In reinforced concrete design, an "indirect support" occurs when a secondary beam is supported by a primary beam rather than a column or wall. This configuration creates a complex stress state at the intersection where the load from the secondary beam must be "lifted" into the top of the primary beam.
Failure to provide adequate "hanger reinforcement" (links or U-bars) can lead to severe diagonal tension cracking and potential shear failure at the intersection. Eurocode 2 (EN 1992-1-1, Clause 9.2.5) mandates specific reinforcement to ensure a safe load path.
The Truss Analogy
To understand why we need additional reinforcement, we use a Strut-and-Tie model. Imagine the internal forces as a truss: the load from the secondary beam enters the bottom or middle of the primary beam. Since concrete cannot resist significant tension, we need vertical steel "ties" (hanger bars) to pull that load up to the compression zone at the top of the primary beam.
Without these ties, the compression struts in the primary beam would have no "node" to push against, and the bottom of the primary beam would simply tear away from the top.
Calculating Additional Reinforcement
The reinforcement provided should be able to resist the total factored reaction force $F_{Ed}$ from the secondary beam. According to EN 1992-1-1, this reinforcement should be placed in addition to the shear reinforcement required for the beams themselves.
The required area of hanger reinforcement $A_{s,link}$ is calculated as:
Where:
- $F_{Ed}$ is the design support reaction from the secondary beam.
- $f_{yd}$ is the design yield strength of the reinforcement ($f_{yk} / 1.15$).
Curtailment and Placement
Placement is as important as quantity. Hanger reinforcement should be concentrated in the intersection zone. Ideally, these bars should be:
- Distributed: Placed within a distance equal to the width of the supporting beam.
- Anchored: Fully anchored in the compression zone of the primary beam (usually by hooking over the top longitudinal bars).
- Symmetrical: Placed on both sides of the secondary beam web.
Key Takeaways
- Don't double-count: Hanger reinforcement is additional to the standard shear links required by $V_{Ed}$.
- Check Top Bars: Ensure the primary beam's top longitudinal bars are sufficient to act as the "node" for the hanger ties.
- Intersection Congestion: High reaction forces can lead to heavy congestion. Using larger diameter links (e.g., 12mm or 16mm) can help maintain spacing for concrete pouring.
Designing complex beam intersections manually is prone to error. Our StrucTalogue EN 1992 Indirect Supports Module automatically detects indirect supports and calculates the required hanger bar area based on the secondary beam's reaction forces.