Structural Engineering

Load and Resistance Factor Design (LRFD)

Strength design in which factored loads must not exceed the nominal strength multiplied by a resistance factor.

LRFD requires φRₙ ≥ Rᵤ: the nominal strength Rₙ, reduced by a resistance factor φ below 1.0, must equal or exceed the required strength Rᵤ found from loads multiplied by load factors above 1.0. Each factor reflects the uncertainty in what it multiplies.

The basic gravity combinations are 1.4D and 1.2D + 1.6L; others add wind, snow and earthquake. Steel design under AISC and concrete design under ACI both use this format, with φ = 0.90 for flexure in both and lower values for brittle or less predictable failures such as concrete shear.

How much of the FE Civil exam is Structural Engineering?