This question relates to the design principles for purlins as specified in the Indian Standard IS 800:2007, particularly concerning the achievement of reduced design forces.

Purlin Functionality in Roof Structures

Purlins are secondary structural members used in roof construction. Their primary function is to support the roof sheeting (like metal sheets or tiles) and transfer the loads acting on the roof (such as dead load, live load, wind load) to the primary structural members like rafters or roof trusses. Purlins typically span the distance between these main structural elements.

IS 800:2007 Purlin Guidance

The Indian Standard IS 800:2007, which provides the code of practice for general construction in steel, offers guidelines for designing various structural elements, including purlins. The standard acknowledges that efficient structural design can lead to reduced material usage and cost. In this context, it suggests methods that allow for reduced design forces.

Continuous Beam Advantages for Purlins

When purlins are designed as continuous beams, they span over multiple supports (rafters or trusses). This configuration offers several structural advantages compared to simply-supported beams:

• Load Distribution: Continuous beams distribute loads more effectively across multiple spans.
• Reduced Bending Moments: The continuity helps in redistributing moments, resulting in lower maximum bending moments at the supports and within the spans compared to a series of simply-supported beams of the same span length. For a uniformly distributed load, the maximum bending moment in a continuous beam over several equal spans is significantly less than that for a simply-supported beam (e.g., $\frac{wL^2}{8}$ for simply supported vs. approximately $\frac{wL^2}{12}$ or lower at intermediate spans for continuous beams, depending on the number of spans).
• Reduced Deflection: Continuous beams generally exhibit lower deflections.
• Efficient Material Use: Lower bending moments and reduced deflections allow for the use of lighter sections or sections with less material, thus achieving economy and reduced design forces.

Reduced Design Forces with Continuous Purlins

By designing purlins as continuous beams, engineers can leverage the structural benefits mentioned above. This allows the design forces (like bending moments and shear forces) acting on the purlins to be lower than if they were designed as simple spans. IS 800:2007 recommends this approach to take advantage of these reduced forces, leading to a more economical and efficient structural design for the roof system.

Designing purlins as simply-supported beams would result in higher bending moments ($\frac{wL^2}{8}$ for uniformly distributed load) and would not fully utilize the potential for economy offered by continuity, especially when purlins naturally span over multiple intermediate supports.

Cantilever beams are generally not suitable for the typical configuration of purlins spanning between main structural members.

Tension members are designed primarily to resist tensile forces, whereas purlins primarily resist bending moments due to transverse loads.

Therefore, the recommendation in IS 800:2007 for taking advantage of reduced design forces is to design purlins as continuous beams.