Scan-to-BIM for Ceilings and Plenums: LOD, RCP and Clash Checks

Lift a ceiling tile in a building that has had three tenants and you are looking at a part of it that no current drawing describes. The ductwork was rerouted by the first fit-out, the conduit was stacked on top by the second, a sprinkler branch was added during a fire protection upgrade and never drawn, and the record set still shows the plenum as the original engineer designed it. This guide covers scan-to-BIM for ceilings and the space above them: how the plenum is scanned, what is modeled and at what level of development, what the reflected ceiling plan carries, and how the model is used to coordinate the new work with the old.

Why the plenum is undocumented

The ceiling plane is where the trades finish their work and where later trades add to it, and little of that reaches a drawing. A design team that allocates clearances from the record set may be designing into space that does not exist: a corridor drawn with a clear plenum may have a duct, two conduits and a cable tray in it by the time the new work arrives.

A laser scan records the plenum as it is. A terrestrial scanner of the Trimble X7 class measures up to 500,000 points a second with a 3D point accuracy of 2.4 mm at 10 m (one standard deviation) by its datasheet, so a duct run, a pipe and the deck above them are captured as measured surfaces from each position the scanner can see them from. The registered point cloud is a record of the plenum that the mechanical, electrical, plumbing and fire protection designers can share, and the model built from it is what their new work is coordinated against.

How a plenum is scanned

Scanning above a ceiling is a question of access. In a lay-in grid, tiles come out on a pattern agreed with the facility and the scanner goes up to the opening to look along the runs; under a hard ceiling the systems are reached through access panels and from the mechanical rooms, shafts and risers that connect them. The scope settles which ceilings may be opened and who opens them before the visit.

Ducts cast shadows. A rectangular duct hides the pipe behind it from one side, and a crowded plenum takes more stations than an open one. A scanner of the Trimble X7 class takes one station in two to seven minutes without images, by its datasheet, and in its standard modes gives a point spacing of 11, 5 or 4 mm at 10 m, so the density can be chosen to suit small-bore pipe and conduit. Stainless pipe and galvanized duct are reflective; the datasheet lists a high sensitivity mode for dark and reflective surfaces, stainless steel among them, at roughly twice the station time. What the scanner cannot see from any opening is not in the cloud, and the model marks those runs as unverified.

What is modeled and at what LOD

The model holds the systems the scanner saw: ductwork with its diffusers, grilles and air handling units, hydronic and plumbing piping with its valves, conduit, cable tray and panels, fire protection mains and branch lines with their sprinkler heads, and the beams and deck the systems hang below where they set the clearance. The level of development is set per element category. At LOD 300, in the BIMForum Level of Development Specification, an element is represented “such that its quantity, size, shape, location, and orientation can be measured”, which is the usual order for a plenum model that new work will be designed around. At LOD 350 the element also shows its “interfaces with adjacent or dependent Model Elements”, the level the specification intends for construction-level coordination, and it is ordered for the runs that the new work has to pass. A quick model for feasibility is LOD 200, where elements are “generically and graphically represented” with approximate quantity, size, shape, location and orientation. The specification does not prescribe a level for any step of a project, so the scope names the LOD per category, and the quote lists them.

A scan model is geometry. It records where a run is and how big it is; it does not carry flows, loads or pressures. The MEP BIM page describes the model and its drawings, and the Scan-to-BIM page the architecture and structure modeled on the same scan.

The reflected ceiling plan

The reflected ceiling plan is the sheet a renovation usually needs from a ceiling scan. When a model is ordered, the plan is cut from it, so a diffuser on the plan and the same diffuser in the model are one object; a 2D-only order for a small space is drawn straight from the cloud. The sheet shows the ceiling grid and heights, the diffusers, the lights, the sprinkler heads and the access panels where the scan found them, and the sections through the plenum show the depth available above the ceiling along each cut. The accuracy of that line work is stated in the U.S. Institute of Building Documentation’s Level of Accuracy terms, as a standard deviation at the 95 percent confidence level; a typical renovation set is drawn to LOA30, between 15 mm and 5 mm, and the quote names the level. The as-built drawings page describes the full sheet set.

Coordinating the new work

The reason to model the plenum is new work that has to pass it. Navisworks, Autodesk’s coordination tool, defines a hard clash as one where “the geometry of selection A intersects that of selection B by a distance of more than the set tolerance” and a clearance clash as one where selection A “comes within a distance of less than the set tolerance” of selection B; the second is the test we use for insulation thickness and maintenance access. The existing-conditions model is what the design team’s new ductwork and piping are tested against. We deliver it coordination-ready, with the runs modeled so that the team can run its own tests, and we run the tests on the client’s new-work model when the scope includes them; resolving a clash is a design decision and stays with the engineer of record.

Price and time

A ceiling and plenum model is priced inside the building ranges by how many systems are in scope and how dense the plenum is. Typical ranges from recent projects: an office or a commercial floor with its systems, $3,000 to $9,000, with several systems and a crowded plenum toward the upper end; a house or an apartment, $2,000 to $4,500; large buildings run from $12,000 and are quoted on their own. The exact price comes in a quote within one business day of your sending the address, the systems you need and the LOD. The scan is scheduled as soon as the project needs it, often within days of the order, a floor usually takes a day on site, and the model usually follows in four to eight weeks for a commercial floor, depending on the systems and the LOD. We scan in all 50 states.

Sources

• Trimble X7 3D laser scanning system datasheet: scan speed, 3D point accuracy, station durations and point spacing, high sensitivity mode.
• BIMForum, Level of Development Specification 2023, Part I: the LOD 200, 300 and 350 definitions.
• Autodesk Navisworks help, glossary: hard clash and clearance clash
• USIBD Document C120, Level of Accuracy Specification Guide, version 2.0 (2016): the LOA levels as standard deviations at the 95 percent confidence level.