🕑 Reading time: 1 minute
The slenderness ratio of a reinforced concrete (RC) column is the ratio between the length of the column, its lateral dimensions, and end fixity. It assesses the ability of the reinforced concrete column to resist buckling pressure. The slenderness ratio is calculated by dividing the column length by its radius of gyration.
The slenderness ratio differentiates short column from long or slender column. The design of the former is controlled by column dimension and material strength whereas the design of the latter is governed by column slenderness.
A column is said to be slender if its cross-sectional dimensions are small compared to its length. If the slenderness ratio of a column is high, it will collapse under a smaller compression load in contrast to a short column with the same cross-sectional dimensions. So, the slenderness effect should be taken into consideration during the design process.
The slenderness ratio of reinforced concrete columns can be computed according to the procedures and specifications of applicable codes such as ACI 318-19 and IS 456.
Contents:
- How to Calculate Slenderness Ratio Based on ACI 318-19?
- Unsupported Length of the Member (lu)
- Radius of Gyration of Column Cross-section (r)
- Effective Length Factor (k)
- ACI Criteria for Slenderness Ratio Effect
- Alignment Chart for Effective Slenderness Factor
- FAQs
How to Calculate Slenderness Ratio Based on ACI 318-19?
The degree of slenderness is generally expressed in terms of the slenderness ratio:
where:
lu: unsupported length of the member
r: radius of gyration of its cross section
K: constant to reflect end conditions of the column; distance between two inflection points (Alignment chart is used to calculate K).
Unsupported Length of the Member (lu)
- The unsupported length (lu) of a column is measured as the clear distance between the underside of the beam, slab, or column capital above, and the top of the beam or slab below, as shown in Figure-1.
2. The unsupported length of a column may be different in two orthogonal directions depending on the supporting elements in respective directions.
Radius of Gyration of Column Cross-section (r)
The radius of gyration introduces the effects of cross-sectional size and shape on slenderness. For the same cross-sectional area, a section with a higher moment of inertia produces a more stable column with a lower slenderness ratio. The radius of gyration r is calculated using the following formula:
Where:
Ig: moment of inertia of gross concrete section about centroidal axis, neglecting reinforcement, mm4
Ag: Gross area of column, mm2
It is possible to use the approximations of r = 0.3h for square and rectangular sections, and r = 0.25h for circular sections.
where,
h: overall sectional dimension in the directional stability is being considered.
Effective Length Factor (k)
The effective length factor (k) reflects the end restraint (support) and lateral bracing conditions of a column. If a column is hinged at both ends, it follows a half-sine wave when it buckles, and the value of (k) factor for such type of column is 1.0. So, the effective length of the column (klu) is equal to the unsupported length of the column (lu).
A column with fully restrained end conditions develops deflected shape as shown in Figure-5. The portion of the column between the points of contraflexure follows a half-sine wave. The effective length factor k for this case is equal to 0.5.
Columns in real structures are rarely either hinged or fixed but have ends partially restrained against rotation by abutting members. The value of k, in this case, varies between 0.5 and 1.0 for laterally braced columns, Figure-6. For unbraced columns, the value of k varies between 1.0 and ∞, Figure-7.
The majority of reinforced concrete columns are considered to be laterally braced columns. The exact value depends on the degree of end restraint, that is, on the ratio of the stiffness (EI/l) of the column to the sum of stiffnesses (EI/l) of the restraining members at both ends.
ACI Criteria for Slenderness Ratio Effect
The effect of slenderness ratio for columns braced against sideway can be neglected when:
Note: As a first approximation, k may be taken equal to 1.0 in this equation
Where:
M1 :lesser factored end moment on a compression member, N·mm
M2 :greater factored end moment on a compression member.
M1/M2 : negative if the column is bent in single curvature, and positive for double curvature. M1/M2 is negative if the column is bent in single curvature, and positive for double curvature, as illustrated in Figure-8:
The effect of slenderness ratio for columns not braced against sideway can be neglected when:
Alignment Chart for Effective Slenderness Factor
If slenderness is found to be important, refine the calculation of k based on the alignment chart as shown below:
The Ψ factor at one end of the column equals the sum of the stiffness ∑(EI/L)of the columns meeting at that joint, including the column in question, divided by the sum of all the stiffnesses of the beams meeting at the joint.
Where:
E: Modulus of Elasticity
I: Moment of Inertia
L: span length measured center to center of joints.
ΨA, ΨB : Values of (Ψ ) at each end of the column
The modulus of elasticity for normal concrete is computed as:
The moment of inertia used to compute Ψshould be based on cracked section, so ACI 318-19 section 6.6.3.1.1 provides Table-1:
Table-1: Moment of Inertia Used to Compute Ψ Factor
| Member and Condition | Moment of Inertia |
| Columns | 0.70Ig |
| Uncracked walls | 0.70Ig |
| Cracked Walls | 0.35Ig |
| Beams | 0.35Ig |
| Flate Plates and Flat Slabs | 0.25Ig |
FAQs
How do you determine if a column is short or slender?
