What makes a structure strong and stable, and how can a designer stop it from collapsing or tipping?
Describe how structures resist forces and stay stable, using techniques such as triangulation and a low, wide base
A clear answer to the N(A)-Level D&T outcome on structures. Forces of tension and compression, making structures strong through triangulation and material shape, and keeping them stable with a low centre of gravity and a wide base.
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What this dot point is asking
SEAB wants you to explain how structures resist forces and stay stable, and to name techniques that make them stronger and harder to tip, such as triangulation and a low, wide base. The topic splits into two ideas: strength (not breaking or collapsing) and stability (not tipping over).
The answer
Forces in a structure
A structure carries loads, and the members inside it feel forces:
- Tension is a pulling force that tries to stretch a member.
- Compression is a squashing force that tries to shorten a member.
A good structure carries these forces without breaking or bending too much.
Making a structure strong
Several techniques make a structure stronger without just adding more material:
- Triangulation. Adding a diagonal to a square frame makes triangles. A rectangle can be pushed sideways into a leaning shape (it racks), but a triangle cannot change shape unless a side changes length, so it stays rigid. This is why bridges and roof trusses are full of triangles.
- Shaping the material. A folded or tubular shape is stiffer than a flat sheet of the same material; folding card into an L or tube resists bending far better.
- Reinforcing joints. Gussets or brackets at corners stop a frame from twisting.
Making a structure stable
Stability is about not tipping over. Two ideas matter:
- A low centre of gravity. The centre of gravity is the point where the weight seems to act. The lower it is, the more stable the object, because the weight stays over the base.
- A wide base. A wide base lets the object lean further before its weight falls outside the base and it tips. Tall, narrow things tip easily; low, wide things are stable.
A structure tips when its centre of gravity passes outside the edge of its base, so keeping the weight low and the base wide keeps it upright.
Examples in context
Example 1. A desk lamp base. The lamp has a heavy, wide base so that when the head extends to one side, the centre of gravity stays over the base and the lamp does not tip, combining a low centre of gravity with a wide base.
Example 2. A model bridge. Made from card strips, a plain rectangular truss sags and leans, but adding diagonal members to triangulate each section makes it rigid, so it carries far more load before collapsing.
Try this
Q1. Name the two types of force members feel in a structure. [2 marks]
- Cue. Tension (pulling) and compression (squashing).
Q2. State two ways to make a structure more stable. [2 marks]
- Cue. Lower the centre of gravity and widen the base.
Q3. Explain why triangulation makes a frame stronger. [3 marks]
- Cue. It adds a diagonal to form triangles; a triangle cannot be pushed out of shape unless a side changes length, so the frame resists racking and stays rigid.
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 student builds a tall, narrow shelving unit that wobbles and tips easily. Explain two changes that would make it more stable, and explain why each works.Show worked answer →
Change 1: make the base wider. A wider base means the unit can lean further before its weight falls outside the base, so it is harder to tip.
Change 2: lower the centre of gravity, for example by putting heavier items on the lower shelves. A lower centre of gravity makes the unit more stable because its weight acts lower down and stays over the base.
A third valid change: fix it to the wall, or add triangulation (a diagonal brace at the back) to stop it racking and leaning.
What markers reward: two stability improvements (wider base, lower centre of gravity, fixing to a wall, triangulation) each with a correct reason linked to keeping the weight over the base or stopping the frame from leaning.
Original4 marksExplain what triangulation is and why it makes a frame structure stronger.Show worked answer →
Triangulation is adding a diagonal member to a square or rectangular frame so it forms triangles. A rectangle can be pushed sideways into a leaning shape (it racks), but a triangle cannot change shape without changing the length of its sides. So the diagonal stops the frame from collapsing sideways and makes it rigid and stronger.
What markers reward: triangulation as adding a diagonal to make triangles, the point that a triangle is a rigid shape that cannot be pushed out of shape, and the result that the frame resists racking and is stronger.
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