In which concrete system is the slab designed to span in both directions directly into columns?

In a flat plate concrete system, the slab is designed to span in both directions directly into the columns. This design allows for a more efficient use of materials, as the flat plate does not require beams to support the slab. The load from the slab transfers directly to the columns, distributing the weight evenly. This configuration is beneficial in terms of ceiling height and offers a sleek, unobtrusive appearance since no beams interrupt the space.

Utilizing a flat plate system can also simplify construction since fewer elements are involved, leading to potentially lower labor and materials costs. However, this system is typically used for lighter loads and spans, as it may require careful consideration of potential punching shear issues around the columns. The architectural flexibility it provides, along with fewer structural elements, makes it a popular choice in certain building types.

The other options, such as flat slab, open-web steel joist systems, and trussed wood joists, define different structural systems that involve beams or trusses to support loads rather than allowing slabs to span directly into columns like in a flat plate system. Each of those systems has its own advantages and design considerations but does not match the specific characteristic of being a straightforward span from the slab to the columns.