Rafters: Beams Supported by Purlins

The question asks to identify the specific name for beams that rest on purlins within a structural context, typically in roof construction.

Let's break down the terminology involved:

• Purlins: These are horizontal structural members used in roof framing. They are positioned perpendicular to the primary rafters or trusses and serve to support the roof decking or sheeting. Purlins distribute the roof load to the main supporting elements.
• Beams resting on purlins: In standard roof framing, the sloping structural members that extend from the ridge of the roof down to the eaves, and which are directly supported by purlins, are called rafters.

Identifying the Correct Structural Member

The options provided are different types of structural components. Understanding their functions helps identify the correct answer:

• Rafters: These are the inclined beams forming the primary slope of a roof. They are commonly supported by purlins, which are themselves supported by trusses or walls. Therefore, rafters are the beams that rest on purlins.
• Spandrel Beam: This term usually refers to a beam located between columns, often supporting a spandrel wall or parapet, typically at floor levels or below windows. It is not directly related to the beams resting on purlins in roof framing.
• Trimmers: Trimmer beams frame around openings in floors or roofs, such as stairwells or chimneys. They support the ends of joists or rafters that are cut to accommodate the opening.
• Stringers: In stair construction, stringers are the side rails that support the treads. In other contexts, like bridges, they can be longitudinal members. While sometimes used loosely in framing, 'rafters' is the precise term for roof beams resting on purlins.

Understanding the Roof Framing System

The connection between rafters and purlins is fundamental in many roof designs. Purlins act as intermediate supports for the rafters, reducing the span and allowing for lighter construction. The rafters directly carry the roof load down to the purlins.

Therefore, the beams resting on purlins are known as rafters.