Module 3 · Lesson 3.1
What a parameter is
Independent and dependent quantities, rules instead of coordinates, and why the choice of parameters is the design.
Why this matters
A model built by typing coordinates answers one question: what does this structure do. A model built from rules answers a family of them.
That sounds like a change of technique. It is really a change of what you are designing. Once the model is parametric, the object you are designing is the rule set — and the rule set is where the engineering judgement now lives.
Independent and dependent
An independent parameter is one you set: number of storeys, grid spacing, roof pitch, tower height.
A dependent parameter is derived: node coordinates, member lengths, the number of purlins, the total structural volume.
The distinction matters because a model that treats a dependent quantity as independent will contradict itself. Set the height and the storey count and the storey height and one of the three has to lose. A well-built parametric model makes exactly two of those three independent and computes the third — and says which.
Rules instead of coordinates
Compare two ways of describing the same portal frame.
As coordinates: twelve numbers, correct for one frame, and nothing about them says what a frame is.
As rules: span, eaves height, roof pitch, and the statement that the ridge sits at mid-span, at eaves height plus half the span times the tangent of the pitch. Three numbers and a relationship — and the relationship is the part that survives a change.
The rule version generates the coordinate version. The reverse is not true, and that asymmetry is the whole reason to work parametrically.
Choosing parameters
The temptation is to parameterise everything. It should be resisted, for two reasons.
The space grows multiplicatively. Five parameters at ten values each is 100,000 combinations. Exploring that needs a search strategy, not a sweep — which is Module 16.
Most parameters do not matter. A sensitivity study — Module 14 — usually finds that two or three inputs carry almost all the variation in the output. The rest are noise dressed as flexibility.
Parameterise what you intend to vary and what you are uncertain about. Fix everything else and say that you have fixed it.
The readability problem
A parametric model is a program, and it has the maintenance properties of one. If the rules are buried in a chain of nested operations that only its author can follow, then the author is now a single point of failure on the project, and the model will be rebuilt from scratch the first time they are unavailable.
Name things. Group the parameters. Write down what each one means and its valid range. It is not tidiness — it is what makes the model an asset rather than a liability.
Try it
Portal frame generator
Three independent parameters generate the whole frame. Everything else on the readout is computed.
Set to zero for a flat roof and watch the ridge height collapse onto the eaves.
- Span (set)
- 24.00 m
- Eaves height (set)
- 7.00 m
- Pitch (set)
- 6.0°
- Rise (computed)
- 1.261 m
- Ridge height (computed)
- 8.261 m
- Rafter length (computed)
- 12.066 m
- Frame member length
- 38.13 m
half-span × tan(pitch)
eaves + rise
two columns plus two rafters — a first proxy for material
What to notice
- Three numbers are set and four are computed. The computed ones cannot be edited, because editing them would let you express a frame that does not exist.
- Set the pitch to zero and the ridge collapses onto the eaves. The rule still holds; the geometry has simply become a flat portal.
- The member length is not a design quantity. It is a comparison quantity, and its whole value is that it updates the instant a parameter moves.
What this shows: Rules generate coordinates; coordinates do not generate rules — and only two of a related triple may be set independently.
Check yourself
A parametric tower model exposes height, number of storeys and storey height as three independent sliders. What is wrong?
Worked example
Which of these is actually a parameter
Given
- A parametric model of a warehouse is proposed with: span, bay spacing, eaves height, roof pitch, number of bays, portal frame or truss, and the steel grade
Find
Which are parameters, which are constants, and which is neither
Worked example
A parameter with no consequence
Given
- A parametric tower model exposes twelve parameters
- Varying one of them — the fillet radius on a connection plate — changes nothing in the analysis output
Find
What to do about it
Practice
A parametric model has 5 parameters, each taking 6 values. How many distinct designs does that define?
Check yourself
What distinguishes a parametric model from a model you can edit?
Check yourself
Why should a parametric model state what it has fixed?
Check yourself
A rule in a parametric model contains an error. How does that differ from the same error made by hand?
Summary
- A parameter is an input others are derived from
- Independent parameters are set; dependent ones are computed, and only one of a related triple may be derived
- Rules generate coordinates; coordinates do not generate rules
- Parameterise what you will vary or are uncertain about, and fix the rest explicitly
- An unreadable parametric model has one user and a short life
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint