How do structures carry loads through ties and struts, and how are members made stable?
Structural members in tension and compression (ties and struts), triangulation, beams and the forces in simple frameworks.
A CCEA A-Level Technology and Design answer on how structures carry loads, the difference between ties (tension) and struts (compression), triangulation for stability, beams and bending, and resolving the forces in a simple framework.
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What this dot point is asking
CCEA expects you to explain how structures carry loads, to distinguish a tie (tension) from a strut (compression), to use triangulation for stability, to describe beams and bending, and to identify the forces in a simple framework. The tie/strut and triangulation ideas are core.
The answer
Ties and struts
Triangulation and stability
Beams and bending
Worked example: identifying members in a bracket
Examples in context
Example 1. Roof truss and bridge. Trusses and lattice bridges are made of triangles, with each member acting as a tie or strut, the textbook use of triangulation to carry large loads with little material.
Example 2. Steel floor joists. Floors use I-section steel joists because the I-shape puts material in the high-stress flanges, giving great bending strength for little weight, the beam-section argument in construction.
Try this
Q1. State whether a member in tension is a tie or a strut. [1 mark]
- Cue. A tie (tension is a pulling force).
Q2. Why is a triangular framework stable while a square one is not? [2 marks]
- Cue. A triangle cannot change shape without changing a side length, so it is rigid; a square can distort into a parallelogram under a sideways load unless braced.
Q3. For a simply supported beam with a central downward load, which surface is in tension? [1 mark]
- Cue. The bottom surface.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20196 marksExplain the difference between a tie and a strut, describe how triangulation makes a framework stable, and state why a square frame is unstable.Show worked answer →
A tie is a structural member in tension - it is being pulled and stretched, so it carries a pulling force (a cable or a thin rod can act as a tie). A strut is a member in compression - it is being squashed/pushed, so it carries a pushing force and must resist buckling (a column or a thick member acts as a strut).
Triangulation is the use of triangles in a framework. A triangle is the only polygon that cannot change shape without changing the length of a side, so a triangulated frame is rigid and stable: any load is shared as tension and compression in the members, and the joints cannot rotate the shape out of true.
A square (or rectangular) frame is unstable because it can distort into a parallelogram (rack/lozenge) under a sideways load - the joints can rotate and the shape collapses sideways, since the sides do not have to change length to do so. Adding a diagonal brace turns each square into two triangles, making it stable (the diagonal acts as a tie or strut).
Markers reward tie = tension/pulled, strut = compression/pushed/buckles, the triangle-cannot-change-shape stability argument, and the square distorting into a parallelogram unless braced.
CCEA 20214 marksA simply supported beam carries a central point load. Sketch where it is in tension and compression, and state two ways to make a beam stronger in bending without using more material.Show worked answer →
For a simply supported beam with a central downward load, the beam bends downward (sags): the top surface is in compression (the fibres are squashed) and the bottom surface is in tension (the fibres are stretched), with a neutral axis in the middle carrying neither.
Two ways to make it stronger in bending without more material (by changing the shape/section so material is placed where stress is highest):
- Increase the depth of the beam (make it taller) - bending strength rises sharply with depth, so a deep, narrow section resists bending far better than a shallow wide one of the same area.
- Use an efficient section such as an I-beam or a hollow box section, which puts most material in the top and bottom flanges (the high-stress regions) and little in the middle (low stress), or fold/corrugate a sheet to add depth.
Markers want top-compression/bottom-tension for a sagging beam, and two valid shape changes (greater depth, I/box section, folding/corrugating) that improve bending strength for the same material.
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Sources & how we know this
- CCEA GCE Technology and Design specification — CCEA (2016)