Choosing a structural system is one of the earliest and most consequential decisions in building design. Steel, concrete, wood, and masonry each have distinct span capabilities, and those capabilities directly affect column spacing, floor-to-floor heights, building configuration, and construction cost.

Steel moment frames can clear-span large open areas. Wood framing works beautifully for residential and light commercial construction but hits practical limits as spans grow. Reinforced concrete offers a spectrum from flat plates for shorter spans to post-tensioned systems for longer ones. Masonry bearing walls carry gravity loads efficiently but impose constraints on open floor plans.

For PDD, you need to understand how each material system behaves structurally, what span ranges each system can achieve, and how those ranges influence the sizing of beams, columns, joists, and bearing walls. NCARB Objective 1.3 asks you to determine the size of structural systems to meet project goals. That means selecting the right system for a given program, calculating or estimating member sizes, and detailing the connections between horizontal and vertical components.

This topic covers the comparative strengths and limitations of all four material systems, span-to-depth ratios that guide preliminary sizing, deflection criteria that govern serviceability, and the seismic classification system that affects which systems you can even use in a given location.