What is a timber structure?
A timber structure carries loads with wood. The most common types are post-and-beam (vertical posts and horizontal beams form a frame, and walls fill the gaps), light-frame (many small studs close together with sheets over them) and log (stacked logs form the walls).
A post-and-beam frame has these parts: a sill (the beam at the bottom that rests on the base), posts (vertical, mainly squeezed), beams (horizontal, bent by the load), rafters (sloping pieces under the roof) and braces (diagonals). The load path is the usual one: roof, rafters, beams, posts, base, ground.
Why timber behaves differently: grain and moisture
Wood is made of long fibres. The grain is the direction of the fibres. Wood is much stronger when loaded along the grain than across the grain, so posts and beams are cut with the grain along their length.
Wood takes up and gives out water. Moisture content = (wet mass − dry mass) ÷ dry mass × 100%. Timber used in buildings is dried first, to about 12 to 18%. Wet wood shrinks as it dries and can twist or crack. Wood that stays above about 20% moisture can rot and attract termites and fungus.
Joints
Pieces must be joined so loads pass from one to the next. In a mortise-and-tenon joint the end of one piece (tenon) fits into a hole in the other (mortise), and a wooden peg locks it. A lap joint overlaps two pieces. Dowels are round wooden pins. Modern builders also use nails, screws, bolts and metal plates.
Joints are often the weakest part of a timber structure, so they must be cut neatly and fit snugly. Cutting too much wood out of a post at the joint can weaken it.
Bracing against wind and earthquakes
A frame of posts and beams with pin-like joints can lean sideways like a pushed box. A diagonal brace turns the box into two triangles, and a triangle cannot change shape. Braces can be pushed or pulled in different directions of wind, so they are often used in pairs, one on each diagonal. Wooden walls covered with boards or panels also stiffen the frame.
Timber is light, so an earthquake produces smaller forces on a timber building than on a heavy concrete one. Good joints and bracing let it move without breaking.
Beams: why depth matters
When a beam carries a load it bends. How much it bends (its sag) depends on the span, the load, the wood and the shape of the cross-section. For a rectangle of width b and depth d, the stiffness is proportional to b × d³.
So a plank on edge (deep) is far stiffer than the same plank flat. Doubling the depth makes the beam 8 times stiffer, so the sag becomes one eighth. Doubling the width only doubles the stiffness. That is why floor joists are tall and thin.
Durability, fire and sustainability
Keep wood dry: put the sill on a stone or concrete base above the ground, add a roof overhang, let air flow and use treated wood where needed. Protect against termites with treatment and clean details.
Wood burns, but a thick timber beam chars on the outside and the inside keeps its strength for a time, and thin pieces burn faster. Fire stops, fire-resistant boards and safe distances help.
Wood stores carbon taken from the air by the tree. Timber from well-managed forests that are replanted is a renewable material with low energy use compared with steel or cement.
Try it: brace, base, depth
In the 3D, use the last step. Raise the wind to 7 with no brace and read the lean. Tick Brace and see it drop. Tick Stone base and see the sill stay dry. Set beam depth to 1, then 2, then 3, and see how fast the sag number falls.
At home: lay a ruler flat between two books and press the middle. Then stand the same ruler on its edge and press. Which bends less?
Key formulas and definitions
- Moisture content % = (wet mass − dry mass) ÷ dry mass × 100
- Beam stiffness ∝ width × depth³
- Doubling depth → stiffness × 8, sag ÷ 8
- Triangle (brace) = rigid frame
Worked examples
1. A piece of wood has wet mass 600 g and dry mass 500 g. Find the moisture content.
Moisture = (600 − 500) ÷ 500 × 100 = 20%. This is the upper safe limit for building timber.
2. A beam is 100 mm wide and 200 mm deep. Compare its stiffness with the same beam laid flat (200 mm wide, 100 mm deep). Use b × d³.
On edge: 100 × 200³ = 100 × 8,000,000 = 800,000,000. Flat: 200 × 100³ = 200 × 1,000,000 = 200,000,000. Ratio = 4, so on edge is 4 times stiffer.
3. A beam sags 40 mm. Its depth is doubled with the same width and load. Find the new sag.
Stiffness becomes 2³ = 8 times, so sag = 40 ÷ 8 = 5 mm.
4. Why are floor joists made tall and thin?
Stiffness grows with depth cubed but only with width once, so a tall joist gives the most stiffness for the least wood.
5. A frame leans 12 mm in a wind. A diagonal brace makes it about 8 times stiffer sideways. About how much does it lean now?
12 ÷ 8 = 1.5 mm.
6. Give two ways to stop the bottom beam (sill) of a timber house from rotting.
Raise it on a stone or concrete base so it does not touch wet ground, and keep rain off with a roof overhang and good air flow. Treated wood also helps.
Common mistakes
- Making a beam wider to stop sag. Depth matters much more than width.
- Laying the plank flat because it looks stable. A flat plank sags a lot.
- Placing the sill on the bare ground or in a wet spot. Wet wood rots.
- Leaving the frame without braces because the posts and beams look strong. Wind makes the box lean.