5.1 Structural Engineering

Why Structural Engineering Dominates the FE Civil Exam

Structural Engineering is one of the two heaviest content areas, contributing roughly 9 to 14 questions. The National Council of Examiners for Engineering and Surveying (NCEES) keeps these problems short: identify the correct load combination, analyze a determinate member, or apply one design equation from the NCEES FE Reference Handbook.

Loads

Every structural problem starts with a load. The exam references ASCE/SEI 7 (American Society of Civil Engineers minimum design loads).

Dead load is computed from material unit weights (reinforced concrete ≈ 150 lb/ft³, structural steel ≈ 490 lb/ft³). Live load comes from ASCE 7 occupancy tables (office floors ≈ 50 lb/ft²). Wind and seismic on the FE are conceptual: know that wind pressure scales with the square of velocity and that seismic base shear V = C_s·W, where W is the seismic weight.

ASD versus LRFD

Two design philosophies appear on every structural-steel and concrete topic.

• Allowable Strength Design (ASD): compare the service-level (unfactored) demand to nominal strength reduced by a safety factor Ω. Requirement: R_a ≤ R_n / Ω.
• Load and Resistance Factor Design (LRFD): compare a factored demand to nominal strength multiplied by a resistance factor φ. Requirement: R_u ≤ φ·R_n.

Memorize the core LRFD load combinations (ASCE 7):

1. 1.4D
2. 1.2D + 1.6L + 0.5(Lr or S or R)
3. 1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W)

For an ordinary gravity floor beam, 1.2D + 1.6L controls.

Analysis of Determinate Beams and Frames

The FE expects fast recall of standard simple-beam results, all tabulated in the Reference Handbook "Beam" tables.

For frames, sum reactions with ΣF_x = 0, ΣF_y = 0, ΣM = 0. A frame is statically determinate when reactions plus internal releases equal the available equilibrium equations.

Reinforced Concrete and Steel Fundamentals

ACI 318 governs concrete. For a singly reinforced rectangular beam where tension steel yields:

• Equivalent stress block depth: a = A_s·f_y / (0.85·f'c·b)
• Nominal moment: M_n = A_s·f_y·(d − a/2)
• Design strength: φM_n with φ = 0.90 for tension-controlled flexure

The balanced reinforcement ratio ρ_b marks the steel amount at which concrete crushing and steel yielding occur simultaneously; ACI limits the practical ratio below ρ_b to keep beams ductile (tension-controlled).

AISC 360 governs steel. For a compact, laterally braced beam, the nominal flexural strength is the plastic moment M_p = F_y·Z, where Z is the plastic section modulus. Tension-member yielding capacity is φ_t·F_y·A_g with φ_t = 0.90 (LRFD).

Reference Handbook Tip

Load combinations sit under "Design Loads / ASCE 7"; beam formulas under "Mechanics of Materials — Beams"; the concrete stress block and φ factors under "Reinforced Concrete (ACI 318)." Practice locating these by section name, not memory.