The two primary design methods used in modern reinforced concrete design are:
Strength Design Method (or Ultimate Strength Design - USD): This is the most common method used today. It focuses on ensuring the structure can withstand the ultimate loads it is expected to encounter during its lifetime. Loads are multiplied by load factors, and the material strengths are reduced by strength reduction factors to provide a margin of safety.
Allowable Stress Design (ASD) / Working Stress Method (WSM): This older method designs structural members so that stresses in the concrete and steel under normal service loads remain below pre-defined allowable limits. While less common for the primary design of new structures today, it is still used for checking serviceability requirements like deflections and cracking.