ComNav Technology: Building Sweden’s Tallest Tower

When completed, the Karlatornet will be Sweden’s tallest building and redefine the skyline of the city of Gothenburg, rising to 74 stories and 246 meters (the Burj Khalifa in Dubai, currently the tallest human-made structure, is 828 meters high). Ensuring that the building remains stable even when deformed by very strong winds, sun exposure, seismic activity, settling or shrinkage will require very high precision construction methods. To ensure its vertical alignment, the engineers are using the core wall control survey (CWCS) method, which relies on active GNSS control points, and SinoGNSS T300S high accuracy GNSS receivers.

A SinoGNSS multi-constellation T300 GNSS receiver and a 360° prism mounted at the top of a building under construction. (Photo: ComNav Technology)

The CWCS method was first used during the construction of the Burj Khalifa and was subsequently applied in the construction of many other high-rise buildings around the world. Swedish surveying company Teodoliten decided to apply this method for the Karlatornet project. Core walls, which are key structural components of high-rises, require a layered construction approach, with multiple concrete pours for each core wall element. CWCS makes it possible to precisely align these core walls with the vertical axis of the building during construction, using GNSS receivers, total stations, inclinometers, and other tools.

When constructing a tall building, it is essential to continuously measure the positions of various elements at its summit to ensure their vertical alignment. This is typically done by placing at the top of the building four GNSS receivers — referred to in this context as active GNSS control points — each with a 360° prism at the bottom. By sighting the prisms and using the 3D coordinates from the GNSS receivers, a surveyor then sets up a total station. This obviates the need for an extensive array of ground control points, which are often not visible from the top, and for holes in the slabs to accommodate vertical laser plummets. Additionally, observations from a nearby reference station are used to post-process the data from the GNSS receivers in post-processed kinematic (PPK) mode to achieve an accuracy of a few millimeters. The Karlatornet project uses four SinoGNSS multi-constellation T300 GNSS receivers for the active control points.

It is also necessary to correct for the movement of the survey platform as the building’s main axis is flexed by the loads applied to it during construction. This is achieved by placing a series of high-precision dual-axis inclinometers along the core walls and then applying a least squares adjustment that takes into consideration the floor height of the measuring devices.

The SinoGNSS T300 receivers operating in GNSS-RTK mode also will be used to monitor and document post-construction building deformations.