Structural Analysis and Design Methods
Choosing a method
Analysis converts loads into internal forces and deflections. Candidates must distinguish statically determinate systems (forces from equilibrium alone) from indeterminate systems (forces depend on relative stiffness). Methods include the portal and cantilever approximations, moment distribution, slope-deflection, and matrix stiffness analysis. Modern practice relies on computer stiffness models, so the exam tests whether you can sanity-check output.
Modeling assumptions and their consequences
Key concepts include tributary area, influence lines for moving loads, and the difference between first-order and second-order (P-Δ) analysis. Slenderness effects matter for slender columns and unbraced frames. Approximate methods are permitted for regular structures but require continuity, and the assumptions (inflection point locations, equal bay stiffness) must be stated.
Traps
- Applying tributary-area load to a continuous beam as if it were simply supported.
- Ignoring pattern loading: live load on alternate spans can produce worse moments than full loading.
- Forgetting that a hinge at a support changes a beam from fixed to simple, increasing midspan moment.
- Assuming a computer model's reactions are correct without checking global equilibrium.
Worked rule of thumb
For a simply supported beam, maximum moment is wL²/8; fixing both ends reduces it to wL²/12 at supports. A quick equilibrium check: total applied load must equal the sum of reactions. If a model reports reactions summing to 90 percent of applied load, suspect a missing support or a units error before trusting member forces.