Constructability reviews and prefabrication

A design can be correct and still be hard to build. A detail may need a crane where none can reach, a sequence that traps one trade behind another, a tolerance that formwork cannot achieve or a connection that a worker cannot get a tool into. Constructability is the degree to which a design can be built safely, efficiently and to the required quality with the resources and methods available. Field engineers are well placed to judge it, because they see the drawings in detail and they see how the work is actually done. This lesson covers how to run a constructability review, how to turn findings into useful proposals, how constructability differs from value engineering, and how prefabrication changes the field engineer's job.

What constructability means

The idea has a long history. Industry research organizations such as the Construction Industry Institute in the United States promoted constructability programs from the 1980s, defining constructability as the optimum use of construction knowledge and experience in planning, design, procurement and field operations to achieve overall project objectives. The core idea is simple: people who build should influence the design early, when changes are cheap, rather than late, when changes are expensive.

Delivery methods affect how much influence builders have. In design-bid-build, the contractor often sees the design only when it is complete, so constructability input comes late, through questions and proposals during construction. Construction management at risk, design-build and integrated delivery bring the contractor in during design, so constructability review becomes part of the design process (CON-340). On Larkspur Commons, Thornbury was engaged as construction manager during design development, so Mei and Dana reviewed the drawings at the 50 % and 90 % construction document stages as well as during construction.

Running a constructability review

A constructability review is a structured look at the design from the builder's point of view. It can cover the whole project or focus on a system, such as the podium transfer slab or the wood-frame stories. A useful review is planned, uses checklists, involves the right people and produces findings with proposed solutions.

Reviewers should include the people who will build the work: the superintendent, field engineers, key subcontractors and, for specialized systems, suppliers. A ninety-minute review of the wood-frame stories with the framing subcontractor's estimator and foreman often finds more practical problems than a week of office review, because the framer knows exactly how panels are built, lifted and fixed.

Ranking and raising findings

A review produces a list of findings. Not all are equally important, and the design team has limited time. Ranking helps focus attention. A simple approach scores each finding for its impact on cost and schedule, the likelihood that it will cause trouble, and how easy it is to fix now compared with later. The example ranks the findings from the review of the wood-frame stories.

# Rank constructability review findings for the wood-frame floors (invented scores, 1 to 5)
findings = [  # (issue, schedule/cost impact, likelihood it causes trouble, ease of fixing now)
    ("Shear wall hold-down rods clash with plumbing risers at stack 4", 5, 4, 4),
    ("Corridor bulkhead too shallow for sprinkler main and duct", 4, 5, 3),
    ("Window rough openings sized without shim space", 3, 4, 5),
    ("Balcony membrane detail needs work over a finished slab", 3, 3, 2),
    ("Crane cannot reach the northeast corner for roof trusses", 5, 2, 3),
    ("Prefabricated wall panels exceed truck width for one stack", 2, 5, 5),
]
ranked = sorted(findings, key=lambda f: f[1] * f[2] * f[3], reverse=True)
print(f"{'Score':>5}  Issue")
for issue, imp, lik, ease in ranked:
    print(f"{imp * lik * ease:5d}  {issue}")
top = [f for f in ranked if f[1] * f[2] * f[3] >= 50]
print(f"Raise now with the design team: {len(top)} items")

Score  Issue
   80  Shear wall hold-down rods clash with plumbing risers at stack 4
   60  Corridor bulkhead too shallow for sprinkler main and duct
   60  Window rough openings sized without shim space
   50  Prefabricated wall panels exceed truck width for one stack
   30  Crane cannot reach the northeast corner for roof trusses
   18  Balcony membrane detail needs work over a finished slab
Raise now with the design team: 4 items

The clash between hold-down rods and plumbing risers ranks highest: it is likely, its impact is large and it is easy to fix on the drawings now, before walls are framed. The crane reach issue scores low because the reviewers judged it unlikely to cause trouble, but its impact would be severe if they were wrong. Scores are an aid, not a decision. Mei lists the crane issue separately for Dana to confirm with the crane supplier, because high-impact items deserve attention even when they seem unlikely.

Findings should be raised in the same way as questions (lesson 2): specific, illustrated, with a proposed solution and a date by which a decision is needed. During design, findings go to the design team through the review process; during construction, they become questions or proposals. The design team owns the design and decides whether to accept a proposal. A field engineer must never implement a constructability improvement that changes the design without approval, however sensible it seems.

