📚 Year 8 Edexcel Engineering: Bridge Design Case Study Practical Walkthrough | 八年级爱德思工程：桥梁设计案例实战演练
Engineering is about solving real-world problems through a structured design process. A case study gives you the chance to apply all the stages—from identifying a need to testing a prototype—just like a professional engineer. In this walkthrough, we will follow a bridge-building challenge that is typical for Year 8 Edexcel Engineering students, exploring how to turn a client brief into a successful working model.
工程学就是通过结构化的设计过程解决现实世界的问题。案例研究让你有机会应用从需求识别到原型测试的所有阶段，就像一位专业工程师那样。在这次实战演练中，我们将跟随一个八年级爱德思工程课程中常见的桥梁建造挑战，探究如何将客户需求转化为成功的可工作模型。
1. Introduction: The Client Brief | 案例简介：客户要求
A local council needs a footbridge to cross a small stream. The bridge must span a gap of 300 mm, support a central load of at least 500 g without collapsing, and be constructed from only dried spaghetti and hot glue. The project budget is limited, and the bridge should be as light as possible while remaining strong. Your task is to design, build, and test a model bridge that meets these requirements.
当地议会需要一座跨过小溪的步行桥。该桥必须跨越 300 mm 的间隙，至少能承受 500 g 的中央载荷而不倒塌，并且只能使用干意大利面条和热熔胶来建造。项目预算有限，桥梁应尽可能轻巧但又要足够坚固。你的任务是设计、建造和测试一座满足这些要求的模型桥梁。
Before starting, you must analyse the brief carefully. The client's priorities are safety (load capacity), span, material restrictions, and cost (weight of materials used). This is the first step in the engineering design cycle: defining the problem.
在开始之前，你必须仔细分析需求。客户的优先事项是安全性（承载能力）、跨度、材料限制以及成本（所用材料的重量）。这就是工程设计循环的第一步：定义问题。
2. Product Design Specification (PDS) | 产品设计规格
A product design specification turns the client's wishes into measurable technical targets. For our spaghetti bridge, a typical PDS might include the following criteria:
产品设计规格将客户的愿望转化为可衡量的技术目标。对于我们的意大利面桥，典型的 PDS 可能包含以下标准：
- Span: exactly 300 mm between supports.
- 跨度：支点之间精确为 300 mm。
- Maximum load: sustain a point load of 5 N (approximately 500 g) at the centre for at least 10 seconds.
- 最大载荷：在中心处承受 5 N（约 500 g）的点载荷至少 10 秒。
- Materials: only dried spaghetti (any type) and hot glue may be used.
- 材料：只能使用干意大利面条（任何种类）和热熔胶。
- Weight limit: the bridge itself must weigh no more than 200 g.
- 重量限制：桥梁本身重量不得超过 200 g。
- Access: a 50 mm × 50 mm clearance below the centre for water flow (simulated by a block).
- 通行：桥下中心处需留有 50 mm × 50 mm 的通水空间（用块体模拟）。
- Construction time: completed within two 60-minute workshop sessions.
- 建造时间：在两个 60 分钟的工作坊课时内完成。
The PDS acts as a checklist throughout the project. Engineers constantly refer back to it to make sure the design stays on track.
PDS 在整个项目中充当核查清单。工程师会不断回看它，以确保设计始终不偏离轨道。
3. Researching Bridge Types | 研究桥梁类型
Before sketching any ideas, it is essential to investigate existing solutions. The three most common bridge types suitable for our span are beam bridges, truss bridges, and arch bridges. Each has advantages and disadvantages in terms of strength, material usage, and construction complexity.
在绘制任何草图之前，研究现有解决方案非常重要。适合我们跨度的三种最常见桥梁类型是梁式桥、桁架桥和拱桥。每种在强度、材料用量和建造复杂性上都有各自的优缺点。
A beam bridge is simplest—a flat deck supported at both ends. However, a simple beam made of spaghetti would snap easily under bending. A truss bridge uses triangles to spread forces, dramatically increasing stiffness for little extra material. An arch bridge transfers load through compression into the supports, which works well with spaghetti's high compressive strength but is difficult to build with straight sticks.
