What civil engineers do: loads and the load path
Civil engineering plans, designs and builds structures and networks: buildings, bridges, roads, railways, tunnels, dams, water supply and drainage. It works with architecture, the environment and the land (territory).
- Dead load: the weight of the structure itself.
- Live (imposed) load: people, furniture, cars, snow.
- Wind and earthquake loads: mostly sideways (lateral).
The load path is the route a force takes to the ground. If any link is missing or too weak, the structure can crack or fail.
The frame: columns, beams, slabs and bracing
- Column: vertical member, mostly in compression (squeezed).
- Beam: horizontal member that bends: top squeezed, bottom stretched (tension). That is why concrete beams have steel bars at the bottom.
- Slab: flat plate forming floors and roofs; passes loads to beams.
- Bracing (diagonals), shear walls or cores resist sideways wind and earthquake loads. A triangle cannot change shape, a rectangle can.
Main materials: wood (light, renewable), steel (strong in tension and compression, fast to build), reinforced concrete (concrete for compression + steel for tension, fire-resistant), and masonry.
For a beam resting on two supports with an even load w (kN/m) over span L (m): maximum bending moment M = wL²/8, each support reaction R = wL/2.
Foundations and retaining walls
Shallow foundations
Used when firm soil is near the surface: pad (isolated) footings under columns, strip footings under walls, a raft (one big slab) on weaker soil. Pressure = load ÷ area, so a wider footing lowers the pressure on the soil.
Deep foundations
Piles are long columns driven or bored into the ground. They carry load by end bearing on a hard layer and by friction along their sides. Used for tall buildings, bridges and soft or wet soil.
Retaining walls
They hold back soil or water on a slope. Types: gravity (heavy mass), cantilever (reinforced concrete L-shape), gabion (stone in wire baskets), anchored walls. Drain holes (weep holes) let water out so pressure does not build up.
The building envelope
The envelope is the skin between inside and outside: walls, windows, roof and the ground floor. It must keep out rain and wind, hold heat in winter or keep it out in summer, block noise, resist fire and let in daylight.
- Load-bearing walls carry loads (brick, stone).
- Infill walls fill a frame.
- Curtain walls: light glass and aluminium panels hung on the frame; they carry only their own weight and wind.
- Envelopes in wood (timber frame panels), steel (cladding sheets with insulation) or concrete (precast panels).
Good insulation and shading cut energy use, which matters for climate and for bills.
Key formulas and definitions
- Load path: roof → slab → beam → column → foundation → soil
- Soil pressure p = load ÷ footing area
- Simply supported beam, even load: M = wL²/8
- Each support: R = wL/2 (total load = wL)
- Column: compression; beam: bending (top compression, bottom tension)
- Triangle (bracing) keeps its shape; rectangle can sway
Worked examples
1. A column carries 600 kN and sits on a 2 m × 2 m pad footing. What pressure does the soil feel?
Area = 4 m². p = 600 ÷ 4 = 150 kN/m² (150 kPa).
2. A 6 m beam on two supports carries 10 kN/m evenly. Find the support reactions and the maximum bending moment.
Total load = 10 × 6 = 60 kN; R = 30 kN at each end. M = 10 × 6² ÷ 8 = 45 kN·m at mid-span.
3. The soil can safely take 100 kN/m². A column carries 900 kN. What square footing size is needed?
Area = 900 ÷ 100 = 9 m², so a 3 m × 3 m footing.
Common mistakes
- Thinking walls always hold up a framed building. In a frame, columns and beams carry the load; walls may only fill gaps.
- Putting steel bars at the top of a simple beam. Tension is at the bottom, so main bars go at the bottom.
- Forgetting sideways loads. Wind and earthquakes need bracing, shear walls or a core.
- Thinking a curtain wall carries floors. It only carries its own weight and wind.