Constructability and value engineering

Value engineering is a related but different process. It is a structured method for improving the value of a project, usually defined as function divided by cost, by examining alternatives that deliver the required functions at lower life-cycle cost. Value engineering often happens when a project is over budget and can turn into cost cutting under another name. Constructability review focuses on how the design will be built; value engineering focuses on what is built and what it costs.

The two overlap: a constructability improvement often saves money, and a value engineering proposal must be buildable. The field engineer's contribution to value engineering is usually practical: whether an alternative can be built with the available methods, what its schedule effect would be and whether it creates new interface or tolerance problems. Value engineering proposals during construction are usually handled under the contract's change provisions, and many contracts share the savings between owner and contractor.

Prefabrication and modular construction

Prefabrication moves work from the site into a factory or a yard. Examples include precast concrete, steel assemblies, prefabricated wall and floor panels, roof trusses, bathroom pods, mechanical and electrical racks, and whole volumetric modules. Prefabrication can improve quality, shorten schedules and reduce site labor and risk, but it changes the field engineer's job. A factory-built panel is made to exact dimensions; it cannot be trimmed on site as easily as site framing. The site must be ready for it, within tolerances that the factory has assumed.

CON-410 covered prefabrication and design for manufacture and assembly from a lean perspective. From a field engineering perspective, the key points are that the as-built conditions must be known before fabrication, that interfaces must be designed with enough adjustment, and that the layout on which prefabricated elements sit must be more accurate than for site-built work.

Measuring before fabrication

Prefabrication rewards the habit of measuring the as-built condition before components are made. For Larkspur Commons, the framing subcontractor builds the level 4 and above corridor walls as factory panels. The panels are detailed from the drawings, but before releasing them for fabrication, Arjun measures the actual distance between the end walls on level 4, which were framed on site.

# Will the prefabricated wall panels fit the level 4 corridor wall line? (inches)
measured_line = 1_226             # as-built distance between the end walls, from the floor layout
panels = [307, 307, 307, 305]
joint = 0.25                        # design gap at each panel joint and at each end
n_gaps = len(panels) + 1
needed = sum(panels) + n_gaps * joint
slack = measured_line - needed
print(f"Panels plus gaps: {needed:.2f} in, measured line: {measured_line} in, slack: {slack:+.2f} in")
min_gap, max_gap = 0.125, 0.5          # acceptable range for each gap
avg_gap = (measured_line - sum(panels)) / n_gaps
ok = min_gap <= avg_gap <= max_gap
print(f"Average gap if slack is shared: {avg_gap:.2f} in  {'fits' if ok else 'does not fit - adjust a panel before fabrication'}")

Panels plus gaps: 1227.25 in, measured line: 1226 in, slack: -1.25 in
Average gap if slack is shared: 0.00 in  does not fit - adjust a panel before fabrication

The panels and their design joints need 1,227 1/4 inches, but the actual wall line is only 1,226 inches, 1 1/4 inches short. If the slack were shared, the joints would be closed completely, which is not acceptable. The end walls were framed slightly inside their design positions, within the framing tolerance, but the panels assumed exact positions. Because the check happened before fabrication, the framer simply shortens the last panel by 1 1/4 inches. Without the check, the panels would have arrived and the last one would have had to be cut on site, or the end walls moved.

The same principle applies to many trades: stairs fabricated after floor-to-floor heights are surveyed, curtain wall brackets designed after slab edges are surveyed, millwork measured after drywall is up, and elevator equipment ordered after the shaft is surveyed. Lesson 10 covers the tools for measuring as-built conditions quickly.

Logistics and installation

Prefabricated elements must be delivered, lifted and set. Constructability for prefabrication therefore includes logistics: whether trucks can reach the site with oversized loads, whether the crane can lift the heaviest element at the required radius, where elements can be stored, and in what order they must arrive. CON-325 covers site logistics, cranes and temporary works in depth. The field engineer's contribution is accurate information about positions, weights from shop drawings, and the readiness of the bearing surfaces, so that each element can be set on the first attempt.

Setting prefabricated elements typically requires the field engineer to lay out bearing lines and reference points, check bearing elevations and shims, check connection embeds or anchors, and survey the element after setting. For volumetric modules and large panels, the tolerances can be tight and the cost of a misfit high, because a crane and a crew wait while the problem is solved.

Lessons learned

Constructability knowledge is easily lost when a project ends. Contractors that capture lessons learned, such as details that caused trouble, tolerances that did not work and sequences that saved time, can bring them to the next project's constructability review. A short note in a shared database, with a photo and a sketch, is often more useful than a long report. On Larkspur Commons, Mei keeps a simple lessons-learned list that becomes part of Thornbury's constructability checklist for the next podium project.