Steel Design

STEEL BEAM DESIGN
Laterally Unrestrained Beam

Dr. A Aziz Saim 2010 EC3

Unrestrained Beam

1

Non-dimensional slenderness
Beam behaviour analogous to yielding/buckling of columns.

M Wyfy

Material yielding (in-plane bending)

MEd

MEd

Elastic member buckling Mcr

Lcr

1.0
Dr. A Aziz Saim 2010 EC3

Non-dimensional slenderness
Unrestrained Beam

 LT
2

Lateral torsional buckling
Lateral torsional buckling
Lateral torsional buckling is the member buckling mode associated with slender beams loaded about their major axis, without continuous lateral restraint. If continuous lateral restraint is provided to the beam, then lateral torsional buckling will be prevented and failure will occur in another mode, generally in-plane bending (and/or shear).
Dr. A Aziz Saim 2010 EC3 Unrestrained Beam 3

Eurocode 3
Eurocode 3 states, as with BS 5950, that both crosssectional and member bending resistance must be verified:

MEd  Mc ,Rd

Cross-section check (In-plane bending)

MEd  Mb,Rd
Dr. A Aziz Saim 2010 EC3 Unrestrained Beam

Member buckling check

4

Dr. A Aziz Saim 2010 EC3

Unrestrained Beam

5

Laterally Unrestrained Beam
The design of beam in this Lecture 3 is considering beams in which either no lateral restraint or only intermittent lateral restraint is provided to the compression flange

Dr. A Aziz Saim 2010 EC3

Unrestrained Beam

6

Lateral Torsional Buckling

Dr. A Aziz Saim 2010 EC3

Unrestrained Beam

7

Lateral Torsional Buckling
Figure 3-1 shows an unrestrained beam subjected to load increment. The compression flange unrestrained and beam is not stiff enough. There is a tendency for the beam to deform sideways and twist about the longitudinal axis. The failure mode which may occur to the beam is called lateral torsional buckling.

Dr. A Aziz Saim 2010 EC3

Unrestrained Beam

8

Involves both deflection and twisting rotation

Out-of plane buckling.
Bending