Using 3D solutions to improve design tolerances on building projects gone awry

By Kevin Corbley

A mixed-use, multi-structure development project in South Africa had gone terribly wrong. Halfway through the construction, the builder went out of business. What was left behind may have given a hint as to why. The quality of construction that had been performed was poor, and the owner was unsure how to move forward with the project.

As-built documentation created by the defunct builder had been generated with measuring tape. Although the as-builts indicated the work aligned with the design plans, experts brought to the scene visually observed quite the opposite. A structural engineer expressed no confidence in the integrity of the work he inspected.

“Documentation said everything was fine, but you could see things weren’t right,” said Brent Godfrey, civil engineer and 3D scanning specialist at Cape Survey of Cape Town.

The project owner requested Cape Survey try to get the construction back on track by assessing the conformance of the existing structures to the original designs. The objective was to evaluate every beam, column and other building element that had been erected to determine whether each was in or out of tolerance. If a structural element was out of alignment, the owner wanted to know how far so decisions could be made to fix or replace it.

The future of the project rested with the accuracy of the new as-built because the cost to repair or reinstall features would be significant. This evaluation would dictate if and how the construction could commence.

Cape Survey has provided professional land surveying services and consulting to South Africa for 55 years. It deploys the latest technologies, including 3D laser scanning and digital construction validation, to bring innovation to its clients.

The firm’s plan was to first scan the entire site and generate an accurate 3D as-built document in digital format. This as-built would then be digitally compared in 3D to the design drawings with the automated Verity construction validation software from ClearEdge3D. Cape Survey had used the software on several projects and knew it would provide exacting tolerance data for every element. Just as importantly, the output would be visually understandable to everyone.

The structure was color-coded by each tolerance level starting with green as in-tolerance.

“The project team included the owner, new contractors, architects, engineers – some of whom didn’t have experience looking at 3D data,” said Godfrey. “We wanted to make [the results] as visual as possible.”

At the outset, the 3D modelling and construction validation seemed to be a straight-forward assignment…until Cape Survey encountered two challenges – one minor, the other major – that caused the team to adopt an entirely new approach to its work.

Project challenges

2D model for a 3D reality

The first challenge Cape Survey faced on the project site was the type of design documents. They had been created in AutoCAD as 2D files, not 3D as the survey and scanning firm was accustomed to using. There were a couple of different options for dealing with this issue, according to Godfrey.

Cape Survey could perform the 3D scanning of the site and take 2D slices of the scan data for comparison with the CAD drawing. This more traditional method was the way the firm would have done the work prior to the availability of Verity. However, the comparison would be mostly visual and use manual measurements to identify discrepancies between the site conditions and the design – requiring hours of tedious work. Worse, the results of this 2D process would be much harder for the client to interpret once all was done.

“Why go to the trouble of making a 3D scan and then doing a 2D analysis?” asked Godfrey.

Instead, Cape Survey took the time to create 3D building information models (BIM) from the original 2D files in AutoCAD. While this may sound like an extreme step, creating a structural BIM from 2D CAD files can be done efficiently as there is no design being done in the process. In a matter of a few weeks, Cape Survey was able to model the structures based solely on the design drawings, survey the site, and scan all the work in place. This allowed them to begin the process of analysing the installed construction work in Verity.

Flawed starting points

The second problem related to the construction site itself, and Cape Survey uncovered the issue while examining the 2D as-built drawings left by the original builder. This document was not tied to any site control or gridlines. Further evaluation revealed the buildings themselves had not been constructed in their correct positions as per design plans. The initial surveying that was performed at the outset of construction work was done incorrectly, leading to variances many times beyond any acceptable tolerance.

This created a unique problem for Cape Survey, because comparing the new as-builts against the original design plans would produce wildly out of tolerance results since the structures hadn’t been built where they should have been. In absolute terms, every element onsite was out of alignment with the plans. As a result, the software would correctly flag every element captured in the scans as out-of-tolerance because the entire buildings were out of alignment with their designed locations.

“We couldn’t just take the original design and use that [for comparison with the as-built] because the deviations were too far off to start with,” said Godfrey.

The 3D as-built model was imported to Navisworks, where the conformance analysis was conducted with Verity.

