Industrial building roof design: purlins placed over roof trusses to carry dead, live, and wind loads are, as per IS practice, assumed to behave structurally as which support condition?

Introduction / Context:
Purlins are secondary structural members spanning over truss rafters to support roof sheeting. Their idealization affects bending moments, deflection, and connection design in industrial buildings.

Given Data / Assumptions:

• Purlins are placed over multiple truss panel points.
• They are connected at each support, producing multi-span behavior.
• IS practice commonly models purlins as continuous over several spans.

Concept / Approach:
With more than two spans, purlins develop negative moments over supports and reduced mid-span positive moments compared to simply supported members. This continuous-beam action is closer to real behavior in typical industrial roofs and is reflected in codal design assumptions and moment coefficients.

Step-by-Step Solution:
Identify load path: sheeting → purlins → trusses → columns.Recognize multi-span continuity: connections at successive trusses enable continuous action.Select the support condition that matches: 'Continuous'.

Verification / Alternative check:
Design tables for purlins provide continuous-beam coefficients and reduction factors due to continuity.

Why Other Options Are Wrong:

• Simply supported ignores continuity and overestimates mid-span moments.
• Cantilever/fixed do not represent typical purlin layouts in trussed roofs.

Common Pitfalls:

• Treating every span independently, leading to conservative but uneconomical designs.

Final Answer:
Continuous