Chicago firm collaborates to design Spain’s tallest building

The Torre Repsol high-rise building was designed by the architectural firm Foster and Partners to be the new corporate headquarters for Repsol YPF S.A., Spain’s largest oil company. The tower—located in Madrid on the former training grounds of the Real Madrid soccer team—is part of a new business park called Cuatro Torres, which includes three other new office towers. At 250 meters (820 feet), Torre Repsol will be the tallest of the four new buildings, as well as the tallest in Spain.

Halvorson and Partners of Chicago collaborated with Foster and Partners to design a unique and iconic building, which would be used to consolidate the oil company’s many smaller offices into one central location. Ultimately, the tower’s design would include five parking levels below grade and 34 office floors (a total of approximately 110 square meters) divided into three distinct office blocks of 11, 12, and 11 floors. Each office block is supported on a set of two-story steel trusses that span between two reinforced concrete cores.

The trusses transfer all of the tower’s gravity loads to the two cores, which are the only vertical load-carrying elements that extend to the foundation. The trusses also link the cores together, and in essence, behave as a large moment-frame to resist east-west lateral forces. The typical office floor plate cantilevers to the north and south of the cores with only two exterior columns on the north and south faces.

Foundation

Buildings in Madrid are typically founded on drilled piers that bear on a stiff clay layer called Tosca. At the Cuatro Torres site, the Tosca clay is approximately 20 meters below grade, and it was presumed that a mat foundation supported on drilled piers would be the appropriate foundation.

However, since there are five basement levels on the site, the bottom of the mat foundation would be located about 22 meters below grade, within the stiff Tosca clay. Further study of the possible foundation systems proved that a mat supported on the drilled piers would be approximately the same size as a mat bearing directly on the Tosca clay. The latter foundation was chosen and a single reinforced concrete mat (72 meters by 44 meters by 5 meters) was used to support the two reinforced concrete cores of the tower. Although settlements for the drilled pier-supported mat were marginally less, the anticipated settlements for the shallow mat foundation system were found to be acceptable (approximately 5 centimeters).

Concrete cores and transfer trusses

The two reinforced concrete cores, located on the east and west sides of the building, are the only vertical load-carrying elements of the tower that extend down to the mat foundation; achieving one of the owner’s objectives—a column-free lobby. The eight gravity-load columns on the typical office floor plate are transferred to the cores by three sets of two-story-deep trusses. In plan, each core measures 22 meters in the north-south direction and 10 meters in the east-west direction; with wall thickness of 1,200 millimeters at the base to 400 millimeters at the top.

North-south lateral loads are resisted by pure cantilever action of two cores, and since the gravity load for the entire building is carried by the cores, there is no uplift or tensions in the core walls, even with an aspect ratio of 11 to 1.

For east-west lateral loads, the cores are too narrow to provide adequate strength and stiffness as pure cantilevers, and the transfer trusses are used to link the two cores together, such that the system behaves like a large moment-frame to resist lateral forces.

At each of the three truss levels, the system of trusses consists of the following: two primary trusses that span east-west—32 meters between the cores; and two secondary trusses that cantilever 10 meters north and south from the primary trusses and transfer the eight gravity columns back to the primary trusses. Ideally, the primary trusses would be simple span between the cores; however, since the primary trusses also interact with the cores to resist lateral loads, the top chord of the truss would need to be connected to the core. Connecting the top chords of the truss to the core walls would induce negative bending moments in the truss under gravity loads, resulting in top-chord tensions at the connection to the core. To minimize the gravity-load negative moments, the top-chord connection of the primary trusses to the core has been detailed to allow horizontal movement; this connection was not fully tightened until the full structural dead load had been applied to the truss. Therefore, in the permanent condition, top-chord tensions only result from live loads and east-west lateral loads.

The connection of the primary trusses to the cores is one of the most critical in the building. Transmitting the large gravity and lateral loads to the cores is accomplished with a robust and positive connection of the truss chords to an embedded, built-up steel column within each core (four total). During erection, the tension force that would develop in the bottom chord of the primary truss actually resolves itself as a horizontal thrust against the cores, since the bending stiffness of the cores is larger than the axial stiffness of the truss chord. The thrust on the cores caused complexity with the diaphragm-to-core connection details of the floors above and below the truss levels. To eliminate this thrust, post-tensioning tendons are provided along the bottom chord of the primary truss and anchored to the embedded column in the cores. In addition to minimizing the axial thrust, the post-tensioning provides a level of redundancy for the critical truss to core connection.

