What span length is considered most economical for a box beam structure?

Answer: B — 40 to 90 feet

The span length that is considered most economical for a box beam structure typically falls within the range of 40 to 90 feet. This span length is optimal because, at these dimensions, the structural efficiency of the box beam can be maximized while keeping material costs and overall construction expenses lower. In this range, the strength-to-weight ratio of the materials used is such that they can efficiently support the loads encountered in typical bridge applications without necessitating excessive reinforcement. Additionally, spans longer or shorter than this range could lead to increased fabrication and support costs or may encounter design limitations that could affect performance and safety. Thus, focusing on spans between 40 and 90 feet allows for a balance of cost-effectiveness, structural integrity, and practical construction methods, making it the best choice for economical design in this context.