Structural engineers do the final calculations, but architects must understand how beams and girders get sized. Your design decisions about spans, floor loads, and clearances directly determine member sizes, which in turn affect floor-to-floor heights, plenum space, and coordination with mechanical systems.

Three forces govern beam sizing: bending moment (the tendency of the beam to flex under load), shear (the tendency for one section to slide past an adjacent section), and deflection (how much the beam physically moves under load). For most architectural spans and loading conditions, deflection controls the design rather than strength.

Deflection limits are codified in IBC Table 1604.3 and expressed as fractions of span length. A floor beam supporting plaster ceilings, for example, is limited to L/360 for live load deflection. That means a 30-foot beam can deflect no more than 1 inch under live load. Stricter limits like L/480 or L/600 apply when brittle finishes are involved.

Span-to-depth rules of thumb let you estimate member depth before any calculations happen. A steel wide-flange beam spanning 30 feet in a typical office building needs a depth of roughly 30/20 = 1.5 feet, or about 18 inches. These ratios differ by material and loading. Knowing them keeps your designs realistic from the start and prevents expensive surprises when the structural engineer runs final numbers.