What is a horizontal structural member used to support floors or roof decking?

The correct answer is a joist. Joists are specifically designed as horizontal structural members that support floors or roof decking. They are commonly used in residential and commercial construction to create a framework that holds up the flooring or roof, distributing weight and providing stability.

Joists are typically spaced evenly apart to provide a strong support system for the floor or roof above, ensuring that loads are transferred efficiently down to the walls or foundation of the structure.

While beams can also serve a supporting function in a structure, they are usually larger and can span longer distances than joists. In contrast, pillars and posts are vertical structural elements that provide support from below, rather than primarily serving as horizontal members. Understanding the role of each component in construction helps in comprehending the overall framework of buildings.