Which design concept is prevailing worldwide for bridge design?

• A. Load Factor Design
• B. Allowable Stress Design
• C. Load and Resistance Factor Design
• D. Level I Earthquake Motions

Answer: (C)

Modern bridge design relies on a reliability-based, factorized approach that applies separate factors to loads and to resistances. This is the Load and Resistance Factor Design method. By using load factors to inflate the actual stresses that loads can produce and resistance factors to reduce the nominal strengths of materials and members, engineers aim for a consistent level of safety across different bridge elements and loading scenarios. The factors are calibrated from statistical data on material strength and load variability, so the resulting design has a uniform target reliability rather than being over- or under-conservative in different situations. This approach has become standard worldwide and is embedded in major design codes, enabling safer, more economical bridges across diverse environments. In contrast, older methods like Allowable Stress Design use a single allowable stress without the probabilistic weighting, and Level I Earthquake Motions refer to seismic input categories rather than the overarching design philosophy.