ECB’s Double Tower Challenges Surveyors

The European Central Bank (ECB) is constructing its new headquarters on the site of the historic Großmarkthalle (wholesale market) in Frankfurt. The design of Vienna-based architects COOP HIMMELB(L)AU consists of three main elements: the former Großmarkthalle with new internal structures; a 185-m (610-ft) asymmetric double office tower with connecting atrium; and an entrance building. The entrance building creates a visual and functional link between the high-rise and the Grossmarkthalle and houses the press conference area.

Once ready for occupancy, the facility will provide employment for around 2,300 workers, mainly in the office towers. The restored Großmarkthalle will largely retain its appearance and provide a historic venue for the ECB’s public functions, with a lobby, visitor center, conference area, library, exhibition spaces and restaurant. Gross floor area of the premises—scheduled for completion by the end of 2013, with staffing by 2014—will total approximately 185,000 m² (45.7 ac or 1.99 million ft²)

Creative Design

The design of the double office tower is creative, to say the least. The north tower, whose east side is inclined by 9 degrees, begins on a rectangular base with sides of 16x79 m (52x260 ft); by the 45th floor, it has become a trapezium with side lengths of 8x59x25x56 m (26x193x82x184 ft). The south tower is just the opposite: It starts at the base as a trapezium with side lengths of 5x55x27x59 m (16x180x89x194 ft), changing to a rectangle with sides of 15x55 m (19x180 ft) by the 43rd floor.

The special architecture of the towers, with inclines, overhangs and continuously changing floor plans, means that the whole structure looks completely different when viewed from different angles. It also places high demands on the surveying team.

Werneck-based engineering company Gemmer & Leber was responsible for the survey work. “A structure with such unusual architecture would be a challenge for any surveyor,” says Dipl. Ing. Willi Almesberger, who developed the surveying concept for the new ECB premises at Gemmer & Leber. “But there was an additional difficulty in the case of the ECB building: The deconstructivist design of the two towers meant that, as construction progressed, the increased loads not only caused the usual settling, but also tilting and twisting of the towers.”

These unavoidable deformations were particularly noticeable in the lower floors, and by level 25 they had reached a maximum value of 16 cm (0.52 ft). To compensate for them and finally achieve a dimensionally stable structure, the whole building had to be specifically constructed to oppose the expected deformations. The structure was therefore predeformed during construction—in other words, the plans were modified to proactively compensate for the torsion-induced deformation—so it would achieve the specified dimensions when all the loads acted together.

The Right Equipment

The complex geometry and required measurement accuracy of +/- 8mm (0.03 ft) in core areas, +/- 6mm (0.02 ft) for other setout points and +/- 5mm (0.02 ft) for flatness presented a significant surveying challenge. To meet this challenge, Gemmer & Leber used five calibrated S6 total stations from Trimble which achieve an accuracy of 0.3 mgon (1 arcsecond) for direction measurements, enable distance measurements to be made with an accuracy of 2 mm+2 ppm and compensate for levelling errors up to an accuracy of 1.5 mgons (5 arc seconds).

“Trimble total stations are very user-friendly and have an almost self-explanatory menu operation,” says Dipl. Ing. Bernhard Engelbreit, project manager for the surveying work. “Using robotic total stations with cable-free Trimble TSC2 controllers, our surveyors could control the instruments remotely in most cases, enabling us to be at the actual measurement point instead of with the instrument.” Points could therefore be surveyed very quickly, making it possible for the surveying work to keep up with the progress of ongoing construction. Adds Engelbreit: “It would not have been possible to meet the requirement to complete a new floor every six days using different equipment.”

Engelbreit’s team first set up a coordinate system for the site. At the beginning of construction work, reference points were established in the form of fixed posts; these reference points were later placed in the structure, when building progress meant they could no longer be seen. To do this, the surveyors set up four plumbing points in the basement of the south tower, six in the north tower and four in the atrium to form the new reference network. The coordinate system was aligned on the grid of the lowest level, with height control carried out by fine levelling.

By means of openings in each slab above the plumbing points, permanently mounted zenith lasers could transmit reference points to the relevant working level. The vertical (or “plumb”) lines allowed reference points to be transmitted to the top edge of the concrete slab when a floor was completed.

The plumb lines were set up to be as free from distortion as possible, and were isolated from the towers’ deformation due to solar radiation, wind or the action of loads from cranes and concrete pumps. This meant the Gemmer & Leber surveying team had to start carrying their total stations to work at 4:00 am to ensure they were ready when construction work began at 7:00 am.

Dimensional Stability

To ensure the required predeformation was achieved, the deformation parameters of each individual floor had to be considered when setting out the individual points. The surveyors received these parameters from the technical office of Ed. Züblin AG from Stuttgart, the company in charge of construction for the ECB project.

The initial point was shown in a CAD plan, which contained the specified dimensions of the completed building.

Almesberger explains the procedure: “The structural engineers specified the deformation parameters for each level, such as rotation points, rotational angles and translation vectors. We then transferred these parameters to the measured points of the CAD plan, and changed the original floor plan to compensate for the expected load deformations.”

The resulting adjusted reference points were measured out from the CAD plan and entered into the total stations. A whole range of measurement points could now be set out on slab edges, columns, recesses or concrete cores, all corrected by the amount of predeformation.

Due to predeformation, almost none of the approximately 25 columns per floor were poured exactly vertical. Each column was erected with a unique inclination; even columns that were to be upright in their final state sometimes had to be poured with inclinations of up to 1.5 cm (0.6 in).

Each individual column therefore had to be individually measured. The measurement points referred to the upper edge of the next concrete slab. The predeformation parameters were accounted for in each of the measurement points. That presented an additional challenge: The top points of the columns were indicated in the plans to align with the column formwork, but in fact they were present only after the 30-cm (12-in) slab had been completed.

Saving Time and Effort

“To set out these initially virtual points and place the formwork in the correct position, we used the total station’s ‘3D axis’” mode,“ explains project manager Engelbreit. “This made it possible to set out inaccessible virtual points, as they were correctly calculated in the total station from real points which were accessible on the formwork.”

This conversion could have been done at a desk, but that approach would have entailed an additional effort of roughly 24 hours per floor. Construction work would have been significantly delayed, as individual work steps were closely interlocked. For example, all concreting work could only be carried out after measuring the column formwork. Delays in surveying would therefore have inevitably caused delays in the whole construction project.

“The ECB project was extremely demanding in all respects, with unusually intensive surveying requirements and a tight schedule,” concludes Almesberger. “The fact that we could manage this huge and complex project without significant problems is due to our team’s commitment and our excellent equipment. The well-designed, practical functions of Trimble total stations made our work considerably easier, saving us a lot of time and effort. They also helped us stay on schedule and complete the preliminary building work for the new ECB premises in the best possible time.”