At each level where the truss top and bottom chords attach to the core, a 1,900-millimeter-thick slab is provided within the core. The thick slabs provide a means of engaging the full cross-section of the core to resist the truss chord forces. The 1,900-millimeter slabs are reinforced with both mild reinforcement and post-tensioning tendons in two directions.

Floor-framing system

The structural system for all floors above-grade consists of steel wide-flange beams supporting a composite metal deck slab. All steel floor framing is S355 K2G3/G4 steel (approximately equivalent to ASTM A992). The office floor slabs have 75-millimeter deck plus 75 millimeters of lightweight concrete. The office floor slabs at levels 1, 12, and 24—which correspond to the top chords of the primary trusses—are 75-millimeter deck plus 150 millimeters of normal-weight concrete; the thicker slab was provided to minimize sound transmission from the mechanical rooms.

The floor framing is supported on four interior and four exterior columns, as shown in Figure 1. The girders that span east-west between the columns not only support the floor framing, they also provide a tension tie between the cores, along with the two diagonal members that span from each column to the cores. The tension tie provides a positive connection within the floor diaphragm between the two cores. Since the east-west lateral loads induce bending in the cores and the primary trusses, the bending forces ultimately induce axial forces in the floor diaphragms for several floors above and below the primary trusses. The tension tie provides a load path for these forces.

Vierendeel frame

The architectural design intent was to minimize the number of exterior columns on the typical office floors and eliminate corner columns. This was achieved by providing only two columns on the north and south faces of the building; the columns are spaced 18 meters apart with a 7-meter cantilever to the east and west of each column. To eliminate the columns from the corners, spandrel beams on the east and west side of the building would span from the cores out to the 7-meter cantilevers on the north and south sides.

The two exterior columns on the north and south sides are supported directly on the secondary trusses at the three truss levels; and to minimize the depth of the 7-meter cantilevers, the spandrel beams on the east and west are moment-connected to the core.

A moment connection of the steel spandrel beam to the concrete core wall would have been difficult to erect, so a steel column was placed 150 millimeters away from the core wall to provide moment fixity for the spandrel beam at the core (see Figure 1). The column that is adjacent to the core is connected with a simple shear connection, an easier detail to construct. The perimeter spandrel beams and exterior columns form the vierendeel frame, which minimizes the depth and weight of the steel frame and helps control deflections.

With a steel column located just 150 millimeters away from the core wall, the effects of creep and shrinkage of the concrete core had to be addressed. Since the steel column would not creep or shrink with the concrete core, the core would be transferring axial load to the column over time and overstressing the column and the connection between the column and the core. Because the adjacent steel column is only required to provide bending stiffness for the cantilevered spandrel beam, the axial loads could be released, allowing the core to creep and shrink without overstressing the columns. A vertical slip detail was provided at the mid-depth of the column, approximately the inflection point. The slip detail still allowed for a shear transfer, such that the column could provide bending stiffness for the cantilevered spandrel beam; see Figure 2.

Construction

Construction began in July 2004 with installation of a perimeter slurry wall and excavation of the basement levels that cover the entire site (7,500 square meters). Below-grade construction was completed in June 2005. The super-structure was topped off in mid-October 2007, and completion of the tower is anticipated in 2008. As the four office buildings within the Cuatro Torres business park near completion, the value of the property has increased. The increase was substantial enough that Repsol YPF already sold the tower to Caja Madrid, one of Spain’s largest banks, and has begun designing a new corporate office complex elsewhere in Madrid.

Design & Construction Team

Owner: Repsol YPF (recently sold building to Caja Madrid)
Architect: Foster and Partners, London
Structural engineer: Halvorson and Partners, Chicago
Structural engineer for garage: Gilsanz, Murray and Steficek, New York
Local structural engineer: Arquing
Geotechnical engineer: Ground Engineering Consultants, Chicago, and SGS Tecnos, Madrid
Project management: Gerens
General contractor: FCC and Dragados

Gregory J. Lakota, S.E., P.E., is a principal at Halvorson and Partners in Chicago. He can be reached at 312-274-2403 or at glakota@halvorsonandpartners.com. Arantzazu Alarcon, Ph.D., is a staff engineer at Halvorson and Partners. He can be reached at 312-981-3331 or at aalarcon@halvorsonandpartners.com.

PHOTOS AND CAPTIONS

Figure 1: Typical floor framing plan
Credit: Halvorson and Partners

Figure 2: Vertical slip connection detail
Caption: Gravity loads from the cantilevered floor framing are supported by a simple shear connection to the core walls; moment restraint is provided by the steel column adjacent to the core wall (see inset).
Credit: Halvorson and Partners