What are rigid horizontal planes that transfer lateral forces to vertical resisting elements called?

The term for rigid horizontal planes that transfer lateral forces to vertical resisting elements is "diaphragms." In structural engineering, diaphragms are critical components that act to distribute lateral loads, such as wind or earthquake forces, from the roof or floor system to the vertical elements of a building, such as walls or columns.

Diaphragms work by allowing the roof or floor structure to behave as a unified surface that can resist and transfer forces effectively. This is essential for maintaining structural stability and ensuring that loads are appropriately handled and managed within a building. Their rigidity is a key characteristic, which enables them to perform this function.

In comparison, beams primarily serve to support loads over spans and transfer forces vertically. Slabs are horizontal structural elements that provide surfaces for floors or roofs but do not usually engage directly in lateral load transfers in the same way diaphragms do. Columns primarily carry vertical loads and support other structural elements but do not specifically address the lateral forces transferred from horizontal planes. Therefore, the unique role of diaphragms in transferring lateral forces distinguishes them as the correct answer to the question.