This was a critical discovery for the eventual success of the project. In reviewing the initial results from Verity, Cape Survey realised that while all elements were out of tolerance from their planned absolute locations, many features were shifted in the same direction and actually in relative alignment to each other, once the positioning error was eliminated. The team began to believe that not as much rework would have to be done on the site after all.

Workflow to create a solution

Once onsite, Cape Survey scanned the exterior of the construction site with Trimble SX10 laser scanners integrated with Trimble R2 GNSS receivers to establish control. Trimble TX8 scanners captured the interior structures. Technicians registered the scans in Trimble RealWorks and exported the structured E57 file to Autodesk Recap. With the 3D as-built model ready, the file was imported to Navisworks where the conformance analysis with Verity would occur.

Before items could be reported on, the final “new” design position was calculated using Verity Bias error from the summary report.

Before evaluating the feature deviations, the firm had to compensate for the positioning error in construction. Fortunately, the software has a built-in feature to calculate how far off the buildings were from their designed positions. Verity performed an analysis of the entire as-built model against the theoretical position of where the buildings were supposed to be in the 3D design model. The software generated a summary report, including Bias Error metrics, for all the analysed elements.

“This is exactly what we needed,” said Godfrey. “It’s the average shift of all elements in the analysis, but weighted based on the surface area of the items.”

Smaller features, such as beams and columns, had less influence than big features like shear walls. Floor slabs were discounted because they didn’t affect 2D positioning of the elements.

Beams and columns had less influence than features like shear walls, while floor slabs were discounted because they didn’t affect 2D positioning of the elements.

“We shifted the design model by this weighted Bias Error value and reported on deviations based on it,” said Godfrey.

Even with the positioning errors eliminated, Verity revealed many features in the construction project far exceeded the 15 mm tolerance from design specifications considered acceptable. This presented a new challenge in presenting the results so everyone could understand. If the software output results by simply coding out-of-tolerance features in one colour, too many elements would appear in that colour.

“We wouldn’t be able to understand what was happening in the building,” said Godfrey, explaining they had to subdivide the deviations to express their severity.

Deviations were evaluated in Verity in several ways depending on the feature. For example, the horizontal displacements of beams were measured in millimetres. Vertical walls and columns were also measured by deviation from vertical, and columns were also assessed by degrees of rotation. The thicknesses of floor slabs were measured against the design in millimetres.

Since out-of-tolerance deviations were measured in different units for various elements, Cape Survey created a report in Verity that categorised them by their magnitude (2X, 3X, 4X, 5X). Each was represented by a different colour in the output report that presented the analysis results in a 3D model of the as-built structure. Green was selected as the colour showing items in tolerance.

For project participants unfamiliar with 3D models, the colourised report was an illustration of construction quality. This digital model created with the software allowed team members to click on a section of a building or an individual element to receive numerical details of its deviations. To supplement the graphical visualisation, Cape Survey generated many spreadsheet reports isolating specific features or categories of deviations.

Aside from the visual impact, the precise measurements made by the software gave the engineering team the information they needed to determine the structural integrity of the buildings. For example, the thicknesses of walls and floor slabs enabled them to calculate whether the buildings could actually support the weight for which they had been designed.

After reviewing the conformance data, members of the evaluation team confirmed that while there was much work required to get the construction back on track, it was much less than originally anticipated.

Saving money and resuming work

“By shifting the design to new gridline positions, [we] improved the tolerance to design by 400%,” said Godfrey. “We had a base to move forward and work from.”

The X, Y position summary showing the design tolerance improvement.

Cape Survey presented the colourised Verity as-built deviation report to the project engineers. Together, the engineers and surveyors reviewed each element of the construction to determine which could stay, had to be repaired, or needed to be replaced entirely. Although many elements had to be replaced, the work was relatively small in comparison to the amount of rework that would have been required had the design position error not been corrected.

For the sake of comparison, Cape Survey compared the Verity results from the uncorrected as-built to design validation against the report made with the error-adjusted as-built model. The uncorrected verification concluded just 10% of features were in tolerance, but this number jumped to 42% for the corrected version. More importantly, the percentage of features that that required replacement or major repair went from 56% down to 6%.

“It was a great savings…and gave the owner a basis to resume construction,” said Godfrey. “By doing the scanning and analysis, we reduced the cost of the rework substantially.”

About the author: Kevin Corbley is a business development consultant who specialises in the geospatial profession. He is based in Denver, Colorado, and may be reached at www.corbleycommunications.com.

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