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The difference between Buckling, Compression & Shear



  • A column is a structural element that transfers the weight of the structure above to other structural elements below through compression.


  • Column history goes back to the Ancient Egyptians where stoned columns were firstly constructed


  • Nowadays most of the columns are made of a combination of concrete and steel ( reinforced concrete)


  • Three main failure mode of concrete columns are Buckling, Pure Compression and Shear.


1.Buckling:


Slender structural members loaded axially in compression will experience buckling. A relatively slender compression member (e.g. a column) may deflect laterally and fail by bending rather than failing by direct compression.


The behaviour can be demonstrated by compressing a plastic ruler.


When lateral bending occurs, we can say that the column has buckled.




Buckling is one of the major causes of failures in structures, and therefore the possibility of buckling should always be considered in the design.

Definition of Bending Moment


A bending moment (BM) is a measure of the bending effect that can occur when an external force (or moment) is applied to a structural element that causes the element to deform and bend.


This concept is important in structural engineering as it is can be used to calculate where, and how much bending may occur when forces are applied determining the maximum deformation a structural member will undergo.


A simple way to visualise bending moment is a diagram of a simply supported beam which means both ends of the beam can rotate and hence there is no bending moment at those locations.


The diagram below shows that maximum bending moment is governed in the mid-span of this simply supported beam and maximum shear occurs at the supports.

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2.Pure Compression:


Compression is one of the fundamental mechanics of deformable bodies.


DEFINITION:


compression /kəmˈprɛʃ(ə)n/

noun

  • the action of compressing or being compressed.

  • the reduction in volume (causing an increase in pressure) of the fuel mixture in an internal combustion engine before ignition.



The axial force P acting at the cross-section is the resultant of the continuously distributed stresses. Elements that are pushed together or carry a load, that tend to compress, the forces are called compressive stress.



3.Shear:


Tension is about pulling and compression is about pushing, then shear is about SLIDING.

Shearing forces are unaligned forces pushing one part of a body in one specific direction, and another part of the body in the opposite direction.






  • Shear strain = angle through which material is distorted as a result of shear stress N.


William A. Nash defines shear force in terms of planes: "If a plane is passed through a body, a force acting along this plane is called a shear force or shearing force."


Sources: Wikipedia , Mechanics of Materials , Structures , designingbuildingsWiki

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