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No matter how carefully work is planned, sometimes designs don’t easily translate into reality. Often, the more sophisticated the design, the more difficult it is to render into actual form. In the case of one very modern, innovative design for a pedestrian/cycle bridge, laser scanning helped keep the project on-time, on-budget and aided in turning the design into reality. By employing terrestrial LIDAR, material scrap rates were significantly reduced and project delays were kept to a minimum.
Implementing a Transportation Solution
The City of Pickering is a rapidly expanding suburban city of 90,000 people approximately 20 miles (32km) east of downtown Toronto, Ontario, Canada. Because the Toronto "region" is a sprawling metropolis of about 5 million people, large highways and mass transit are essential commutation conduits to and from the downtown core. Pickering sits just on the north shore of Lake Ontario and is bisected by Highway 401 (the busiest highway in Canada and one of the busiest in all of North America), as well as major passenger and freight rail lines.
In 2011, with funding from multiple levels of government and the provincial rail agency, Metrolinx, a bridge project was undertaken to provide pedestrians, cyclists and commuters with the ability to easily traverse these major east-west transportation corridors in a comfortable and, some might say, stylish fashion. (See Figure 1 for an example of the artistic nature of the design.) At over $20million (CDN) the span is intended to help connect the City of Pickering to the major transportation systems in Southern Ontario, and is eyed as an impetus to help transform the downtown area of the city by creating pedestrian access across the transportation corridor. Additionally, the bridge is designed to allow easy access to the commuter rail station/platforms, parking lots and shopping centers that are adjoining the highway and rail lines.
Modern Design and construction Methods
AECOM Canada is responsible for the overall design and construction of the two-phase project, with Teeple Architects providing the preliminary design and APlus General Contractor acting as the lead construction firm. AECOM used Revit Building Information Modeling (BIM) software to model the complex design and employed Integrated Project Delivery (IPD) techniques for construction. BIM-dependent IPD aids in the communication with the sub-contractors/sub-trades to estimate material delivery quantities and to provide a method for feedback on design. IPD helps smooth the "hand-off" points between the trades as well.
Generally, BIM’s are leveraged throughout the design-build and even extend into the full life-cycle of the construction project continuing on as a "facilities management tool" extending into operations and maintenance aspects of the facility after completion of construction. The BIM can sometimes be used as an accurate as-built document, especially when scan data is available to compare to design and then used to update the model layers. While using BIM-based IPD makes the handoff between sub-trades much easier and minimizes confusion, sometimes there are complexities in the design where adjustment is required to meet the requirements during construction. Using a LIDAR scanner can aid in determining if there are slight discrepancies between the design model and the actual as-built conditions. This is especially helpful where tight tolerances are concerned.
A Unique Design
The fully accessible and completely enclosed Pickering Bridge is the first of its kind in Canada. It stretches over 800 feet (about 240meters). A standard truss-based steel structure with concrete flooring and membrane-on-steel-deck roof, the bridge’s interior has customized glazing that provides a comfortable interior environment. It is also the first pedestrian bridge to be covered in highly specialized perforated aluminum sheeting (cladding) called Kalzip.
Utilizing a passive ventilation approach, the Kalzip material resists solar gain, easily sheds snow/ice (before accumulating), reduces wind-loading, and due to the perforations, serves as a transmissive material for light and air. In addressing the unique construction challenges of the glazing within the structure and also the compound curves required in the design of the Kalzip cladding, it fell to the use of terrestrial LIDAR to assist in providing project cost and time savings by verifying the as-built situation for two different aspects of the construction.
Verification Through Measurement
Applied Precision Incorporated (API, www.appliedprecision.ca) is a growing multi-disciplinary 3D digitizing services firm located in Mississauga, Ontario. They specialize in all aspects of 3D measurement science. Complicated metrology projects across the globe keep them very busy.
API was requested by the subcontractor (Flynn Canada) responsible for the glazing enclosure system to provide high-accuracy 3D data for the areas where the glazing was to be installed. The glazing is custom manufactured and then delivered to the project site for installation. Comprehensive and precise measurement data was required to safely and accurately install large glass panels for the walkway. If a discrepancy in dimensions between the BIM and the as-built condition occurs then the glass cannot be used and it has to be scrapped, thereby increasing materials costs and potentially causing delays while waiting for replacement sections to be manufactured.
API are in the vanguard of an increasing number of metrology and scanning firms who are figuring out how to navigate the messy world of extracting models from point clouds, and delivering them in a useful format for architecture/ engineering/construction firms (A/E/C). They are learning how to successfully contend with delivering solutions to companies in an environment where there are a plethora of third-party software packages, assorted plug-ins, and many CAD platforms. The demand for 3D information by contractors and sub-contractors is increasing, especially as more and more design-build projects move to a BIM-based approach. According to Sean Belshaw, Technical Services Manager at API, they are seeing a significant increase in requests from the A/E/C community to play a role in documenting as-built conditions to help verify tolerances and ensure quality.
