When New York City’s 432 Park Avenue building was completed in 2015, it was the third tallest tower in the United States and the Western hemisphere’s tallest residential building. It also had the distinction of being the first supertall pencil tower in New York.
The structure rises straight up from a perfectly square (“The purest geometric form,” according to architect Rafael Viñoly) footprint measuring just 28.5 meters (93.5 feet) per side. That’s about 814 sq. meters (8,760 sq. feet)—for comparison, the Burj Khalifa’s footprint is about 8,000 sq. meters.
A building this tall and this skinny is very nearly in a class by itself and calls for advanced construction techniques. One task in particular is simple in conception but extremely challenging in practice: keeping the building plumb.
Working with Gravity
Many confounding factors affect supertall verticality and most are dynamic, changing from hour to hour. Some of the most important include thermal load (the differing expansion rates of sunlit and shaded sides of a building), wind pressure (remember, each side of the completed building will be like a giant, 131,000 sq. foot sail), and crane loading and movement during construction. More subtle factors include slight variations in concrete settling, and even the variations, within tolerance, of steel work.
During the building’s construction, Adam M. Cronin, lead surveyor for Roger and Sons Concrete on 432 Park Avenue, really needed to know—in real time if possible—where the building was compared to design, and how it was responding to various loads.
For conventional urban construction, even skyscrapers, this task is relatively simple. Ground level control is transferred to permanent marks on surrounding buildings, and those marks are used as references for formwork positioning, steel assembly, and other layout tasks. Sometimes buildings are kept plumb with sightings through slab penetrations. But these methods won’t work on a supertall. For one thing, they don’t scale well—the need for vertical alignment information is so critical and urgent that optical measurements are simply not fast or accurate enough. It can take several hours in typical ground reference systems to take all needed measurements and perform calculations for a high altitude positional fix.
More obviously, 432 Park Avenue quickly rose well above nearby buildings, making nearby optical references useless. “We’re literally in the clouds up here,” Cronin pointed out while construction was under way. “Some days, we can’t see the street or even other buildings.”
So on this project, Cronin used the most recent iteration of the core wall survey control system first developed by Leica Geosystems for use on the Burj Khalifa, and proven several times since, most notably on One World Trade Center.
Real-Time Data Streams
In essence, the system combined real-time data streams from several sources:
- Positional data from four GPS/GNSS receivers, posted near the corners of 432 Park Avenue’s outer formwork platform (also known as the “cocoon”).
- Continuously monitored optical data, derived from total station shots on 360-degree prisms mounted just beneath the GPS/GNSS receivers. This data gave feedback on the building’s frame and shape, and the prisms were also used as resection points when doing layout and form positioning work. The prisms were “active” control points, moving upward as construction progressed.
- Dual-axis inclinometers installed in the building’s basement and at regular intervals of about 10-12 floors. Able to measure displacement to +/-0.2 inch of arc, the inclinometers measure movement due to the weather, crane loads, concrete placement, cocoon jumping and so on and can be left in place after construction to provide continuous monitoring.
- As construction progressed, a weather station was added to tie in real-time wind and temperature information with the GNSS/RTK observations and tiltmeter data.
All this data was combined and processed in a customized implementation of an advanced network RTK solution that automatically applied the complex transformation between “ellipsoid normal” (vertical relative to the WGS84 ellipsoid) and “gravity vertical” (vertical on the job site, better known as “plumb”).
“As an exercise, we performed the same positioning work at twilight and in the afternoon and found that the differences were negligible. And even during the polar vortex weather, which was super cold, we always had good signal and good results. It was a very reliable system.”
All results could be accessed continuously, and a “solid solution” was provided each hour. Cronin could check the figures and be confident he knew exactly how the building is placed within two hundredths of a foot. Over time, he could develop a sense of how the various construction and weather loads affect verticality from day to day. If needed, he could adjust form positioning to make corrections. It was a surprisingly speedy process. “We were completing a lift every three days,” Cronin says. “And that’s fast for this type of construction.”
In sum, the core wall survey control system frees supertall construction teams from the need to tie to ground references. Building control is independent of ground control, and the team can generate precise coordinates as needed, compare these to design coordinates, and correct the building’s vertical alignment incrementally to keep walls plumb.
The core wall survey control system consists of consultation, training, installation and ongoing management of the data. It’s a modular system, and the components can be acquired based on the user’s needs. As BIM and real-time monitoring increasingly become tightly integrated in supertall construction, the core wall survey control system will continue to be refined to be even more effective.
Proving GPS for Building Construction
432 Park Avenue was Cronin’s first opportunity to use a GNSS-based system on a major building construction. He admits he was a little uncomfortable at first. “I didn’t want to “flip the switch” too soon,” he says. “Coming from a more traditional surveying background, it was important to me to test the system against ground references.”
Fortunately, he was able to do just that for several months. Cronin essentially doubled up on verticality control during construction of the first 20 floors. That is, he started with ground control and a network of prisms on nearby buildings while also installing and using the core-wall alignment system.
“We compared results every floor,” he explains. “And by the 20th floor, as I lost the ability to use references I was used to, I was already super confident—the GNSS system always checked out.” In fact, as the building rose, multipath issues were eliminated and GNSS coordinates became even more reliable.
Cronin established some good routines for working with the core wall system. For instance, he learned to use overnight results as the basis for layout work to balance thermal loads and avoid cranes shifting a lot of weight around. He preferred to do precise layout work in the twilight hours, when the site was calmer and there was less movement in the structure. Still, he found that he could get consistent results at any time of day.
“As an exercise, we performed the same positioning work at twilight and in the afternoon, and found that the differences were negligible, which surprised me,” Cronin says. “And even during the polar vortex weather, which was super cold, we always had good signal and good results. It was a very reliable system.”