What a building structure does
The structure is the skeleton of a building: the parts that hold it up and stop it falling or shaking too much. Its job is to carry every load safely to the ground.
Main loads: dead load (the weight of the building itself: slabs, walls, beams), live load (people, furniture, vehicles that come and go), wind load (sideways push), earthquake load (shaking of the ground) and, in cold places, snow load. Loads are measured in newtons (N) or kilonewtons (kN).
A good structure is strong (does not break), stiff (does not bend or sway too much) and stable (does not tip).
The load path
Every load must have a clear route to the ground, called the load path. For a normal building: roof or floor slab → beams → columns (or walls) → footings → ground. If the path is broken, for example a column that stops halfway, the load finds a weaker way and may cause damage.
Each part takes the load from above and adds its own weight. So lower parts carry more, which is why ground-floor columns are thicker than top-floor ones.
Structural systems: wall and frame
In a wall (load-bearing) structure the walls carry the floors and roof, as in many brick or stone houses. Walls cannot easily be removed and openings must be small.
In a frame structure columns and beams form a skeleton and the walls only fill the gaps. Walls can be moved or large openings made. Most apartment blocks and offices use frames of reinforced concrete or steel, and traditional houses may use frames of timber.
A frame has joints between columns and beams. Joints that do not rotate are rigid, which helps the frame resist sideways loads. Other forms such as arches, domes and shells carry loads through curved shapes.
Wind, earthquakes and bracing
Wind and earthquakes push sideways. A plain frame of columns and beams can sway like a box being pushed, even when the loads from above are fine. To stop this we add bracing (diagonal bars that make triangles), shear walls (strong walls in the frame) or rigid joints.
A triangle cannot change shape without changing the length of its sides, which is why diagonals work. The taller the building, the more important sideways loads become.
Foundations
The foundation passes the building load into the ground. The ground can carry only a certain pressure, so the foundation must be wide enough.
Pressure = load ÷ area. A wider footing has more area, so the pressure on the soil is less. On soft ground use wide footings or a raft under the whole building. If the soil is very weak, use piles: long posts that go down to hard layers. Water in the ground, loose soil and trees nearby also affect foundations.
Try it: lean and sink
In the 3D, go to the last step. Pick "Frame" with wind 8: it leans a lot. Press "Frame + brace" and the lean drops. Now keep the footing at 1 on soft ground and see the sinking number; slide it to 3 and watch it fall.
At home: build a square frame from four ice-cream sticks and pins. Push the top sideways: it folds. Add one diagonal stick and push again.
Key formulas and definitions
- Total load = dead load + live load (+ wind, earthquake, snow)
- Pressure on soil = load ÷ footing area
- Load path: slab → beam → column → footing → ground
- Triangle = rigid shape (bracing)
Worked examples
1. A floor has dead load 50 kN and live load 20 kN. Find the total vertical load.
Total = 50 + 20 = 70 kN.
2. A column carries 120 kN onto a square footing of 1 m by 1 m. Find the soil pressure.
Area = 1 × 1 = 1 m². Pressure = 120 ÷ 1 = 120 kN/m² (120 kPa).
3. The footing is made 2 m by 2 m, same load of 120 kN. Find the pressure now.
Area = 4 m². Pressure = 120 ÷ 4 = 30 kN/m². Double the width gives one fourth of the pressure.
4. Name the load path in a frame building, from the roof to the ground.
Roof slab → beam → column → footing → ground.
5. Why is a plain four-bar frame unstable in wind, and what fixes it?
Its joints can turn, so the shape folds sideways like a box. A diagonal brace forms two triangles, which cannot fold, and so the frame stays upright.
6. Soil can safely take 60 kN/m². A column load of 180 kN needs a footing of at least how much area?
Area = load ÷ safe pressure = 180 ÷ 60 = 3 m². For a square footing, side = √3 ≈ 1.75 m.
Common mistakes
- Thinking walls always carry the load. In a frame building the walls only fill the gaps.
- Looking only at downward loads and forgetting wind and earthquake pushes.
- Believing a bigger footing makes the building heavier on the ground. It spreads the same load over more area, so the pressure falls.
- Making the ground-floor columns thin because they look the same as upper ones. They carry the weight of every floor above.