The ratio of rise to span of truss represents the pitch in roof truss.

• Span is Large
• Loads are heavy
• Bending Moment is very high.
• Load cannot be carried by rolled section economically.
• Even I-section with cover plate also become insufficient to support the loads.
• Need deeper sections having higher stiffness to carry heavy loads.

The bending moment of simply supported beam under UDL occurs maximum at center of span and reduces gradually towards supports. Since the bending stress is resisted by flange area; therefore, for the effective and economical use of plate, it is necessary to use a smaller flange area towards the supports which is done by curtailment of flange plate.

• Span is long
• Large Bending Moment are generated
• Available Rolled Section are not sufficient to resist external bending.
• Depth of beam is restricted due to head room requirements.

To account for lateral torsional buckling.

For Beam – L/300, where L is span of beam

For Purlin – L/180, where L is span of purlin

(Note: Since deflection is serviceability criteria, we have to provide serviceability at working loads. So, while calculating the deflection, we use service/working load not factored load)

Web buckling occurs when intensity of compressive stress at center of cross section is greater than critical buckling stress. Web act as a column. Buckling depends upon slenderness ratio of web d/tw.

Due to high stress just below the vertical concentrated load and above the reactions from support, web tends to fold at radius of root. This phenomenon is called web crippling.

Web buckling can be controlled by providing stiffeners. Web crippling can be controlled by providing more bearing area and stiffeners as well.

Buckling and Crippling can be reduced by providing thicker web.

(Note: As per IS Code, if beam is safe in crippling, it will be safe for buckling too.)

In bending, Load is perpendicular to longitudinal axis, under loading beam deflects in vertical position

In Buckling, intensity of vertical compressive stress near the center of section becomes greater than critical buckling stress for the web acting as column. Depends upon slenderness ratio.

Fig: Bending of beam

Fig: Buckling of beam

Beam – I section (Bending and Shear) (Bending – Flange, Shear – Web)

Column – Channel Section (Compression)

Roof – Angle Section / I section / Channel Section (Biaxial Bending)

As a Tension Member – Angle Section (Tension Force)

Compression Flange is restrained laterally.