Horizontal members that carry the heaviest load of attached horizontal members are known as what?

The correct answer is girders, which are crucial structural elements in building construction. Girders function as primary horizontal supports that carry the significant loads from other horizontal members, such as joists or beams, and transfer these loads down to the vertical supports like columns or walls.

In construction, girders are typically larger and stronger than the other horizontal members mentioned. They are designed to handle heavy weight, making them essential in maintaining the overall stability and integrity of a building. This is why they are chosen to bear the heaviest loads, ensuring that all the structural elements function effectively together.

Joists, while important, serve a different purpose. They are typically smaller and used for spanning shorter distances to support floors or ceilings; joists are supported by a girder or beam. Similarly, rafters are inclined members used primarily in roofing structures, and beams, while they do carry loads, typically are secondary to girders in a structural system where the heaviest loads are concerned. Therefore, girders stand out as vital components for load-bearing in residential construction.