How do forces act on a structure, and how is a structure designed to resist them?
Types of force (tension, compression, shear, bending, torsion) and stress in structures, and techniques for reinforcing and strengthening structures (triangulation, webbing, beams, folding and laminating).
A focused answer to WJEC A-Level Design and Technology Unit 3 structures, forces and stresses, covering the types of force (tension, compression, shear, bending, torsion), how stress acts on a structure, and techniques for strengthening and reinforcing structures such as triangulation, beams, webbing, folding and laminating.
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What this dot point is asking
WJEC wants you to name and describe the forces that act on a structure and to explain how a structure is reinforced to resist them efficiently. The exam reliably asks you to identify forces on a named structure (a bridge, a shelf, a chair) and to explain strengthening techniques such as triangulation and efficient cross-sections. You need the forces, the idea of stress, and several reinforcing techniques with reasons.
The answer
Types of force
Real structures carry several of these at once: a loaded shelf bends, its bracket is in tension and compression, and its fixings are in shear.
Stress
Strengthening and stiffening structures
The skill examined is getting strength and stiffness for little added weight:
- Triangulation - add a diagonal member so a frame is made of triangles. A triangle is rigid (its shape cannot change without changing a side length), so triangulated frames resist racking. Used in roof trusses, bridges, pylons and brackets.
- Efficient cross-sections (beams) - an I-section or box section places material far from the neutral axis where bending stress is greatest, giving high stiffness for little material. Used in steel I-beams and box-section bike frames.
- Folding, corrugating and webbing - folds, corrugations or an added web stiffen a thin sheet hugely against bending, as in corrugated card, roofing sheet and folded panels.
- Lamination - gluing thin layers (often around a former) makes a strong, stable member that holds a curve, as in plywood and laminated beams.
Examples in context
Example 1. An electricity pylon. It is built almost entirely from triangulated steel members, so the wind and cable loads are carried as tension and compression in straight members, giving a very strong, very light lattice - far lighter than a solid tower of the same strength.
Example 2. Corrugated cardboard packaging. A flat sheet of card bends easily, but adding a corrugated fluting between two liners makes it stiff against bending while staying light and cheap, which is why almost all shipping boxes are corrugated.
Try this
Q1. Name the force that tends to twist a structural member and give one product where it occurs. [2 marks]
- Cue. Torsion; a drive shaft, an axle, a screwdriver shaft or a door handle spindle.
Q2. Explain why an I-section beam is stiffer in bending than a solid rectangular bar of the same mass. [3 marks]
- Cue. The I-section places most of the material in the top and bottom flanges, far from the neutral axis where bending stress is greatest, so it resists bending more for the same amount of material.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20194 marksName and describe two different types of force that act on the structure of a footbridge.Show worked answer →
A footbridge experiences several forces, and a good answer names two and describes how each acts.
Compression is a squashing force that tends to shorten a member. The vertical supports and the deck under the weight of people are in compression, pushed together by the load they carry.
Tension is a stretching force that tends to lengthen a member. In a suspension or cable-stayed footbridge the cables are in tension, pulled tight as they hold up the deck. Bending also acts on the deck as the load tries to make it sag, and shear acts where the deck meets the supports.
Markers reward two correctly named forces with a clear description of the direction of each (squashing or shortening for compression, stretching or lengthening for tension) and, ideally, where on the bridge each acts.
WJEC 20216 marksExplain, with examples, three techniques a designer can use to strengthen or stiffen a structure without greatly increasing its weight.Show worked answer →
A strong answer gives three distinct techniques, each with how it works and an example.
Triangulation: adding a diagonal member turns an unstable rectangular frame into rigid triangles, because a triangle cannot be deformed without changing the length of a side. This is used in roof trusses, bridges, pylons and shelf brackets.
Using an efficient cross-section (a beam): an I-section or box section places material away from the neutral axis where bending stress is greatest, giving high stiffness for little material, as in steel I-beams and box-section bicycle frames.
Folding, corrugating or webbing: putting folds or corrugations into a thin sheet, or adding a web, greatly increases stiffness against bending, as in corrugated cardboard, roofing sheet and folded metal panels. Lamination (gluing thin layers) is a further valid technique.
Markers reward three genuinely different techniques, a correct explanation of why each works, and a relevant example for each.
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Sources & how we know this
- WJEC AS/A Level Design and Technology specification — WJEC (2017)