梁式桥最简单——一个两端支撑的平直桥面。然而，由意大利面条制成的简单梁在弯曲下很容易折断。桁架桥使用三角形来分散力，以很少的额外材料大幅提高刚度。拱桥通过受压将载荷传递到支座，这很好地利用了意大利面条的高抗压强度，但用直条建造比较困难。
From this research, a truss design appears the most practical for a lightweight, strong model. However, the team also considers a combination truss–arch to take advantage of both tension and compression behaviour.
从这项研究来看，桁架设计对于轻质且坚固的模型似乎最为实用。不过，团队也考虑采用桁架与拱结合的形式，以同时利用受拉和受压特性。
4. Generating Design Concepts | 生成设计概念
Using the research, you now sketch at least three different bridge concepts. Quick freehand drawings should show side views and top views, labelling key components such as top chord, bottom chord, verticals, and diagonals. Concept 1: Warren truss with alternating diagonals. Concept 2: Pratt truss with verticals in compression and diagonals in tension. Concept 3: an arched truss where the top chord curves downward to form a shallow arch.
借助研究成果，你现在至少绘制三种不同的桥梁概念草图。快速的徒手画应当展示侧视图和俯视图，并标注关键部件，如上弦杆、下弦杆、竖杆和斜杆。概念 1：Warren 桁架，采用交替斜杆。概念 2：Pratt 桁架，竖杆受压、斜杆受拉。概念 3：拱形桁架，其上弦杆向下弯曲形成浅拱。
This stage is about creativity and quantity, not perfection. Every sketch is assessed later against the PDS to see which one best satisfies the specifications.
这一阶段关乎创意和数量，而非完美。后续会根据 PDS 对每张草图进行评估，看哪一个最符合规格要求。
5. Selecting the Optimal Design | 选择最优设计
A decision matrix helps remove bias when selecting a final design. You list the key criteria from the PDS, give each a weighting based on importance, and score each concept (1–5). For example:
决策矩阵有助于在选择最终方案时消除偏见。你列出 PDS 中的关键标准，根据重要性赋予每个标准权重，然后对每个概念打分（1–5 分）。例如：
| Criterion (Criteria) | Weight | Concept 1 (Warren) | Concept 2 (Pratt) | Concept 3 (Arch-truss) |
|---|---|---|---|---|
| Strength (Strength) | 0.4 | 4 | 5 | 5 |
| Ease of build (Ease of build) | 0.3 | 5 | 4 | 2 |
| Material efficiency (Material efficiency) | 0.2 | 4 | 4 | 3 |
| Aesthetics (Aesthetics) | 0.1 | 3 | 4 | 5 |
| Weighted total | 4.2 | 4.5 | 3.7 |
Based on the weighted totals, Concept 2 (Pratt truss) scores highest. It balances strength and buildability, making it the selected design for detailed development.
根据加权总分，概念 2（Pratt 桁架）得分最高。它在强度和易建性之间取得了平衡，因此被选为进行详细开发的设计方案。
6. Understanding Forces in Structures | 理解结构中的力
Even simple bridge models follow basic structural principles. When a load is applied at the centre, the top chords experience compression (being squeezed), while the bottom chords experience tension (being stretched). The diagonal members in a Pratt truss are in tension, and the verticals in compression. Knowing this helps you decide where to bundle spaghetti strands for extra strength.
即使是简单的桥梁模型也遵循基本的结构原理。当载荷施加在中心时，上弦杆受压（被挤压），而下弦杆受拉（被拉伸）。Pratt 桁架中的斜杆受拉，竖杆受压。了解这一点有助于你决定在何处绑扎意大利面条以获得额外强度。
For an evenly loaded beam, the maximum bending moment at the centre is given by:
M = F × L ÷ 4
对于均匀受载的简支梁，中心处的最大弯矩由下式给出：
M = F × L ÷ 4
culate stresses in detail, this formula shows why a deeper truss (greater distance between top and bottom chords) resists bending more effectively.
