This question relates to the structural design guidelines provided in the Indian Standard code IS 800-1984, specifically concerning the use of angle sections as purlins in roof trusses.

Understanding Purlins and Angle Sections

Purlins are horizontal structural members that are supported by the main roof trusses or rafters. Their primary function is to carry the roof covering (sheeting) and transmit the loads (like dead load, wind load, etc.) to the main structural supports. Angle sections, such as equal or unequal angles (e.g., ISA 90x90x8), are commonly used as purlins due to their availability, ease of connection, and cost-effectiveness, particularly for smaller spans.

IS 800-1984 Guidelines for Angle Purlins

The IS 800-1984 code provides specific recommendations for the design and use of structural steel members. When using single angle sections as purlins, their behavior under load is somewhat complex due to the asymmetry of the section and the inclination of the roof. The code specifies certain limitations to ensure the stability and performance of the purlin.

According to IS 800-1984, when angle sections are employed as purlins, the slope of the roof truss should generally not exceed a specific limit. This limitation is primarily to avoid issues related to the buckling stability of the compression flange (the part of the angle facing uphill) and to simplify the stress analysis. The recommended maximum slope for the roof truss, when using angle sections as purlins, is typically stipulated as being less than 30 degrees.

Analysis of Options

Let's examine the given options in light of the IS 800-1984 guideline:

• Option 1: Between 50° and 70° - This slope is significantly steeper than the recommended limit and is generally unsuitable for single angle purlins.
• Option 2: Less than 30° - This option aligns with the guideline specified in IS 800-1984 for the use of angle sections as purlins.
• Option 3: Greater than 30° - This contradicts the code's recommendation for limiting the slope.
• Option 4: Less than 45° - While less restrictive than Option 1, this limit is still greater than the specific recommendation provided by IS 800-1984 for angle purlins.

Conclusion

Based on the provisions of IS 800-1984, an angle section can be effectively and safely used as a purlin when the slope of the roof truss is maintained at a value less than 30°.