3D Laser Scanning for Structural Renovations: Columns, Slabs and Embeds

A structural renovation connects new work to old: a transfer beam bolted to embeds in a concrete core, a steel frame tied into a masonry wall, a new opening cut through a slab. The new steel is fabricated in a shop to drawings, and the old structure was built on site to a tolerance, so the two meet with an error that someone has to measure before the fabrication order goes out. This is the job of 3D laser scanning in a structural renovation, and this guide covers what the scan captures of a structure, what it cannot, and what the structural engineer and the fabricator get from it.

Why the drawings are not enough

The construction drawings of an existing building show the structure as designed. Columns were placed within the tolerances of their formwork, steel was erected to its code tolerances, slabs were never perfectly flat, and concrete creeps and buildings move in the decades after. None of that makes the structure unsound, and all of it puts the drawings off by a few millimeters here and a few centimeters there. When the renovation is a new partition, trim absorbs those errors; when it is a transfer beam whose bolt holes have to meet embeds cast forty years ago, they decide whether the beam bolts up or goes back to the shop.

A laser scan records the structure as it stands. The terrestrial scanner described on the 3D laser scanning page, of the Trimble X7 class, is rated by its datasheet at a 3D point accuracy of 2.4 mm at 10 m and 3.5 mm at 20 m (one standard deviation), so every column face, slab edge and exposed beam within its view is in the cloud as a measured surface. The registered point cloud is the base the engineer designs the connection to and the fabricator checks the shop drawings against.

What the scan captures

A structural scan is planned around what the new work has to meet. Inside the building it records the columns and the beams where they are exposed, the undersides of the slabs, the slab edges at openings and shafts, the stair and elevator cores, and the embeds, base plates and anchor bolts wherever a ceiling, a finish or a spandrel is opened to show them. Outside, it records the facade and the perimeter structure where they are visible, the roof and its equipment, and the ground the new footings have to come out of. A building about to be renovated structurally is often scanned twice when the schedule allows: once before demolition, to plan, and again when the finishes are off and the structure is in view, with the second visit quoted together with the first.

What the scanner cannot see is not in the cloud. Reinforcement inside a slab, an embed behind a finish or a beam above a closed ceiling is recorded only when it is exposed, and until then the model carries it from the drawings and marks it as unverified. A scan is a surface measurement; where the engineer needs to know what is inside a member, a probe, a scan of the opened element or a separate ground penetrating radar survey fills that gap.

How the site is scanned

Renovation sites are scanned around the work. The stations go where the structure is visible, which on a site mid-demolition means scanning when the ceilings are down and before the new framing goes up, often in a window agreed with the superintendent before the trades arrive. In the scanner’s standard modes a station takes two to seven minutes by its datasheet, depending on the point density chosen; at connection zones the stations are set closer and denser, so that a bolt group or an embed plate is captured with enough points to be modeled, and on open floors they are set wider. The stations are registered to each other in the field with a report of the error per station and the overlap between scans, and the cloud is tied to the building’s grid when the project has survey control, so that the model’s grid lines are the surveyor’s.

What the engineer and the fabricator get

The delivery begins with the registered point cloud in E57 and in RCP/RCS for Revit, with the registration report. From it, the structural deliverables are drawn or modeled: framing plans and sections showing the existing columns, beams and slabs at their measured positions, the elevations of the walls the new work ties into, and, when a model is ordered, a Revit model of the existing structure at LOD 300, where each element is represented so that its quantity, size, shape, location and orientation can be measured, or at LOD 350 where the model must also show how the elements meet, which the BIMForum specification intends for construction-level coordination; the Scan-to-BIM page describes the model. The accuracy of the line work is stated in the quote in the U.S. Institute of Building Documentation’s Level of Accuracy terms, a standard deviation at the 95 percent confidence level, and a tighter level can be specified for individual connection zones measured at short range.

With the cloud in Revit or Navisworks, the engineer measures what the drawings never showed: how far a column stands from the grid line the project sets, how much a slab edge bows between columns, where an embed sits in relation to the beam that has to reach it. Those measurements and the tolerances they are judged against are the engineer’s work, and so is the connection design that follows from them. The scan does not produce the fabricator’s model either: LOD 400 in the BIMForum specification is the element “with detail sufficient for fabrication, assembly, and installation”, and that is the fabricator’s model, made to the measured base the scan supplies.

Price and time

A structural scan is priced on the floors and the connection zones in scope and on what is built on the cloud. Typical ranges from recent projects: an office or a commercial floor, $3,000 to $9,000; a house or an apartment, $2,000 to $4,500; large buildings run from $12,000 and are quoted on their own; the point cloud alone starts at about $1,200. The exact price comes in a quote within one business day of your sending the address, the connection zones and the deliverables you need. The scan is scheduled as soon as the project needs it, often within days of the order, which matters on a site where the structure is exposed for a short window, and the drawings or the model usually follow in two to eight weeks depending on the scope. We scan in all 50 states.

Sources

• Trimble X7 3D laser scanning system datasheet: 3D point accuracy, station durations, registration report.
• BIMForum, Level of Development Specification 2023, Part I: the LOD 300, 350 and 400 definitions.
• USIBD Document C120, Level of Accuracy Specification Guide, version 2.0 (2016): the LOA levels as standard deviations at the 95 percent confidence level.
• ASTM E2807, Standard Specification for 3D Imaging Data Exchange: the E57 file format.