The column is considered to be slender if its length is higher compared to its cross-sectional dimension i.e. slenderness ratio is used to differentiate slender column from short column.
What is slenderness ratio of a reinforced concrete column?
Slenderness ratio of reinforced concrete (RC) column is the ratio between column length, its lateral dimensions, and end fixity. It assesses the ability of reinforced concrete column to resist buckling pressure.
What is the use of slenderness ratio?
It is used to find out the design load as well as in classifying various columns in short/intermediate/long. Theslenderness ratioof a column gives an indication of buckling failure in the column. More theslenderness ratio, the more is the tendency of the column to fail by buckling effect in that direction.
How do you calculate slenderness ratio of a column?
1. Calculate the effective length factor (K) that ranges between 0.5 to 1.0 for the reinforced concrete column. It can be considered to be 1.0 conservatively.
2. Compute the unsupported length of the column (lu) which is measured as the clear distance between the underside of the beam, slab, or column capital above, and the top of the beam or slab below.
3. Estimate the radius of gyration (r). It is possible to use the approximations of r = 0.3h for square and rectangular sections, and r = 0.25h for circular sections.
4. Calculate slenderness ratio which is equal to effective length factor times unsupported length divided by the radius of gyration.
What is buckling of column?
Buckling of the column is a form of deformation as a result of axial compression forces that leads to a bending of the column, as a result of the instability of the column. This mode of failure is quick and undesirable.
What is radius of gyration of column?
The radius of Gyration is used to describe the distribution of cross-sectional area in an RC columnaround its centroidal axis.
Read more:
Tips and rules for design of reinforced concrete column
Euler's theory of column buckling
What is tributary area in columns?
FAQs
How do you find the slenderness ratio of a column? ›
The length of a column is the distance between points unsupported against lateral deflection. The slenderness ratio is the length l divided by the least radius of gyration k.
How do you find the slenderness of a concrete column? ›The slenderness ratio is calculated by dividing the column length by its radius of gyration. The slenderness ratio differentiates short column from long or slender column. The design of the former is controlled by column dimension and material strength whereas the design of the latter is governed by column slenderness.
What is RC column in construction? ›A reinforced concrete column is a structural member designed to carry compressive loads, composed of concrete with an embedded steel frame to provide reinforcement. For design purposes, the columns are separated into two categories: short columns and slender columns.
What is R in slenderness ratio? ›lu: is the unsupported length of the column. Combining the above equations, we can calculate the slenderness ratio by the following formula. r: is the cross-sectional radius of gyration.
What is the minimum slenderness ratio of RCC column? ›Explanation: Short Column - Short columns are those whose slenderness ratio is less than 32 or length to diameter ratio is less than 8. Such columns are always subjected to direct compressive stress only. Long Column - Long columns have a slenderness ratio more than 120 or length to diameter ratio more than 30.
What is the value of slenderness ratio for long column? ›The medium column slenderness ratio is between 32 to 120. Long columns have a slenderness ratio of more than 120.
What is slenderness ratio of RCC wall? ›For Load-bearing walls, the maximum permissible slenderness ratio (λ) is: λ = 27. For non-Load bearing walls, the maximum permissible slenderness ratio (λ) is: λ = 30.
What is the maximum slenderness limit for RCC columns? ›As per IS 456-2000 it is less than 80. For RCC column, it's 40. According to IS 456-2000 if slenderness ratio is less than 40 then it is a short column. Slenderness ratio 50 is for steel column not for RCC column.
What does slenderness mean? ›the quality of being thin and delicate, often in a way that is attractive : He emphasised his elegant slenderness in close-fitting garments of black satin. We tend to associate slenderness with all kinds of positive attributes, from self-control to intelligence.
How do you calculate reinforced concrete column? ›The area of concrete within the column cross section is found by subtracting the steel area from the gross cross-sectional dimensions; that is, Ac = Ag – As.
How is RCC column calculated? ›
RCC quantity estimation in the footing
RCC quantity in footing can be calculated by = 2 m x 2 m x 0.45 m =1.8 cum.
A short column is defined as one in which the ultimate load is not reduced by the bending deformations because the additional eccentricities Δ=e2 are negligible. A slender column is defined as one in which the ultimate load is reduced by the amplified bending moment caused by additional eccentricity.
What is the slenderness ratio for steel columns? ›A short steel column is one whose slenderness ratio does not exceed 50; an intermediate length steel column has a slenderness ratio ranging from about 50 to 200, and are dominated by the strength limit of the material, while a long steel column may be assumed to have a slenderness ratio greater than 200 and its ...
When slenderness ratio in a column lies between 32 to 120 it is known as? ›The medium column slenderness ratio is between 32 to 120.
Long columns have a slenderness ratio of more than 120.
Short columns have a slenderness ratio of less than 32. Such columns are always subjected to direct compressive stress only.