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Compression of

Steel Structures

Ana García Durán

METAL STRUCTURES

Compression members in trusses

Introduction

Stress in the column cross-section

f = P/ A

Structural elements subjected to axial compressive forces

Design governed by strength and buckling

Columns are subjected to axial loads through the centroid

Tension VS Compression

· Compression beside compression forces causes buckling of member

· Tension in members causes lengthening of them

· Presence of holes in bolted connection reduce Gross area in tension members

Compression Member

Vs

Tension Member

· The bending of tension members probably will not be serious as the tensile loads tends to straighten those members, but bending of compression members is serious because compressive loads will tend to magnify the bending in those members

Compression Member Failure

A compressive material failure

A buckling failure

A combination of both compressive and buckling failures

Compression Member Failure

Torsional Buckling

these columns fail by twisting (torsion) or combined effect of torsional and flexural buckling

Torsional Buckling

Local Buckling

This occurs when some part or parts of x-section of a column are so thin that they buckle locally in compression before other modes of buckling can occur

Local Buckling

Flexural Buckling

(Euler Buckling)

Is the primary type of buckling members are subjected to bending or flexure when they become unstable

Flexural Buckling

(Euler Buckling)

Section used for Compression Member

Numerous shapes can be used for columns to resist given loads.

However, from practical point of view, the number of possible solutions is severely limited by section availability, connection problems and type of structure in which the section is to be used

Buckling

Buckling occurs when a straight column subjected to axial compression suddenly undergoes bending.

Buckling is identified as a failre limit-state for columns.

Stable Equilibrium

The column will go back to its original straight position.

In columns elasticity of column itself acts as a restoring force.

This action contributes stable equilibrium

Stable Equilibrium

Precarious Equilibrium

· The amount of deflection depends on amount of force F

· The column can be in equillibrium in an infinite number of bent position

Precarious Equilibrium

Unstable Equilibrium

· The elastic restoring force was not enough to prevent small disturbance growing into an excessively large deflection

· Depending on magnitude of load P, column either remain in bent position, or will completely collapse or fracture.

Unstable Equilibrium

Compression member buckling

· Buckling occurs when a straight, homogeneous, centrally loaded column subjected to axial compression suddenly undergoes bending

· Buckling is identified as a failure limit-state for columns

Critical load

· The value of P at which a straight column becomes unstable is called the Critical Load

· When column bands at critical load, it is said to have buckled

· Therefore, critical load is also called the buckling load

Elastic Buckling of Columns

· The critical buckling load Pcr for columns is theoretically given by

Elastic Buckling of Columns

· Tendency of compression members to buckling is governed by L/r

Effective lenght in different direcctions

Euler slenderness

· The intersection point P, of the two curves represents the maximum theorical value of slenderness of a column compressed to the yield strength. This maximum slenderness (sometimes called Euler slenderness)

Inelastic Buckling

· In elastic buckling it was assumed that a column made of a metal whose stress-strain curve is linear until a yield plateau reached

· For a column with intermediate length, when buckling occurs after the stress in the column exceeds the proportional limit of the column material and before the stress reached the ultimate stregth. This kind of situation is called inelastic buckling.

Inelastic Buckling

Load Buckling

· If local buckling of the individual plate elements occurs, then the column may not be able to develop its buckling strength

· Therefore, the local buckling limit state must be prevented from controlling the column stregth

Load Buckling

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