其中 F 为点载荷（5 N），L 为跨度（0.3 m）。算得 M = 5 × 0.3 ÷ 4 = 0.375 N·m。虽然你不会详细计算应力，但这个公式说明了为何更深的桁架（上下弦杆间距更大）能更有效地抵抗弯曲。
7. Material Selection and Properties | 材料选择与性能
Spaghetti is chosen not only because it is cheap and accessible, but also because it has interesting mechanical properties. It is brittle in tension but relatively strong in compression. This influences the truss layout: place more spaghetti in members that carry compression, and use multiple parallel strands for tension members to reduce the risk of snapping.
选择意大利面条不仅因为它便宜易得，还因为它具有有趣的力学性能。它在受拉时呈脆性，但在受压时相对坚固。这会影响桁架的布局：在受压构件中放置更多面条，而在受拉构件中使用多股平行面条以降低断裂风险。
Hot glue provides fast bonding, but excessive glue adds unnecessary weight. The rule is to use only enough glue to secure joints, as weight is an enemy of bridge performance.
热熔胶能快速粘合，但过多的胶水会增加不必要的重量。原则是只用刚好足够的胶水来固定节点，因为重量是桥梁性能的敌人。
The average diameter of a spaghetti strand is about 2 mm. The cross-sectional area is roughly:
A = π × r² ≈ 3.14 × (1)² = 3.14 mm²
一根意大利面条的平均直径约为 2 mm。横截面积大致为：
A = π × r² ≈ 3.14 × (1)² = 3.14 mm²
If the compressive strength of spaghetti is approximately 5 MPa, then one strand can theoretically support about 15 N in compression before buckling becomes an issue. In practice, buckling will happen sooner, so bundling and bracing are essential.
如果意大利面条的抗压强度约为 5 MPa，理论上单个面条在受压失稳前可支撑约 15 N。实际上，失稳会更早发生，因此绑扎和支撑至关重要。
8. Detailed Design and Dimensions | 详细设计与尺寸
The selected Pratt truss is now drawn to scale. The bridge will be 350 mm long overall, with a 300 mm clear span between supports. Height of the truss (from top chord to bottom chord) is set at 60 mm, giving a depth-to-span ratio of 1:5, which is efficient. The truss consists of seven panels, each 50 mm wide, with verticals every 50 mm and diagonals sloping at about 45°.
现在按比例绘制选定的 Pratt 桁架。桥梁总长 350 mm，支点间净跨 300 mm。桁架高度（从上弦杆到下弦杆）设定为 60 mm，高跨比为 1:5，这个比例效率很高。桁架由七个节间组成，每个节间宽 50 mm，竖杆间距 50 mm，斜杆倾角约为 45°。
Both sides of the bridge are identical trusses connected by lateral cross-braces to prevent sideways buckling. The deck is formed by laying spaghetti sticks flat across the bottom chords, spaced 10 mm apart.
桥梁的两侧是相同的桁架，通过横向交叉支撑连接，以防止侧向失稳。桥面是将意大利面条平铺在下弦杆上，间距 10 mm 而成。
A bill of materials is prepared: 80 strands of spaghetti, 3 glue sticks. Total estimated mass: 170 g, within the 200 g limit.
准备了物料清单：80 根意大利面条，3 根胶棒。预估总质量：170 g，在 200 g 的限制以内。
9. Manufacturing Process and Risk Assessment | 制造过程与风险评估
A clear step-by-step plan reduces mistakes during construction. The process for building one truss side is:
清晰的逐步计划可以减少建造过程中的错误。搭建单侧桁架的过程如下：
- Print a full-size drawing and place it under a sheet of baking paper.
- 打印一张全尺寸图纸，并将其放在一张烘焙纸下方。
- Cut spaghetti to length for top and bottom chords, verticals, and diagonals.
- 将上弦杆、下弦杆、竖杆和斜杆的意大利面条按长度切好。
- Assemble the bottom chord first, then verticals, top chord, and finally diagonals, using a small dot of glue at each joint.
- 先组装下弦杆，然后是竖杆、上弦杆，最后斜杆，每个节点使用一小点胶水。
- Build two identical trusses; once cured, join them with cross-braces.
- 建造两个相同的桁架；胶水固化后，用交叉支撑将它们连接起来。
- Add the deck planks and any additional bracing.
