Getting a new angle on assessing buildings
With plans underway for a new Metro system in Dublin, the focus is not only on how to excavate the underground tunnel, but what kind of impact the dig might have on buildings overhead. Now a project funded by Science Foundation Ireland has come up with a new angle on predicting the potential for damage, and providing information that can inform engineering strategies for Metro North.
The technology centres on new data collection, processing, and transformation methods for a remote sensing technique called aerial laser scanning or Light Detection and Ranging (LiDAR), which is carried out using low-flying aircraft.
“It operates by firing a light source at something and measuring the time it takes the light to bounce back. Since we know the speed of light, the distance to the object can be determined,” explains Dr Debra Laefer, Head of the Urban Modelling Group at University College Dublin’s School of Architecture, Landscape and Civil Engineering.
Traditionally, LiDAR has been collected and processed to with little interest or capability to capture building facades, explains Dr Laefer. But she had another idea – by flying at 45 degrees to the buildings, employing a calculated overlap, using the aircraft’s position as a new mechanism for more precise data processing it would be possible to generate vertical data with unprecedented detail.
“This data would be far more valuable than that previously collected in terms of determining building features,” she says. “A key component is in window detection as this greatly influences a building’s stiffness, and this characteristic requires gathering high quality vertical data along each building’s façade.”
The Dublin Metro project was one of the key motivating factors in trying to come up with a better way to document building characteristics along its proposed route, says Dr Laefer.
“The best possible method from an engineering point of view to predict potential damage to buildings in advance of tunnelling, is by going to the building and measuring and modelling it in detail (spending up to one week per building for surveying and generation of a computational model), but with thousands of potentially impacted buildings along a tunnel route this is not financially or temporally viable” reported Dr. Laefer.
So Dr Laefer had the idea that aerial data collected from a helicopter-based LiDAR could be used to automatically generate the data meshes.
“The flight paths can be planned in order to best collect vertical façade data on buildings of interest,” she explains. “The data is then ‘cleansed’ – a process being still worked on and perfected – so that data points related to each building can be identified individually.” Then the data points can be linked to boundary walls and used to automatically develop a data mesh and model to predict the structure’s response to various ground movements.
“Dublin’s strict zoning laws with respect to height and its high prevalence of unreinforced masonry buildings makes this the perfect testing ground for this technology as buildings are not heavily shadowed by each other and their structural features are largely visible, in contrast to a place like New York City, where the combination of short and tall buildings and the predominance of reinforced concrete and glass-clad steel structures pose further challenges,” says Dr Laefer, who notes the potential scale of the savings.
“Given that €4.6 million was paid out in damages following the construction of the Dublin Port Tunnel, it is clear that costs associated with damages from the Metro North project could be much higher. The funding for this project by SFI was €460,000, setting a relatively low damage payout threshold to benefit the taxpayer for damage prevention during Metro North’s installation. The other benefits are that this technology can place Ireland right out at the forefront of creating engineering-enabled 3D urban systems, and this can help industry and public bodies to do better work.”
Dr Laefer has now filed a patent on the data-processing portion of this work to exploit latent information in the LiDAR data, and she suggests that the technique could also lay the basis for a new generation of tools that could help provide emergency services with better data to prioritise interventions after disasters such as earthquakes.
