👉This image explains the "Purpose of Stirrups during construction?" in the context of reinforced concrete beams, specifically by illustrating two common types of failure in beams:  Failure and Shear Failure. Stirrups are crucial for preventing the latter.

🏷Let's break down each part:

📜Overall Context:

⏺The image focuses on a beam supported by two columns, a common structural element in buildings.

⏺The text aims to explain why steel stirrups (small, closed loops of rebar) are incorporated into concrete beams.

🚧Part (a): Flexure Failure

* Illustration: Shows a beam bending significantly downwards in the middle, creating a curved shape.
    * "Beam" is labeled, indicating the horizontal structural member.
    * "Column" are the vertical supports at each end.
    * The bottom face of the beam is depicted with a mesh-like pattern, and small vertical lines extend upwards, representing cracks.
* Description: "Bottom face stretches in tension and vertical cracks develop."
    * When a beam is subjected to a load (e.g., from above), its top face is compressed (in compression), and its bottom face is stretched (in tension).
    * Concrete is very strong in compression but weak in tension. Therefore, as the bottom face stretches, it cracks vertically.
    * The "Flexure Failure" label highlights that this type of failure is due to bending moments.
* Relevance to Stirrups: While longitudinal (horizontal) rebar at the bottom of the beam is primarily designed to resist these tensile forces and prevent flexure failure, stirrups play a secondary role in maintaining the integrity of the concrete cover around this rebar, especially after significant cracking. However, their main purpose is not primarily for flexural strength.

🚧Part (b): Shear Failure

* Illustration: Shows a beam near one of its supporting columns.
    * A prominent "Inclined crack" is visible, typically forming at an angle of approximately 45° to the beam's longitudinal axis. This crack starts near the column and propagates upwards and inwards.
* Description: "Shear Failure"
    * Shear forces are concentrated near the supports of a beam. These forces tend to cause one part of the beam to slide past another.
    * Concrete's weakness in tension also makes it susceptible to diagonal tension stresses caused by shear forces. When these stresses exceed the concrete's tensile strength, inclined cracks form.
    * These inclined cracks are characteristic of shear failure and can lead to a sudden and brittle collapse of the beam if not properly reinforced.

⭐️Purpose of Stirrups (Implicit in the explanation):

    * The smaller diagram on the left shows a multi-story building frame with a highlighted area indicating a beam-column joint. An arrow points to the beam, suggesting that the integrity of these connections is crucial.
    * Stirrups (also known as shear reinforcement or ties) are typically vertical or U-shaped closed loops of rebar placed perpendicularly to the main longitudinal rebar within the concrete beam.
    * Their primary purpose is to resist the diagonal tension stresses that cause shear cracks. They "tie" the concrete together, preventing the inclined cracks from widening and leading to failure.

⏺By providing a tension resistance in the direction perpendicular to the inclined crack, stirrups help the beam withstand shear forces.
    * Without adequate stirrups, shear failure can be sudden and catastrophic, making them essential for the safety and ductility of reinforced concrete structures.

📜In summary, the image effectively conveys that while longitudinal rebar addresses flexural (bending) stresses, it is the stirrups that are critical for resisting shear stresses and preventing brittle shear failure in concrete beams, particularly near their supports where shear forces are highest.

@etconp