How to Calculate Helix Bar for Reinforced Concrete Design

 

How to Calculate Helix Bar for Reinforced Concrete Design

A helix bar is a spiral reinforcement used in reinforced concrete design to resist torsional and axial loads in elements like columns and piles. Helix bars are particularly effective for columns and piles that are subjected to axial loads and twisting forces.

Steps to Calculate Helix Bar in Concrete Design

In order to calculate the required helix bar reinforcement, the following parameters and formulas are typically used:

Formula for Helical Reinforcement Design:

The basic formula for calculating the required helical reinforcement is:

e = (V²) / (g × R)

Where:

• e: Superelevation (in meters or feet)
• V: Design speed (in meters per second or feet per second)
• g: Gravitational acceleration (9.81 m/s² or 32.2 ft/s²)
• R: Radius of the curve (in meters or feet)

Step-by-Step Calculation Process:

1. Determine Axial and Torsional Loads:

First, calculate the axial load P and torsional moment T acting on the column or pile. These values depend on the loads acting on the structure, including dead load, live load, and any applied moments.

2. Select Reinforcement Material:

Choose the yield strength of the reinforcement material. For example, the yield strength can typically be taken as:

• 250 MPa for mild steel
• 500 MPa for high-strength steel

3. Calculate the Helical Reinforcement Ratio (\rho):

The reinforcement ratio is calculated using the following formula:

ρ = P_u / (F_y × A_column)

Where:

• P_u: Ultimate axial load
• F_y: Yield strength of steel
• A_column: Cross-sectional area of the column

4. Design the Helix Pitch:

The pitch of the helix is the distance between turns of the helix. It is calculated using the formula:

P = 2πR / N

Where:

• R: Radius of the column
• N: Number of turns in the helix

5. Determine the Number of Turns:

The number of turns in the helix is based on the height of the column H and the pitch P. It is given by:

N = H / P

6. Calculate the Required Area of Helical Reinforcement:

The area of helical reinforcement is calculated based on the reinforcement ratio and the cross-sectional area of the column:

A_helical = ρ × A_column

7. Check Shear and Torsion Resistance:

After designing the helical reinforcement, ensure that the helical bars provide enough strength to resist the applied shear and torsional forces. The bars should be spaced and sized to prevent failure due to torsion.

Example Calculation:

Let's consider the following design parameters for a column:

• Column Diameter (D): 12 inches (1 ft)
• Axial Load (P_u): 50 kips (50,000 lbs)
• Yield Strength of Steel (F_y): 60,000 psi
• Column Length (L): 10 ft
• Torsional Moment (T): 2 k-ft
• Column Radius (R): 0.5 ft (half of 12 inches)

Step 1: Calculate Reinforcement Ratio (ρ):

First, we calculate the reinforcement ratio:

ρ = 50,000 / (60,000 × 1) = 0.8333

Step 2: Calculate Helical Pitch (P):

The pitch is calculated for 2 turns:

P = 2π × 0.5 / 2 = 1.57 ft

Step 3: Check the Area of Helical Bars:

Now, calculate the area of the helical bars:

A_helical = 0.8333 × 1 = 0.8333 sq ft

Step 4: Choose the Bar Diameter:

Choose a diameter for the helical bars. For example, we could use #4 bars, each with 0.2 sq in. of cross-sectional area.

Conclusion:

In this example, we calculated the required helical reinforcement for a column subjected to an axial load and torsional moment. The required reinforcement ratio, pitch, and bar diameter were determined to provide adequate strength for the column.

Factors Affecting Helical Reinforcement Design:

• Speed: Higher speeds require greater reinforcement in the case of torsional loads.
• Radius: Smaller column radii require more reinforcement to resist torsional moments.
• Environmental Conditions: Wet or harsh conditions may require adjustments to the reinforcement.