Designing a Sturdy Short Column: A Practical Approach

In the realm of construction, crafting a resilient axially loaded reinforced concrete (RC) short column follows a rather uncomplicated process. The cornerstone lies in the material’s strength and the configuration of the column’s cross-section. When vertical axial forces act precisely at the center of the column’s cross-section, we term it as an axially loaded column.

However, in real-world scenarios, achieving such precision is a rarity. Inaccuracies in column layout, uneven loading due to variations in slab thickness, and alignment imperfections can easily offset the vertical load from the column’s center. This deviation results in what we call eccentricities, which are essentially displacements from the column’s central axis.

Axially loaded columns typically exhibit minimal eccentricity, usually less than 0.1 times the total depth of the column (h). An example is the interior column of a multi-story building, subjected to symmetrical loads from floor slabs on all sides. ACI 318-19 offers guidelines for designers, providing specifications to ensure an economical and secure design.

Step-by-Step Procedure for Design

1. Calculate Axial Load : Begin by determining the total axial load (Pu) on the column. This involves transferring loads from slabs and beams to the columns.

2. Assume Reinforcement Ratio : Select a reinforcement ratio (pg) within the range defined by ACI 318-19 (0.01 to 0.08). This ratio is expressed as the longitudinal reinforcement area (Ast) divided by the gross area of the column (Ag).

pg=AstAgpg=AgAst​

3. Express Steel Area : Express the steel area in terms of the reinforcement ratio times the gross area of the column cross-section.

pg=AstAg(Equation 1)pg=AgAst​(Equation 1)

4. Select Column Dimensions : Use the calculated values from Equation 1 to select column dimensions and round them.

Ag=Pu0.85×0.65×[0.85×fc′×(Ag−pg×Ag)+pg×fy]Ag=0.85×0.65×[0.85×fc′×(Ag−pg×Ag)+pg×fy]Pu​

5. Calculate Longitudinal Reinforcement Area : Compute the longitudinal reinforcement area using the dimensions from Step 4.

6. Assume Bar Size : Select a bar size from Table-1 and determine the number of bars needed for the column.

7. Check Reinforcement Ratio : Verify if the calculated reinforcement ratio is within the specified range.

8. Design Ties : Determine tie size and estimate spacing for the column.

9. Code Requirements Check : Ensure compliance with code requirements, including clear spacing between bars, minimum number of bars, minimum tie diameter, and proper tie arrangement.

Example: Designing a Short Square Tied Column

Problem Statement
Design an axially loaded short square tied column to support a maximum factored load (Pu) of 2600 KN. Material strength: fc′=28fc′=28 MPa and $fy=420$ MPa.

Solution Steps

• Calculate Pu (already provided as 2600 KN).
• Assume $pg=0.02$ (within the acceptable range).
• Express Ast in terms of the gross area of the column.
• Estimate column dimensions.
• Compute Ast using the selected dimensions.
• Assume a bar size and determine the number of bars.
• Calculate the reinforcement ratio and check its validity.
• Design ties, considering tie size and spacing.
• Perform checks for code requirements.

By following these steps, one can systematically design a robust short column capable of withstanding axial loads, adhering to practical and straightforward engineering principles.