Design of Columns

# Design of Columns | RCC & Prestressed Concrete - Civil Engineering (CE) PDF Download

Slenderness ratio (λ)
λ = effective length/least lateral dimension
If λ > 12 then the column is long.

Load carrying capacity for short column
P = σscAsc + σccAc
where, AC = Area of concrete, Ac = Ag - ASC
σSC Stress in compression steel
σCC Stress in concrete
Ag Total gross cross-sectional area
ASC Area of compression steel

Load carrying capacity for long column
P = CrscAsc + σccAc)
where, Cr = Reduction factor

where, leff = Effective length of the column
B = Least lateral dimension
imin = Least radius of gyration and
where, l = Moment of inertia and A = Cross-sectional area

### Effective length of column

Effective length of Compression Members

Column with helical reinforcement
Strength of the column is increased by 5%
P = 1.05(σscAsc + σccAc) for short column
P = 1.05CrscAsc + σccAc) for long column

Longitudinal reinforcement

1. Minimum area of steel = 0.8% of the gross area of the column
2. Maximum area of steel
(i) When bars are not lapped Amax = 6% of the gross area of the column
(ii) When bars are lapped Amax = 4% of the gross area of the column

Minimum number of bars for reinforcement
For rectangular column  4
For circular column  6
Minimum diameter of bar = 12 mm
Maximum distance between longitudinal bar = 300 mm
Pedestal: It is a short length whose effective length is not more than 3 times of lest lateral dimension.

Transverse reinforcement (Ties)

where ϕmin dia of the main longitudinal bar
φ = dia of the bar for transverse reinforcement
Pitch (p)

where, φmin = minimum dia of main longitudinal bar

Helical reinforcement

1. Diameters of helical reinforcement is selected such that
2. Pitch of helical reinforcement: (p)

where, dC = Core diameter = dg – 2 × clear cover to helical reinforcement
A= Gross area = π/4(dg)2
dg = Gross diameter
Vh = Volume of helical reinforcement in a unit length of the column
φh = Diameter of steel bar forming the helix

dh = centre to centre dia of the helix
= dg – 2 clear cover - φh
φh = diameter of the steel bar forming the helix

Some others IS recommendations

1. Slenderness limit
(i) Unsupported length between end restrains 60 times least lateral dimension.
(ii) If in any given plane one end of the column is unrestrained than its unsupported length
2. All column should be designed for a minimum eccentricity of

### Limit state method

• Slenderness ratio (λ) if

λ<12 Short column

1. Eccentricity

If emin ≤0.05D then it is a short axially loaded column.
where, Pu = axial load on the column.
2. Short axially loaded column with helical reinforcement
Pu = 0.4 fckAc + 0.67 fyAsc
3. Some others IS code Recommendations
Pu = 1.05(0.4 fckAc + 0.67 fyAsc)
(i) Slenderness limit
(a) Unsupported length between end restrains  60 times least lateral dimension
(b) If in any given plane one end of the column is unrestrained than its unsupported length
(ii) All column should be designed for a minimum eccentricity of

Where e = 0, i.e., the column is truly axially loaded.
Pu = 0.45 fckAc + 0.75 fyAsc
This formula is also used for member subjected to combined axial load and bi-axial bending and also used when e > 0.05 D.

The document Design of Columns | RCC & Prestressed Concrete - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course RCC & Prestressed Concrete.
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## RCC & Prestressed Concrete

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## FAQs on Design of Columns - RCC & Prestressed Concrete - Civil Engineering (CE)

 1. What are the different types of columns used in civil engineering?
Ans. In civil engineering, there are several types of columns used, including reinforced concrete columns, steel columns, composite columns, timber columns, and masonry columns. Each type has its own advantages and disadvantages depending on the specific project requirements and structural considerations.
 2. How do you design a reinforced concrete column?
Ans. The design of a reinforced concrete column involves determining the required dimensions, reinforcement details, and load-carrying capacity. The design process includes calculating the axial load, selecting an appropriate column section, designing the reinforcement, and checking for stability and serviceability requirements. The design is typically based on the applicable design codes and regulations.
Ans. Column design is crucial in structural engineering as columns play a significant role in supporting vertical loads and transferring them to the foundation. Proper column design ensures the structural integrity and stability of a building or structure. A well-designed column can withstand the applied loads, prevent excessive deflections, and ensure the safety and longevity of the structure.
 4. How do you calculate the load capacity of a column?
Ans. The load capacity of a column is determined by calculating the maximum axial load it can carry without failure. This involves considering factors such as the material properties, column dimensions, reinforcement details, and safety factors. The load capacity can be calculated using various methods, such as the Euler's formula, the Perry-Robertson formula, or through computer-aided structural analysis software.
 5. What are the common failure modes in column design?
Ans. Common failure modes in column design include axial compression failure, buckling failure, and bending failure. Axial compression failure occurs when the column fails due to excessive compressive forces. Buckling failure occurs when the column buckles and collapses under compressive loads. Bending failure happens when the column fails due to excessive bending moments or lateral loads. Proper design considerations and reinforcement can help mitigate these failure modes and ensure the stability and safety of the column.

## RCC & Prestressed Concrete

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