How is anchorage length provided in beams?

Short Answer:

Anchorage length in beams is the length of reinforcement bar that is embedded or extended into the concrete to safely transfer stress between steel and concrete. It ensures that the steel bars do not slip out under load and the beam remains strong and stable.

This length is provided by extending the bars beyond the critical section (like supports or bends), as per the design rules. The anchorage length depends on bar diameter, type of stress, and concrete grade. Proper anchorage helps maintain the bond and prevents failure due to steel slippage or pull-out.

Detailed Explanation:

Anchorage length provided in beams

In reinforced concrete structures, anchorage length is a very important part of beam design. It refers to the minimum length of the reinforcement bar embedded inside the concrete to ensure that the bar does not slip out under tension or compression. The purpose of providing anchorage is to create a strong bond between steel and concrete, so they work together as a single unit when loads are applied.

Concrete holds the steel bars in place using grip and friction. But to make sure the bars do not pull out or move, they must be extended a certain distance into the concrete. This distance is called the anchorage length, also known as development length in some cases.

How Anchorage Length is Provided in Beams

  1. At Supports (Tension Bars)

In simply supported beams, maximum tension is in the bottom steel at the mid-span. However, at the supports, the bars are extended beyond the face of the support to provide anchorage. This extended portion is usually hooked or bent to increase the grip inside the concrete.

  • The bar is projected past the support a length equal to or more than the required anchorage length.
  • Hooks of 90° or 135° are often added at the ends to improve bond strength.
  1. At Laps and Splices

When the beam is longer than the available bar length, two bars are joined using lap splices. In this case, both bars are overlapped for a certain length to transfer stress properly.

  • This overlapping length is also based on anchorage length requirements.
  • For tension bars, lap length is usually more than for compression bars.
  1. In Bent-up Bars

Bent-up bars provided for shear or negative moments are also extended sufficiently into the concrete at the supports or points of maximum stress. The anchorage length ensures these bars resist the applied forces without slipping.

  1. Calculation of Anchorage Length

The required anchorage length depends on:

  • Diameter of the bar (ϕ): Larger bars need more length.
  • Type of stress: Tension bars need more anchorage than compression bars.
  • Grade of concrete (fck): Higher-grade concrete provides better grip, so less anchorage length is needed.
  • Type of bar: Deformed bars have better grip than plain bars.

As per IS 456:2000, the anchorage length LaL_aLa​ is calculated as:

Anchorage length (La) ≥ (ϕ × σ_s) / (4 × τ_bd)
Where:
ϕ = diameter of bar
σ_s = stress in bar
τ_bd = design bond stress depending on concrete grade

A general rule often used in practice is:

Anchorage length = 50 × diameter of bar (for mild steel in tension)

  1. Use of Hooks and Bends

To reduce the required straight length, bars are often hooked or bent at ends. These shapes increase the grip and help reduce space, especially in small beams.

  • 90° or 135° bends are common.
  • Hooked ends count as part of the anchorage length as per IS code.
Conclusion:

Anchorage length in beams is provided by extending the reinforcement bars into the concrete beyond critical sections like supports or bends. It ensures the safe transfer of stress and prevents bar slippage. Anchorage can be provided using straight lengths, hooks, bends, or lap splices. Proper anchorage length, as per design and IS code, is essential for the safety and performance of RCC beams.