Structural Design and Analysis: Concrete, Steel, Masonry, Foundations
Limit-state thinking
Design uses the limit-state method: ultimate limit states (strength, stability, fatigue) and serviceability limit states (deflection, crack width, vibration). Loads are separated into characteristic values and design values via partial factors; resistances are similarly reduced. A typical fundamental combination is 1.3·G + 1.5·Q, while seismic combinations take the form G + ψ·Q ± E. Confusing characteristic and design values is the most common analytical error.
Materials
Reinforced concrete: concrete classes by cube strength, reinforcement classes (A500C being the common ductile ribbed bar), minimum cover by exposure class, anchorage and lap lengths, shear and punching checks, deflection limits. Steel: cross-section classification, buckling and lateral-torsional restraint, slenderness limits for members, weld and bolt design. Masonry: unit and mortar strength determine design compressive strength; slenderness, eccentricity and seismic ties govern. Foundations: bearing capacity and settlement checks, frost-heave depth for shallow footings, and under seismic action tie beams, rafts or piles to resist overturning and differential movement.
Modelling discipline
The analysis model must match the assumed load path. Second-order effects must be included where sway is significant; diaphragms must be modelled as rigid or flexible consistently with the floor construction; seismic analysis needs accidental eccentricity and both directions considered, with the vertical component in special cases.
Traps
- Using service loads in an ultimate limit state check.
- Mixing units: kN versus kgf, MPa versus kg/cm², a legacy of transition between old SNiP tables and current practice.
- Ignoring second-order effects in slender columns or sway frames.
- Designing foundations without checking settlement as well as bearing capacity.
Rule of thumb
Draw the load path first, member by member, from roof to soil; every load must have a continuous, checked route to the ground before any number is calculated.