Which design feature is necessary for spread box beams to function properly?

For spread box beams to function properly, having a deck surface for traffic is crucial. The deck surface serves as the roadway for vehicles and provides the necessary support and structural integrity for the entire bridge system. It distributes loads across the beams, ensuring that the stresses from vehicular traffic are adequately managed. Without a proper deck surface, the beams would not be able to perform their function of bearing loads effectively, leading to potential structural failures or excessive deflections that could compromise safety.

Additionally, while features like shear keys, composite integration, and diaphragm supports may contribute to other aspects of the bridge design and stability, they do not fulfill the fundamental requirement of accommodating traffic loads as the deck surface does. The deck is essential for operational functionality, allowing the bridge to serve its primary purpose: facilitating safe and efficient transportation.