Structural Systems
Limit state and load path
Structure is examined as safe design, not member selection by habit. Design uses limit state philosophy: ultimate limit state against collapse with partial factors (commonly 1.4 on dead and 1.6 on imposed loads) and serviceability checks for deflection and cracking. Trace the load path from slab to beam to column to foundation. Imposed loads come from the code: about 1.5-2.0 kN/m² residential, 2.5-3.0 kN/m² offices, 4.0-5.0 kN/m² corridors and stairs. Concrete grade, cover (about 25 mm for slabs, 40 mm where concrete contacts soil) and reinforcement detailing decide durability.
Preliminary sizing
Rules of thumb for reinforced concrete: beam depth span/12 to span/15 for continuous beams, span/10 for cantilevers; slab thickness about span/30 continuous one-way, span/10 for cantilevers; column area from axial load over permissible stress. Choose foundations from soil bearing capacity: strip for competent laterite, raft for weak or variable ground, piles for soft clay or a high water table.
Worked example
A 150 mm slab: dead load 0.15 x 24 = 3.6 kN/m², finishes and services 1.4, total dead 5.0 kN/m². Imposed 2.0 kN/m². Ultimate load = 1.4 x 5.0 + 1.6 x 2.0 = 7.0 + 3.2 = 10.2 kN/m². A beam with 4 m tributary width carries 40.8 kN/m; over a 6 m span the bending moment is wl²/8 = 183.6 kNm, which sets the reinforcement.
Traps
Using 1.4 and 1.6 for serviceability checks; ignoring lateral stability and bracing; sizing a cantilever with a simply supported rule of thumb; raising concrete grade to compensate for inadequate cover.