Which of the following describes a series of small parallel beams laid on edge to support floor and ceiling loads?

The correct choice identifies joists as the structural elements used to support floor and ceiling loads. Joists are engineered to span distances and provide the necessary support for various loads imposed on floors and ceilings, distributing weight evenly across their length. They are typically made of wood or engineered materials, and their parallel alignment assists in maintaining structural integrity while enabling the construction of open spaces.

Rafters, while also important in building structures, primarily support the roof and are typically angled rather than laid flat like joists. Trusses are more complex structural frameworks used for larger spans that incorporate multiple members in a triangular formation, serving a different purpose than joists. Purlins are horizontal members that provide support for roof sheathing and are placed perpendicular to rafters, which further differentiates them from the role of joists in floor and ceiling applications. Understanding these distinctions highlights why joists are specifically characterized by their function to support horizontal loads in floors and ceilings.