Structural: Loads, Analysis, and Member Design
Structural questions test statics, load combinations, and member behavior. Core concepts: equilibrium (ΣF = 0, ΣM = 0), shear and moment diagrams, beam deflection relationships, ASCE 7 load combinations (e.g., 1.2D + 1.6L), LRFD vs. ASD, and basic member checks: flexure (φMn ≥ Mu), shear (φVn ≥ Vu), and axial (φPn ≥ Pu). Know the difference between allowable stress and strength design, and the role of slenderness (KL/r) in compression members.
Traps
- Reporting maximum moment where shear is zero without checking support conditions - for a simply supported beam with a uniform load, Mmax = wL²/8 at midspan.
- Forgetting self-weight of the member in the load combination.
- Confusing service loads (used in deflection checks) with factored loads (used in strength checks).
- Using the wrong effective length factor K for a fixed vs. pinned column.
Worked example
A simply supported beam spans 20 ft with a uniform factored load of 2.0 klf. Mmax = wL²/8 = 2.0 × 20² / 8 = 100 kip-ft. If φMn = 120 kip-ft, the beam is adequate (120 > 100). Rule of thumb: memorize Mmax = wL²/8 (uniform, simple span) and PL/4 (point load at midspan) - they appear repeatedly, and deflection checks use service loads, not factored ones.