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Factors afecting beam strengthBending strength may be limited by material strength, lateral-torsional buckling or local

buckling.

As was stated previously the strength of a beam depends on its maximum moment and shear

capacities, and these depend on the relevant allowable stresses.

A beam may fail in one of three ways. The three types of failure are material failure causing

a plastic hinge to form, lateral-torsional buckling along the length of the beam, and local

buckling of the beam cross-section.

A plastic hinge forms when the bending stress reaches the material yield strength.

Strength failure of beams

ollapse by formation of a plastic hinge. !here the stress in the beam reaches the yield

stress, the bending moment cannot be increased and the beam collapses as though a hinge

has been inserted into the beam.

"ateral-torsional buckling is associated with the compression developed in part of the beam

cross-section due to bending.

Bending moments cause a pair of internal hori#ontal forces, one force is a tension force and

the other a compression force. The tension force stretches one side of the beam. This force,

like pulling a string, tends to keep the tension side of the beam straight between supports.

$owever, the compression force can buckle the compression side of the beam. Because the

tension force is keeping one side taut, the beam can only buckle sideways and twist the

beam - hence lateral torsional buckling.

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Tension and compression due to bending

%&ect of tension maintaining straight form of beam

%&ect of compression causing lateral-torsional buckling

"ateral-torsional buckling may be resisted by restraining the beam laterally.

This buckling may be prevented by ade'uate lateral restraint preventing sideways movement

of the beam. (f this is at discrete points, the beam may buckle between these points.

Buckling of beams with intermediate restraint

Steel )oor beams are often ade'uately restrained by the )oor slab.

(n building structures the )oor slab is often able to provide e&ective restraint to the )oor

beams, preventing lateral-torsional buckling. (n such cases the beams can be designed on the

basis of the full bending strength.

"ocal buckling can occur if the beam cross-section is very slender.

(f some part of the beam is very slender, then this may buckle locally. (n practice, standard

steel sections are proportioned so that this is not a critical design consideration.

"ocal buckling of beams

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The dominant failure mode depends on a number of factors.

!hich type of failure occurs depends on four factors*

the slenderness ratio of the beam.

the shape of the bending moment diagram.

the proportions of the individual parts +webs, )anges etc. of the beam cross-section.

the presence of a high shear force.

Any of these factors may reduce the allowable stress to below the yield stress to ensure that

the beam is safe against any type of failure.