Who Is Responsible When A Design Changes On Site

No project is built exactly as drawn.

Conditions are found, products are substituted, dimensions do not work out, and something has to change. That is ordinary construction.

The problems come from changes that were made and not recorded, because a structure that differs from its documentation cannot be certified against it.

The three kinds of change

A change the engineer designed. The engineer issues a revised detail or a written instruction, it is built accordingly, and the documentation reflects the structure. Clean.

A change the builder proposed and the engineer accepted. A request for information, an answer, and a record. Also clean, and this is the majority of well-handled changes.

A change the builder made. No question asked, no record created, and the structure now differs from the drawings in a way nobody has assessed.

The third one is the problem, and it is rarely malicious. It is usually a reasonable-looking decision made at speed by someone who did not know what the detail was for.

The changes that matter most

Some site decisions look trivial and are not.

Reinforcement substitution. Swapping bar sizes or spacing because of what is on the rack. Occasionally equivalent, frequently not, particularly where crack control rather than strength governed the original.

A penetration cut where the drawing showed none, which in a post-tensioned slab can sever a tendon. The reason [core drilling](/blog/core-drilling-and-penetrations-in-existing-slabs) is never done without locating the tendons.

Concrete grade changed, which affects durability and cover as well as strength.

A connection modified to make it fit. Holes flame cut, plates trimmed, members forced. Every one of those changes the capacity of a connection designed for specific forces, and it is the recurring risk in [steel fabrication and site fit](/blog/steel-fabrication-tolerances-and-site-fit).

Construction sequence changed, which matters enormously in structures that depend on the order of loading. Props removed early, [backpropping](/blog/backpropping-and-temporary-works) reduced, a transfer structure loaded before it was complete.

A footing founded higher than specified because the excavation hit water or rock was deeper than expected. One of the most consequential and one of the most commonly unrecorded.

Temporary works altered, which is a safety issue as much as a structural one.

Why documentation is the whole answer

An engineer certifies that a structure was built in accordance with the design.

Which is only possible if the design as built is known. An undocumented change means the certifier is either certifying something they have not assessed, or refusing to certify, and both are bad outcomes discovered late.

Practically, the documents that matter are the request for information register, the engineer's written instructions and revised details, the inspection records, and the as-built or work-as-executed drawings where the structure differs from the design.

That last one is the item most often skipped and most often needed, because the next engineer to work on the building will rely on it. It is what prevents the next project starting from [a building with no usable drawings](/blog/assessing-an-existing-building-with-no-drawings).

Inspections are where changes get caught

A change made without asking is usually visible at an inspection, and the inspection is the mechanism that catches it before it is covered.

Which is the practical value of [pre-pour and construction inspections](/blog/construction-supervision-pre-pour-inspections) beyond the checking itself: they are the regular point at which the structure and the drawings are reconciled.

Skipping inspections to save fee reliably costs more than the fee, because the discrepancies are found at completion instead, when the cost of resolving them is highest.

What good practice looks like

Ask before changing anything structural. The answer is usually quick, occasionally a redesign, and always cheaper than the alternative.

Put it in writing, both ways. A verbal approval on site protects nobody, including the person who gave it.

Keep the register current rather than reconstructing it at the end.

Record what was actually found. Founding levels, conditions encountered, substitutions made.

Raise it early. A discrepancy raised the day it is found is a technical question. The same discrepancy raised at handover is a dispute.

The underlying point

Engineering documentation is not paperwork sitting alongside the structure. It is the statement of what the structure is.

A building whose documentation does not describe it is a building whose adequacy cannot be demonstrated, and that shows up years later when somebody wants to add a storey, change the use, or sell it.

ACSES provides structural engineering and project management across Sydney, including construction stage inspection and certification. Get in touch.