Structural Analysis and Design
Structural analysis and design questions test whether you can convert a building or bridge into a stable load path, select an analysis method that matches the structure's stiffness and ductility, and apply ECP 201 load factors without mixing service and ultimate values. The exam favours framed buildings, cantilevers, continuous beams and simple bridges where equilibrium, load placement and code combinations decide the answer.
Core rules and concepts
- Stability and determinacy: a plane structure needs enough reactions and internal continuity to prevent translation and rotation; check for mechanisms before calculating forces.
- Load cases: dead, live, wind, seismic, earth and water pressure are analysed separately and combined using ECP 201 strength combinations. The common gravity combination is 1.4D + 1.6L; lateral combinations add wind or seismic with the code's companion factors. Confirm the edition because ECP 201 has been revised.
- Pattern loading: for continuous beams and slabs, alternate span loading can govern sagging and hogging moments; a single full-span load is not always conservative.
- Effective span: use the code definition based on support width and depth, not the clear distance, when computing moments and deflections.
- Second-order effects: slender columns and unbraced frames require magnification for axial load and sway; ignoring them underestimates design moments.
Worked example
A simply supported beam spans 6.0 m and carries a dead load of 20 kN/m including self-weight and a live load of 15 kN/m. The ultimate load is wu = 1.4(20) + 1.6(15) = 28 + 24 = 52 kN/m. The maximum moment is Mu = wuL²/8 = 52 × 6²/8 = 234 kN·m. The corresponding ultimate shear at the support is Vu = wuL/2 = 156 kN. If the beam were continuous, the support moment and possible pattern loading would have to be checked separately; the simple-beam result is not transferable without reanalysis.
Traps that cost marks
- Designing with service loads and then applying a factor twice.
- Omitting self-weight or secondary dead loads such as finishes and partitions.
- Using the clear span instead of the effective span.
- Assuming full plastic redistribution without checking rotation capacity and section ductility.
- Ignoring lateral-torsional buckling when a steel beam's compression flange is unrestrained.
- Reporting a moment without units or with kN versus kN·m confusion.