What is strut-and-tie model in deep beam design?

Short Answer:

The strut-and-tie model in deep beam design is a simplified method used to represent the flow of forces through concrete. It uses imaginary triangles made of struts (compression members), ties (tension reinforcement), and nodes (connection points) to show how loads are transferred from the point of application to the supports.

This model is especially useful for deep beams, where standard bending theory does not apply due to non-linear stress distribution. The strut-and-tie method helps in placing reinforcement accurately by visualizing the internal load path, ensuring safety and strength in critical regions.

Detailed Explanation

Strut-and-Tie Model in Deep Beam Design

In deep beams, due to their small span-to-depth ratio, the usual flexural theory fails to accurately describe the behavior of load transfer. Instead, the beam behaves more like a truss, where the load travels through direct compression and tension paths. This is where the strut-and-tie model (STM) becomes important. It is a powerful design tool used to simplify and understand the complex stress flow in deep beams and other discontinuity regions.

The strut-and-tie model divides the structural member into a set of triangle-like patterns formed by:

  • Struts: Represent the concrete compression zones where force travels in a diagonal or vertical direction.
  • Ties: Represent the steel reinforcement bars that carry tensile forces in the member.
  • Nodes: Are the junction points where struts and ties meet and forces are transferred.

This model allows engineers to convert the actual stress field into an idealized truss layout, which is easier to analyze and design.

How Strut-and-Tie Model Works in Deep Beam Design

  1. Load Path Visualization
    In deep beams, the loads from the top are transferred to supports not through flexure but through diagonal compressive paths (struts) and horizontal steel (ties). These load paths are visualized as a truss made up of concrete struts and steel ties.

For example, in a simply supported deep beam:

  • The load applied at the top travels diagonally through concrete (strut) to the support.
  • Horizontal tension at the bottom is resisted by steel bars (tie).
  • The interaction point is called the node.
  1. Types of Struts
  • Vertical struts: Transfer load directly to the support.
  • Diagonal struts: Take the load from the load point to support corners.
  • Bottle-shaped struts: Occur where load spreads out from a point, such as under concentrated loads.
  1. Types of Ties
  • Bottom ties: Horizontal reinforcement at the bottom of the beam.
  • Inclined ties: Provided when tensile forces travel diagonally.
  1. Nodes and Node Design
    Nodes are critical because they are the points where forces meet. There are three main types:
  • CCC: All compression (concrete–concrete–concrete)
  • CCT: Compression and tension mix (concrete–concrete–tie)
  • CTT: Nodes with two tension members (rare)

The size and reinforcement around nodes must be adequate to avoid crushing or splitting of concrete.

  1. Design Steps Using Strut-and-Tie Model
  • Identify the load path and sketch the strut-and-tie truss system.
  • Calculate the forces in struts and ties.
  • Design reinforcement to resist tension in ties.
  • Check bearing strength and dimensions of nodes.
  • Ensure minimum concrete strength and detailing are satisfied.
  1. Advantages of STM in Deep Beams
  • Provides a clear understanding of how loads move through the beam.
  • Helps in placing reinforcement exactly where needed.
  • Suitable for irregular shapes or loading conditions.
  • Follows guidelines of IS 456 and advanced codes like ACI 318.
  1. Code Reference
  • IS 456:2000 suggests STM for non-linear regions like deep beams.
  • SP 24 and IS 13920 give extra guidance on how to apply this model practically.
  • ACI 318 and Eurocode also recognize STM for deep beam and corbel design.
Conclusion

The strut-and-tie model in deep beam design is a truss-based method that represents how forces travel through compression and tension within the structure. It simplifies complex stress conditions into a visual and analytical model, making it easier to design safe and effective reinforcement for deep beams. By using STM, engineers ensure that structural strength and load paths are properly managed, especially in short-span, heavy-load applications.