Mechanisms of structural failure

[edit] Introduction

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.

[edit] Compressive and tensile failure

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.

[edit] Bending failure

Bending is the bowing deformation of a beam or other member under load. Bending stress can be slow or sudden depending on the nature of the force applied, whether compressive or tensile forces.

[edit] Shear failure

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.

[edit] Buckling failure

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

For a member to withstand loads against buckling failure it should be designed to reduce its slenderness by increasing its cross-sectional dimension.

Bracing a member at its intermediate points along its length or height

For economical reasons consider bracing a member as cheaper and easier than increasing its cross sectional dimensions this is uneconomical.

[edit] Conclusion

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.

[edit] Related articles on Designing Buildings

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