Punching Shear Strength of Reinforced Concrete Slabs without Transverse Reinforcement

Key Insights: Punching Shear Strength of Reinforced Concrete Slabs without Transverse Reinforcement

A novel failure criterion based on critical shear crack opening and slab rotation offers a more accurate prediction of punching shear strength in concrete slabs.

Research Focus

This research addresses the limitations of current design code provisions for predicting punching shear strength in reinforced concrete slabs, particularly those lacking transverse reinforcement. Understanding and accurately quantifying this failure mode is critical for ensuring the safety and efficiency of flat slab construction, which is prevalent in many building types. The study developed a mechanical explanation and a new failure criterion based on experimental observations and theoretical analysis.

What the Research Found

Finding 1: Critical Shear Crack Theory Explains Failure

The research posits that punching shear failure is initiated by the opening of a critical shear crack. This crack propagates and influences the slab's ability to resist shear forces, directly impacting its ultimate punching capacity. This understanding moves beyond empirical formulas to a more fundamental mechanical basis.

Finding 2: Slab Rotation is a Key Indicator of Strength

Punching shear strength is directly linked to the slab's rotation at failure. The proposed criterion correlates the load-carrying capacity with the degree of slab rotation, providing a more nuanced approach than current code methods that often rely on a fixed critical section.

Finding 3: New Criterion Improves Prediction Accuracy

The developed failure criterion, incorporating critical shear crack opening and slab rotation, demonstrated superior accuracy in predicting punching shear failures across various experimental scenarios, including those with low reinforcement ratios and exhibiting size effects. This suggests a more reliable tool for design engineers.

Why It Matters for Practice

This research challenges the assumption that slab thickness alone is the primary driver of punching shear capacity. It highlights the significant influence of slab rotation and the underlying mechanics of crack propagation. The findings offer the potential for more refined and potentially more economical designs by providing a more accurate assessment of slab performance, especially in situations where current codes might be overly conservative or unconservative.

Putting It Into Practice

Based on these findings, professionals should consider:

• • Evaluating the load-rotation behavior of slabs as a critical factor in punching shear design.
• • Investigating the application of the proposed critical shear crack theory for more accurate strength predictions, particularly for non-standard slab geometries or reinforcement configurations.
• • Exploring how this improved understanding can lead to optimized slab thicknesses and reinforcement layouts, potentially reducing material usage.

Limitations to Note

The proposed model's application requires understanding the slab's load-rotation relationship, for which a simplified mechanical model is provided. Further investigation into the universality of this model across a broader range of slab types and loading conditions may be beneficial.