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Last edited 15 Sep 2020
Mechanisms of structural failure
Structural failure refers to defects in which a load bearing component of the building is unable to support and transfer loads to another element. Structural failure develops due to breakdown in the performance of the materials in a structural component may be caused by:
- Erroneous construction: Failure of the engineer to supervise all the construction activities at the site resulting in faults to construction elements which later develop into failure; issues like use of salty sand in making concrete, use of poor grade steel not as specified, improper tightening of torque nuts, bad welds and so on.
- Improper design: Failure of the engineer to account for all the loads the structure is to carry, application of erroneous design theories, use of inaccurate data, not taking account of the impacts of repetitive or impulsive stresses, improper use of materials, misunderstanding of properties and so on.
- Foundation failures: Failure of the ground on which the foundation rests to carry load, causing displacements, altering the stress distribution to the whole structure and so on.
- Overloading: Excess loads that are applied beyond that which had been anticipated, these loads might be due to vibrating earthquake, heavy snow loads, hurricanes, storage, change of use and so on.
- Innovation: New types of structures subject to unexpected failures.
Compressive failure can be sudden and catastrophic, apparent by cross breaks caused by the excessive longitudinal compression or bending. Tensile failure can be detected by stretching in the sizes of the member. These elongations come along with cracks that orient perpendicular to the direction of the tensile forces.
Shear failure is principally an internal splitting force caused by two forces acting in opposite directions at a distance to one another along the cross section of a structural member. Shear cracks are diagonal, for example, appearing in the web of a beam.
Buckling is a creasing failure by crumpling of a longitudinal structural member loaded eccentrically with a compressive force. It occurs to long and slender members subject to axial compressive stress. Buckling load is a compressive load at which a column or strut begins to buckle. Steel columns are more prone to buckling than concrete columns because they are more slender. Also slender beams and floor joists fail due to compressive stresses resulting from the bending.
Buckling Vs. Bending
|Occurs when the load reaches a certain critical value known as Critical load||Occurs under all magnitudes of loads applied to a structural component.|
|Buckling takes place abruptly.||May be sudden or gradual, it gives warning signs of failure of the structural component before complete collapse.|
|Buckling under load members become unstable, but will generally not fracture||Bending under load member deflects, it may start to develop some cracks.|
|Instability caused by buckling in one member can lead to excessive stresses in other members causing progressive collapse of the whole structure.||The defects developed due to bending stress cause less effect to progressive failure of the whole structure.|
|Buckling is due to axial stresses applied along the longitudinal axis of the member.||Bending is a combination of tensile and compressive stresses.|
Compressive, tensile, bending and buckling are the basic types of structural failure for construction elements. These are caused due to faults in design and construction. To mitigate these failures the structural engineer has to properly study properties of the structural materials, loads to be applied and the use of the structure to propose an appropriate material, adequate cross section and a proper structural system.
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