Module 19 · Lesson 19.2

Decisions, reach, and the record

Which decisions matter most, when they get taken, and how to leave behind something a reviewer can actually disagree with.

Why this matters

There is an uncomfortable asymmetry in design work.

The decisions that shape a building most are taken at the beginning — when the least is known, the fewest calculations exist, and the pressure to decide quickly is highest. The decisions taken later, with far better information, can only adjust what the early ones already fixed.

This lesson makes that concrete by tracing four decisions from this course through the project and measuring how far each one travels. Then it asks what has to be written down for any of it to be reviewable.

By the end of this lesson you should be able to

Trace a decision forward through the stages of a project
Measure how much of the remaining project a decision reaches
Say which decisions can still be revisited and which cannot
Write a record entry that survives review

What you should already know

Actions and their combinations (Module 2)
Bracing and frame stability (Modules 4 and 16)
Composite construction stages (Module 11)
Execution classes (Module 18)

Reach

Take the crane decision — the one made back in Module 2, when the crane was identified as a source of cyclic loading rather than just another load.

Follow it forward:

Scheme. A crane-supporting structure is needed, with its own load path down to foundation.
Member design. Module 17: fatigue sized the girder at 384 kg/m where strength alone wanted 123.
Connections. The weld detail category became a sizing variable rather than a workmanship note — improving it from 71 to 125 saved 133 kg/m.
Execution. Module 18: service category SC2 raised the execution class from EXC2 to EXC3, doubling the volumetric testing.

That decision was taken at the second of seven stages, and it reaches the last. 100% of what remained of the project.

Now take a late decision — preloaded bolts at the splices, chosen at the connections stage. It reaches execution, and that is all there is left. Also 100% of what remains. The percentage is honest, and it is not the interesting number: the span is. One decision reaches five stages; the other reaches one.

Try it

How far a decision reaches

Pick a decision and see which later stages it touches. The pattern is the point: the earlier a decision is taken, the further it travels — and the less information there was when it was taken.

Decision

Decision: Treat the crane as a source of cyclic loading
Taken at: Actions
Stages reached downstream: 5
Of what remains: 100 %
Locks later work: yes
Scheme: A crane-supporting structure is required, with its own load path to foundation
Members: Fatigue sizes the girder at 384 kg/m where strength alone wanted 123
Connections: The weld detail category becomes a sizing variable, not a workmanship note
Execution: Service category SC2 raises the execution class from EXC2 to EXC3

"Treat the crane as a source of cyclic loading" is taken at the actions stage and reaches 5 stages downstream — 100% of what remains of the project. It reaches past analysis, so revisiting it means redoing member design. In practice that makes it irreversible long before anybody calls it fixed.

Things worth trying

Start with the crane decision. It is taken at the actions stage — almost the beginning — and it reaches every remaining stage: 100% of what is left.
Read its four consequences in order. Each one was established by a different module, and none of them was asserted: the 384 kg/m is Module 17's result and the EXC3 is Module 18's.
Now switch to the preloaded bolts. Also 100% of what remains — but what remains is one stage. The percentage is honest and the span is what matters.
That is the asymmetry the lab is for. Both decisions reach everything that is left. One reaches five stages and the other reaches one.
Try the braced-frame decision. It stops at connections, so it does NOT lock member design in the same way — the verdict changes colour.
Try the composite decision. It reaches execution, because propped or unpropped construction is something the erector has to do differently. A choice about a beam has become an instruction on site.
The uncomfortable conclusion: the decisions that travel furthest are taken earliest, when the least is known about the building. No amount of care later recovers a wrong one — it only makes the wrong decision better executed.

Worked example

One decision, followed to the far end

Given

A building with a crane bay, at the actions stage
The question: is the crane a load, or a cyclic action?

Find

What follows from treating it as cyclic

The record

If the early decisions are the consequential ones and they will not be revisited, then the only thing that makes them reviewable is what was written down at the time.

A design record entry needs three things, and most have only one:

1.What was decided. Almost everybody manages this.
2.Why this and not the alternative. Far fewer.
3.What would reopen it. Almost nobody.

The third is the one that turns a record into something useful. "Braced frame with a central core, because a braced bay is roughly seventeen times more efficient per tonne than a moment frame and the core was needed for the lifts anyway — revisit if the architect removes the core or the lift positions move off the stability line" is an entry somebody can act on two years later.

"610×229×113 UB floor beams. It works." is not a record. It is a note, and it cannot be reviewed, because there is nothing in it to disagree with.

Practice

The crane decision is taken at the actions stage — the second of seven stages — and its consequences reach the execution stage, which is the last. How many stages downstream does it reach?

Predict first

Two decisions both reach 100% of what remains of the project: the crane decision taken at the actions stage, and preloaded bolts chosen at the connections stage. Which is more consequential, and why?

Summary

The crane decision, taken at stage 2 of 7, reached the last stage: 100% of what remained
It set the member at 384 kg/m against 123, made a weld detail a sizing variable, and forced EXC3
A late decision can also reach 100% of what remains — but very little remains
Once a decision reaches past analysis, revisiting it means redoing member design
So the honest position is that early decisions stand, which is an argument for effort at scheme stage
A record needs what, why not the alternative, and what would reopen it — the third is the rare one
It has to be written at the time; afterwards the reasons have become justifications

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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