Measurement by LIDaR for Phase I
According to Tarjit Bassi of Applied Precision, Flynn Canada has a longestablished relationship with API covering multiple projects where the value of laser scanning for as-built information was clearly proven. On that basis, when presented with the challenges of installing specialized glazing in the unique box structure of the bridge, Flynn once again turned to API to solve the problem of ascertaining precise dimensions. Without the accurate metrology data, scrap rate on glass installation would have cost time and significantly impacted the tight budget.
Originally, it was felt that perhaps only a total station would be required to perform the metrology. However, upon reviewing the tight dimensional requirements, sophisticated design, and having made reconnaissance visits to the site, the API team realized that the best technology for the job was terrestrial LIDAR.
Measurement by LiDAR for Phase II
After being contracted to provide asbuilt CAD models for the glazing installation in the first phase of construction, API was called back by the general contractor, APlus General Contractor Inc. APlus has extensive professional experience working on complicated and innovative construction projects across Ontario. After seeing how the terrestrial LIDAR information reduced the uncertainty as to the exact dimensions of the glazing openings, APlus believed that a similar approach could be taken to reduce scrap rates in installing the unique Kalzip cladding materials.
The Kalzip cladding is like a sheath or "skin" over the "bones" (properly known as purlins) that are the structural components according to APlus Project Manager, Fiaz Kara. The Kalzip is cut and form-rolled on-site into compound curves, then installed. The individual sheets are then literally "zipped" together with a proprietary tool from the manufacturer. With an intricate perforation pattern and a unique proprietary connection, the individual sheets must be cut extremely accurately to be made to fit the mounts properly and mesh together. Even the slightest difference with respect to the anchor point can lead to an alignment problem and possibly render the material as scrap.
Complex Site and Conditions
Everything from traversing 14 lanes of heavy vehicle traffic and 5 sets of rail lines, avoiding commuters on foot, commuter and freight train traffic, to freeze-thaw weather to working around other sub-contractors presented significant challenges to the API crew. Additionally, they needed to employ a high-lift to reach certain areas to place targets and conduct scanning operations so the complete extent of the "ribbing" (purlins) could be kept entirely within the field-of-view (FOV) of the scanner. Sean Belshaw of Applied Precision pointed out that the scanner allowed the team to collect data safely from many positions without interfering with traffic or putting the team in harm’s way with minimal site visits.
Targeting accuracy
API realized that in order to achieve the tight measurement accuracies required, they would need to install targets throughout the areas to be scanned. Using standalone scans would achieve an approximate measurement accuracy of 20mm. By installing targets throughout the scan scenes, accuracy was increased to around 3-6mm. Once the data is collected, API post-processes it and runs an auto-alignment software tool to generate a uniform point cloud. The point cloud is then run through extraction software where the objects of interest (purlins) are output in solid model format for the contractor.
According to Fiaz Kara of APlus, without laser scanning there would be no other method to get the Kalzip cladding material to accurately fit on the first go. The alternative (to scanning the purlins and generating accurate CAD models from the point clouds) was to iterate test fits through a sort of "trial and error" approach. APlus, who has a great reputation for completing projects on-time and on budget, were concerned that test-fitting without CAD models and purlin radii values of the as-built conditions would lead to a high scrap rate. While there is almost always scrap in any construction process, APlus estimated they would realize a very significant reduction in waste by having a precise model of the purlins to which the Kalzip sheeting would be anchored.
Contractors Benefiting From Use of LiDAR in BIM Methods
To ensure construction projects are on time and on budget, contractors are opting to use relatively low cost scanning to ensure that as-built conditions match the BIM. If there are discrepancies, then the contractor is able to make adjustments, usually on-site, thereby avoiding or at least minimizing wastage/ scrap. Scanning is cheaper than wasting materials at the job site and less costly than delays incurred while performing re-work of the construction. Contractors and sub-contractors are slowly beginning to trust the models (up to a point) and will cut, form, weld and pour materials according to the BIM provided. But for now the construction firms and building trades are taking a cautious approach. As Ronald Reagan once said, the contractors will; "Trust, but verify" Expect to see more and more A/E/C firms and subcontractors hiring scanning/metrology firms like API to do just that.
Bill Gutelius is the President and co-founder of Active Imaging Systems (AIS). He consults on LIDAR technologies and their applications for commercial and government clients.
Contributors:
(From Applied Precision Incorporated) Sean Belshaw, Manager, Technical Services Leo Perez, Application Specialist Monika Sudol, Senior 3D Application Specialist Tarjit Bassi, Technical Sales Specialist
Special thanks to: Fiaz Kara, C.E.T., Project Manager, APlus General Contractor Inc.
A 3.643Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE