<aside> 🚩 An engineering requirement is a precise condition that applies to every possible design that solves the design problem.

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Overview

What are Requirements?

Design problems are typically defined initially in general terms (e.g., as in a Design Brief), not in the language of engineering and science. However, engineers cannot solve problems written in the language of laypersons. Before designing can start, engineers need a list of the conditions that will let them distinguish a satisfactory design from an unsatisfactory one.

This list of appropriately written conditions is called a set of requirements for the design problem at hand.

A satisfactory intervention must satisfy all the requirements; violating even one requirement renders the entire intervention unsatisfactory.

Requirements relate not only to functions needed to use an intervention, but also functions needed to support setting up an intervention before use and putting it away after use, to its manufacture and maintenance, and to how it is disposed of at the end of its useful life.

Form of Requirements

A requirement includes (a) a condition that must be true of any preferred solution to a design problem, plus (b) a justification for the requirement.

The condition of a requirement is written as a sentence of the form:

<aside> 💡 The intervention must do something such that limits on that action.

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The do something part of a requirement is an action verb, and is called a function.

The limits on that action describe the extent to which a function must be provided, and are called constraints.

Some important characteristics of requirements are:

• Functions must be active verbs, like lift, support, accelerate, move, resist, respond, protect, illuminate, etc.
  • Functions must not use the verbs "to be" or “to have”.
    • Example: "The intervention must be lightweight" or "The intervention must have a motor."
  • Functions must not have redundant phrasing.
    • Example: “The intervention must be able to lift up to 100 kg.” Here, the phrase “be able to” is redundant and must be removed.
• A constraint is a condition that specifies a variable and its precise value on an ordinal, interval, or ratio scale. The value may be a precise range of acceptable values.
  • Example: “The operating temperature of the furnace must be (20, 32, 40)C”.
    • In this case, the operating range is between 32 C and 40 C, with an optimal (or “target”) temperature of 32 C.
• Requirements must be phrased positively; do not use "not" in a requirement. For instance:
  • ❌ The intervention cannot carry less than 1kg and more than 100kg.
  • ❌ The intervention must carry no less than 1kg and no more than 100kg.
  • ✅ The intervention must carry between 1kg and 100 kg, inclusively.
• Each function must be its own requirement. Do not use "and" to include multiple functions in a single condition.
• Requirements must not mention parts, assemblies, materials, shapes, or other properties of the intervention. The only exception is if the necessity of the part, assembly, material, shape, or property is well established in existing design decisions, background information, or the Design Brief.
  • Requirements may refer directly to parts, assemblies, materials, shapes, or other things in the environment of the design problem, because those things are not part of the intervention and not under the control of the designer.

The justifications explain why the function and the constraints must be as they are. Justifications can include:

• a reference to a standard or other scientific or regulatory body;
• a reference to some clear statement in the Design Brief;
• a conclusion drawn from background research arising from an area of concern; or
• a rational, evidence-based argument derived from existing information.

Five Attributes that Requirements Cover

There are five key attributes of any good design. Requirements must address them all. However, you may not be able to address them all at this point in your engineering education.

The five attributes that a set of requirements must cover are:

Functionality: Does the design intervention actually do what it's supposed to do? Does it fulfil its intended role in a larger system? Does it bring about a preferred situation?

Usability: Can users use it? Can they install, use, and decommission it safely, easily, even enjoyably?

Producibility: Can the intervention be manufactured/assembled easily and robustly?

Maintainability: Can the intervention be maintained easily and well? Does it even need to be maintained? How inconvenient is maintenance to users?

Sustainability: What impact does the design impose on the environment? How long will the product last? What happens to it at its end of life? How many people can afford it and for how long? Can it be sold at a reasonable price that satisfies users' financial abilities while allowing the company to remain financially solvent?

<aside> 💡 Focus on Functionality and Usability. In one-semester design courses, it is very difficult to cover all necessary requirements for all five attributes. Since quality is more important than quantity in this course, make sure you have functionality and usability as well addressed as possible, even if that means neglecting the others somewhat. Your instructor may have specific tasks you need to complete with respect to the other three attributes. But remember: in real life, all five attributes are essential for a successful intervention.

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