- 加上桥面板和任何额外支撑。
A risk assessment identifies hazards: hot glue gun burns, snapped spaghetti causing sharp edges, and moving heavy test weights. Mitigations include heat-resistant gloves, safety glasses, and working in a clear zone.
风险评估识别出危险：热熔胶枪烫伤、面条折断产生的锋利边缘，以及移动重测试砝码。缓解措施包括耐热手套、护目镜和在洁净区域内操作。
10. Prototype Testing and Data Collection | 原型测试与数据收集
Once the bridge is fully cured, it is set up across two desks with a 300 mm gap. A loading bucket is suspended from the centre of the deck. Masses are added gradually, 50 g at a time, while timing how long the bridge holds each increment. The test stops when the bridge collapses or reaches a deflection greater than 20 mm, which indicates failure.
桥梁完全固化后，将其架跨在两个桌子之间，间隙 300 mm。从桥面中心悬挂一个加载桶。逐渐添加砝码，每次 50 g，同时记录桥梁在每一增量下维持的时间。当桥梁倒塌或挠度超过 20 mm（表明失效）时停止测试。
0 g before a top chord member snapped, triggering progressive collapse. The bridge mass was 168 g, giving a strength-to-weight ratio of 750 ÷ 168 ≈ 4.5.
记录的数据：最大持荷、首次出现开裂声时的质量、失效位置以及桥梁总重。在一次测试中，桥梁在持荷 750 g 时一个上弦杆件折断，引发了连续倒塌。桥梁质量为 168 g，强度 - 重量比约为 750 ÷ 168 ≈ 4.5。
11. Evaluation and Suggested Improvements | 评估与改进建议
Against the original PDS, the bridge surpassed the 500 g load requirement and stayed under the weight limit. However, the failure occurred in a top chord member, indicating that compression buckling controlled the collapse. To improve, you could double-laminate top chords (glue two strands together) or add more lateral bracing to reduce effective length and increase buckling resistance.
对照原始 PDS，该桥梁超过了 500 g 的载荷要求，且重量未超标。然而，失效发生在一根上弦杆件上，表明受压失稳控制了倒塌。为了改进，可以将上弦杆双层叠合（将两根面条粘在一起），或增加更多横向支撑以减小有效长度、提高抗失稳能力。
Another improvement would be to pre-tension the bottom chord during assembly to counteract tensile break. Also, using jigs during construction could improve joint accuracy, reducing eccentric loads that cause premature failure.
另一项改进是在组装时对下弦杆施加预张力，以抵消受拉断裂。此外，在建造过程中使用夹具可以提高节点精度，减少导致提前破坏的偏心载荷。
The design could also be optimised for a higher strength-to-weight ratio by removing material from lightly stressed areas—this is called topology optimisation in real engineering.
还可以通过移除轻应力区域的材料来优化设计，以获得更高的强度 - 重量比——这在真实工程中被称为拓扑优化。
12. Reflective Practice | 反思实践
A crucial part of any engineering project is reflecting on what you learned. By following the full design cycle, you experienced how theory connects to hands-on making. You also saw that real testing often reveals unexpected weaknesses—a top chord failure rather than a bottom chord break highlights the need to understand buckling, not just tensile strength.
任何工程项目的一个关键部分是反思你学到了什么。通过完成整个设计循环，你体验了理论如何与实际制作相联系。你也看到真实的测试往往会揭示意想不到的薄弱点——上弦杆失效而非下弦杆断裂，这突显了理解失稳而不仅仅是抗拉强度的必要性。
Teamwork and communication were essential: sharing sketches, agreeing on the final design, and dividing construction tasks. Documenting every step in an engineering logbook is a habit that will serve you well in future Edexcel assessments.
团队合作与沟通至关重要：分享草图、商定最终设计以及分工建造任务。在工程日志中记录每一步是一个良好习惯，会在未来的爱德思评估中给你带来很大帮助。
Remember, in engineering, failure is not a disaster—it is information. Each broken bridge shows you exactly where to improve next time.
请记住，在工程学中，失败不是灾难——而是信息。每一座断裂的桥梁都明确告诉你下一次该在哪里改进。
Published by TutorHao | Engineering Revision Series | aleveler.com
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