Shear walls, critical structural elements designed to resist lateral forces like wind and earthquakes, can fail in several distinct ways. Understanding these failure modes is essential for proper design and safety.
The different failure modes in the shear wall under lateral loading are sliding shear failure, flexure failure, diagonal tension failure, diagonal compression failure, and hinge sliding failure. These modes typically manifest under significant lateral loads, leading to various forms of damage and potential collapse if not adequately addressed in the design and construction.
Common Shear Wall Failure Modes
Here's a breakdown of the types of failure observed in shear walls:
Flexure Failure
Flexural failure occurs when the bending stress at the base of the shear wall exceeds its capacity. This is similar to how a cantilever beam fails under bending.
- Mechanism: The wall behaves like a vertical cantilever. Tension cracks appear on the side opposite the load, while compression crushing occurs on the loaded side.
- Appearance: Horizontal cracks at the base, spalling of concrete or masonry on the compression side, and potentially buckling of reinforcement bars in tension.
- Contributing Factors: Insufficient vertical reinforcement or inadequate confinement at the base.
- Mitigation: Increasing vertical reinforcement and improving detailing at the base, especially using confinement steel.
Sliding Shear Failure
Sliding shear failure happens when the shear force parallel to the base of the wall exceeds the wall's resistance to sliding.
- Mechanism: The wall base effectively slides horizontally along a plane, often at construction joints or between the wall and the foundation.
- Appearance: A prominent horizontal crack near the base, potentially with observable horizontal displacement.
- Contributing Factors: Lack of adequate shear friction reinforcement crossing the potential sliding plane, poor construction joint preparation.
- Mitigation: Providing dowel bars or shear reinforcement crossing the base joint and ensuring proper surface preparation of concrete joints.
Diagonal Tension Failure
Also known as shear failure, diagonal tension failure occurs when diagonal cracks form across the shear wall due to principal tensile stresses induced by shear forces.
- Mechanism: Shear forces create diagonal tensile stresses. If these stresses exceed the concrete's tensile strength, diagonal cracks appear.
- Appearance: Cracks that run diagonally across the wall panel, typically oriented at approximately 45 degrees to the horizontal, forming an 'X' pattern in cyclic loading.
- Contributing Factors: Insufficient shear reinforcement (horizontal and vertical ties), low concrete tensile strength.
- Mitigation: Increasing the amount and proper detailing of shear reinforcement throughout the wall panel.
Diagonal Compression Failure
Diagonal compression failure occurs when the concrete or masonry within diagonal compression struts, formed by diagonal tension cracks, crushes.
- Mechanism: After diagonal tension cracks form, the load is carried by diagonal compression struts between the cracks. If the compressive stress in these struts exceeds the material's capacity, crushing occurs.
- Appearance: Spalling and crushing of concrete within the diagonal panels, often accompanied by buckling of reinforcement bars in those areas.
- Contributing Factors: High shear stresses combined with insufficient concrete compressive strength or inadequate confinement of the concrete core.
- Mitigation: Using higher strength concrete, providing adequate confinement reinforcement, and ensuring proper detailing of boundary elements in ductile shear walls.
Hinge Sliding Failure
Hinge sliding failure is a variation of sliding shear failure that occurs within a plastic hinge region, typically near the base where flexural yielding has occurred.
- Mechanism: After flexural yielding causes significant horizontal cracking and elongation of vertical bars at the base (forming a hinge), the shear resistance along these cracks is reduced. Sliding then occurs along these yielded horizontal planes.
- Appearance: Significant horizontal cracks at the base accompanied by observable horizontal displacement within the yielded zone. It often follows significant flexural deformation.
- Contributing Factors: Combination of high shear forces and significant flexural yielding at the base, inadequate shear friction across the yielded section.
- Mitigation: Similar to sliding shear, requires adequate shear reinforcement across potential sliding planes, especially detailing within the plastic hinge region to maintain shear resistance.
Understanding these failure modes is crucial for engineers to design robust and safe shear walls that can perform effectively under seismic and wind loading, protecting structures and their occupants.
