Posted by Brandon Garrett
As-built drawings are an essential part of any renovation or restoration project. If accurate drawings aren't available, however, 3D scanning and point cloud integration into BIM can provide fast and precise information of existing field conditions so Revit models are properly detailed.
One of the major undertakings of the two-phase renovation of the Albuquerque Convention Center was the new Territorial-style entry, which was located over an existing basement that was difficult to measure and lacked accurate drawings. This created a challenge for the design team as the required structural columns for the new entry needed to penetrate the existing concrete waffle slab without conflicting with the slab ribs that span in both directions between column heads or band beams. To accomplish this, the team needed to create an accurate Building Information Model (BIM).
To create a 3D scan that could link into BIM, a select group of students from Navajo Technical University made the drive to Albuquerque and spent a morning scanning the basement of the Convention Center.
Rendering of the new entry of the Albuquerque Convention Center
Creating a set of field verified drawings is time consuming, often requiring a full day or more to take simple tape measurements and record dimensions on paper. Ceiling height is often recorded by tape measurement as well, with the locations of electrical, data, and power outlets being documented.
An alternative to this method involves the use of a laser tape. The laser provides faster and more accurate measurements, but does not work when measuring elements on the same plane where the laser cannot hit its surface.
In addition to taking measurements, photographs are often used as a visual reference. All of this information is then converted into a digital format (CAD or BIM) for use in the working drawings. Inaccuracies are often only noticed during this final step, and areas of interest may or may not have been photographed, often resulting in a return visit to the site.
To bypass the time-consuming tape measurements and ensure an accurate BIM, we looked to a new and emerging technology known as 3D scanning.
Unfamiliar with the technology, we connected with Scott Halliday and his students from Navajo Tech. Students gained real-world experience, and we learned about the use of Point Clouds and established internal workflows for managing and implementing the technology for future projects.
There are several types of 3D scanners on the market, and in the case of the Convention Center project, the students used a LIDAR scanner. LIDAR works by shooting a laser and analyzing the reflected light off a surface. This yields some of the most accurate measurements, with some scanners that are accurate down to +/- 2mm. Many scanners can also embed photographic information into the models.
For the large Albuquerque Convention Center space, we needed several scans because each scanner has a limited range. Paper targets were placed around the space and used by sophisticated software in combination with the geospatial data from the scanner to stitch the individual scans into one large image. Once this process was completed, the students sent a digital copy to the design team.
This data was combined to create an accurate 3D representation of the space, which was then imported into various applications for analysis and reference.
The advantage of using this technology to document as-built conditions is that it captures everything and eliminates the need to revisit a site for future verification.
For the design team, this was our first experience with this type of data. What surprised us most was the massive file size, as the completed scan was approaching 40GB. Using software from FARO and ReCAP, we were able to isolate and link the most important scan data into the Revit model. All of the model geometry was then updated to match the scan, which gave the design team the confidence they needed to finalize the exact location of the required structural column penetration through the slab.
This scanning technology is poised to change the way we document existing conditions. The cost of these scanners has been dropping, and we expect this service will continue to be used on most renovation and restoration projects. Besides documenting existing conditions, this has applications in historic preservation work and even during construction when verifying field conditions against Building Information Models.
Brandon Garrett & Scott Halliday presented this collaboration at the 2013 SPAR international conference. View the presentation.