a river.

Design Process:

• Load Analysis: The engineer determines the loads that the bridge will carry, including live loads, environmental conditions, and potential seismic events.
• Material Selection: High-performance concrete and corrosion-resistant steel reinforcement are chosen due to the bridge’s exposure to a marine environment.
• Column Configuration: The design includes various column shapes and sizes to accommodate the bridge’s geometry while adhering to AS 3600’s guidelines.
• Durability: The concrete mix design and cover thickness are carefully chosen to ensure durability against harsh environmental conditions.
• Earthquake Resistance: Special attention is given to seismic design, as the bridge is located in a seismic-prone region.
• Construction Quality: Stringent quality control measures are implemented during the construction phase, including testing for concrete strength, rebar placement, and formwork.

Compliance and Quality Assurance

Quality assurance processes in AS 3600 are essential to ensure that concrete structures are designed, constructed, and maintained to meet the required safety, durability, and performance standards.

AS 3600 includes specific provisions for quality assurance, which aim to control and monitor the quality of materials, workmanship, and construction procedures.

Here are some key aspects of quality assurance processes in AS 3600:

Material Quality Control:

AS 3600 specifies requirements for materials used in concrete structures, including concrete, reinforcement bars, and any supplementary materials. Quality control processes for materials involve testing and certification to ensure they meet the specified standards.

Mix Design and Proportions:

Quality assurance in concrete mix design involves ensuring that the specified proportions of aggregates, cement, water, and additives are accurately calculated and followed during the batching and mixing process.

Testing and Inspection:

The standard prescribes testing and inspection procedures for various aspects of concrete construction, including the measurement of concrete strength, slump, and air content, as well as reinforcement placement and corrosion protection. AS 3600 provides guidelines for sampling and testing procedures to verify that the materials and construction methods meet the project requirements.

Formwork and Shuttering:

Quality control in formwork construction ensures that formwork is built to the required specifications and standards. This includes alignment, dimension, and surface finish requirements.

Reinforcement Installation:

The proper installation and placement of reinforcement bars are critical to the integrity of reinforced concrete structures. Quality assurance processes involve verifying that bars are correctly positioned, secured, and adequately covered by concrete.

Curing and Protection:

AS 3600 emphasizes the importance of proper curing to maintain concrete’s strength and durability. Quality assurance processes include ensuring that the curing methods are suitable and adequate for the environmental conditions.

Documentation and Records:

Maintaining detailed records of all quality control activities, including material tests, inspections, and any deviations from the project’s specifications, is a key aspect of quality assurance.

Quality Audits and Inspections:

Independent audits and inspections may be conducted at various stages of construction to verify compliance with AS 3600 and project-specific requirements.

Adherence to Codes and Standards:

The quality assurance process ensures that the design and construction comply with AS 3600 and any other relevant codes, standards, and regulations.

Traceability and Reporting:

Quality assurance processes include establishing a traceable system for materials and their origins, as well as regular reporting to relevant stakeholders regarding the status of construction and compliance.

Certification and Compliance:

A crucial aspect of quality assurance is obtaining certifications or compliance declarations from materials suppliers, contractors, and testing laboratories.

Corrective Actions:

In case of non-compliance or quality issues, AS 3600 defines procedures for corrective actions to rectify problems and ensure the structure’s long-term integrity.

Overall, quality assurance processes in AS 3600 are designed to prevent defects, ensure safety, and achieve the intended serviceability and durability of concrete structures. These processes are vital for the successful construction and long-term performance of concrete structures in Australia.

Further Reading

For further reading on reinforced concrete column design, consider the following references and publications:

• AS 3600-2018 - Concrete Structures: The Australian Standard itself is an excellent resource for in-depth information on reinforced concrete design.
• Concrete Structures by R.S. Narayanan and A. Halabe: This comprehensive book provides detailed explanations and examples of concrete design to Australian standards, including AS 3600. It covers a wide range of topics, including reinforced concrete columns.
• Reinforced Concrete: Mechanics and Design by James G. MacGregor and James K. Wight: While not specific to AS 3600, this textbook covers reinforced concrete design principles and mechanics, making it a valuable resource for understanding the underlying concepts.
• Design of Concrete Structures by Arthur H. Nilson, David Darwin, and Charles W. Dolan: Another general textbook on concrete design, this book provides a solid foundation for reinforced concrete design concepts, which can be applied to AS 3600.
• ACI 318 Building Code Requirements for Structural Concrete: This is the American Concrete Institute’s standard for concrete design in the United States. Although it’s not specific to AS 3600, it can be a useful resource for learning about reinforced concrete design principles.
• Educational Institutions: Universities and engineering schools often have course materials and publications related to concrete design. These resources are often freely available on the institutions’ websites or libraries.
• Professional Organizations: Organizations like Engineers Australia, the Institution of Structural Engineers (Australia), and the Concrete Institute of Australia may offer publications, webinars, and seminars related to concrete design, including AS 3600.

Conclusion

In conclusion, Australian Standard AS 3600 is an important standard for the design and construction of concrete structures, including columns. It provides guidelines for load calculation and analysis, material properties and selection, compliance and quality assurance, and best practices and optimization techniques.

Structural engineers should continue learning and staying updated with the latest standards and practices to ensure the safety and reliability of their designs. ClearCalcs’ calculators are updated with each new edition of the standards to ensure the compliance of your designs.

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