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How do structures carry loads through struts and ties, and what makes a structure strong and stable?

Identify struts and ties and the forces in a structure, explain how triangulation and a low centre of gravity give strength and stability, and describe ways to reinforce structures

A focused answer to the O-Level Design and Technology outcome on structures. Struts in compression and ties in tension, triangulation, stability and centre of gravity, and reinforcement methods.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to identify struts and ties and the forces in a structure, explain how triangulation and a low centre of gravity give strength and stability, and describe ways to reinforce structures. This is the structures half of the technology strand. You should know which members are in compression and which in tension, why triangles are rigid, and what makes a structure hard to topple.

The answer

What a structure does

A structure supports a load and holds its shape against forces. The members of a frame structure carry the load by being pushed or pulled. Understanding which members are in compression and which in tension lets a designer make a structure strong without wasting material, and lets them choose materials suited to each member's force.

Struts and ties

The two key members are defined by the force they carry:

A strut is being squashed, so it is in compression: the forces push inward along its length. Struts must resist being crushed or buckling.
A tie is being stretched, so it is in tension: the forces pull outward along its length. Ties must resist being pulled apart.

In a loaded bracket, for example, a diagonal that pushes back against the load is a strut in compression, while a member being pulled taut is a tie in tension. Identifying struts and ties is a standard exam task.

Triangulation

A four-sided frame (a rectangle) is not rigid: its corners can change angle, so it can be pushed sideways into a parallelogram and collapse. A triangle, however, is rigid: its shape cannot change unless a side changes length. Triangulation means adding diagonal members to turn rectangles into triangles, making a frame hold its shape under load. This is why bridges, roof trusses, cranes and pylons are full of triangles. Adding one diagonal to a square frame transforms a floppy shape into a rigid one.

Stability and the centre of gravity

Stability is how hard a structure is to topple. It depends on the base width and the centre of gravity (the point where the weight acts). An object topples when its centre of gravity passes beyond the edge of its base. Two design moves increase stability:

Widen the base. The centre of gravity must move further before it passes the base edge, so the object can tilt more before toppling.
Lower the centre of gravity (put heavy parts at the bottom). A lower centre of gravity must be tilted further to pass outside the base, so the object is harder to tip and returns upright more readily.

A wide base and a low centre of gravity together make a structure stable; a tall, narrow, top-heavy shape is easily toppled.

Reinforcing structures

Beyond triangulation, structures are strengthened by: using a larger or hollow cross-section (a tube or I-beam resists bending well for its weight), adding webs or gussets at joints, folding or corrugating sheet material to stiffen it, and choosing materials with the right properties (strong in compression for struts, in tension for ties). The aim is maximum strength and rigidity for minimum material and weight.

Worked example

Analyse and improve a simple rectangular shelf bracket made of three members: a horizontal top, a vertical wall member, and the load hanging at the front of the top.

Step 1: See the problem with a rectangle

A bracket built as an L (two members at a right angle) tends to sag, because the joint at the corner can flex and there is nothing to stop the top bending down under the load. The rectangle (or right angle) is not rigid on its own.

Step 2: Add a diagonal to triangulate

Add a diagonal member from the bottom of the wall up to the front of the horizontal top, forming a triangle. The triangle cannot change shape without a side changing length, so the bracket becomes rigid and stops sagging.

Step 3: Identify the forces in the diagonal

Under the downward load, the diagonal is squashed between the top and the wall: it pushes back against the load, so it is a strut in compression. The horizontal top, pulled at the joint, acts partly as a tie in tension. Knowing this, the strut is made stout enough not to buckle.

Step 4: Reinforce the joints

Add small gusset plates at the corners to spread the load and stop the joints flexing. The bracket is now triangulated and reinforced: the diagonal strut carries the compression, the members are sized for their forces, and the joints are stiffened, giving a strong, rigid shelf bracket from little material.

Examples in context

Example 1. A roof truss. A roof truss is a frame of triangles spanning a building. The triangulated arrangement keeps the roof rigid under the weight of tiles and wind, with some members in compression (struts) and others in tension (ties). If the truss were made of rectangles it would rack and collapse; the triangles make it strong and light, which is why every traditional roof and bridge relies on triangulation.

Example 2. A racing car's low, wide stance. A racing car is built low and wide so its centre of gravity is close to the ground and its base (the track between the wheels) is broad. This makes it very stable in fast corners, because the centre of gravity stays well within the base even when cornering hard. A tall, narrow vehicle with a high centre of gravity would tip over in the same corner, showing the stability principle in action.

Try this

Cue. State the force in a strut and the force in a tie. Answer: a strut is in compression (squashed); a tie is in tension (stretched).
Cue. Explain why a triangle is used to make a frame rigid. Answer: a triangle cannot change shape unless a side changes length, so adding a diagonal to a rectangle (triangulation) stops the frame being pushed out of shape.
Cue. Give two ways to make a tall product less likely to topple. Answer: widen its base and lower its centre of gravity (put heavy parts at the bottom), so its centre of gravity must move further to pass beyond the base.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksA designer is building a frame structure for a shelf bracket. (a) Explain the difference between a strut and a tie, naming the force in each. (b) Explain how triangulation makes a frame structure more rigid.

Show worked answer →

(a) A strut is a member that is being squashed, so it is in compression (the forces push inward along it). A tie is a member that is being stretched, so it is in tension (the forces pull outward along it). In a shelf bracket, the diagonal that pushes back against the load is often a strut in compression, while a member being pulled is a tie in tension.

(b) Triangulation means adding a diagonal member to turn a four-sided (rectangular) frame into triangles. A rectangle can be pushed out of shape into a parallelogram because its corners can change angle, but a triangle is rigid: its shape cannot change without changing the length of a side. Adding a diagonal makes the frame hold its shape under load, so it is far more rigid.

What markers reward: strut as compression (squashed) versus tie as tension (stretched), and triangulation explained as adding a diagonal so the frame is made of rigid triangles that cannot be pushed out of shape like a rectangle can.

Original4 marksExplain two ways a designer can make a tall, narrow product more stable, and explain how each works.

Show worked answer →

First, widen the base. A wider base means the centre of gravity has to move further before it passes beyond the edge of the base, so the product can tilt more before it topples. A wider base therefore makes it harder to tip over.

Second, lower the centre of gravity, for example by putting heavy parts at the bottom. A lower centre of gravity means the product can be tilted further before its centre of gravity passes outside the base, so it is more stable and returns upright more readily. Together, a wide base and a low centre of gravity make a structure much harder to topple.

What markers reward: two valid methods (widen the base, lower the centre of gravity by weighting the bottom) each correctly explained in terms of the centre of gravity having to pass outside the base for the object to topple.