So why has there been a switch away from steel in favour of concrete construction? Concrete construction has always had an inherent advantage for tall building design in that it provides a significant amount of stiffness, mass, and damping for the structural system, three factors that are critical in controlling building motions and accelerations. However, even with these advantages, it was not until the more recent advancements in material technology that concrete systems took the lead in high-rise construction. High-strength and high-modulus concrete mixes required for tall buildings were once difficult and expensive to achieve but have become more conventional and cost effective.
Further, advancements in construction technology have assisted in making concrete easier and faster to place. It is now possible to pump high-strength concrete more than 600 m in a single stage, allowing for supertall applications. Additionally, utilizing such construction methods as self-climbing formwork systems, reusable formwork liners, panellized horizontal formwork systems, prefabricated reinforcement cages, and state-of-the-art concrete pumping systems allows the speed of concrete construction to compete with if not match the speed of steel construction, taking away one of the previous advantages to utilizing an all-steel solution. For these reasons and others (reduced floor-to-floor heights and inherent fire-proofing), it is becoming increasingly common for designers and contractors to choose concrete for high-rise building construction. This article will explore the use of concrete in high-rise construction by focusing on three SOM projects that are currently under construction or have recently been completed: Burj Dubai, Trump Tower (Chicago), and Infinity Tower (Dubai).
The Burj Dubai is the tallest building in the world. It is the centrepiece of a development located just outside of downtown Dubai. The project consists of the tower itself, as well as an adjacent podium structure, and separate twelve-storey office annex and two-storey pool annex. The 280,000 m2 multiuse tower is predominantly a residential and office usage, and also contains retail and a Giorgio Armani Hotel.
The primary design concept of the tower is an organic form with tri-axial geometry and spiralling growth that can be easily seen in the final design. The floor plan of the tower consists of a tri-axial, Y-shaped plan, formed by having three separate wings connected to a central core. As the tower rises, one wing at each tier sets back in a spiralling pattern, further emphasizing its height. The Y-shaped plan is ideal for residential and hotel usage, in that it allows the maximum views outward without overlooking a neighbouring unit.
The spiraling Y-shaped plan was also utilized to shape the tower so as to reduce wind forces, as well as to keep the structure simple and foster constructability. The tower is primarily a reinforced-concrete structure, with concrete construction utilized for the first 155 stories, above which exists a structural steel spire. The structural system can be described as a buttressed core, and consists of high-performance-concrete wall construction. Each of the wings buttresses the others via a six-sided central core, or hexagonal hub. This central core provides the torsional resistance of the structure, similar to a closed pipe or axle. Corridor walls extend from the central core to near the end of each wing, terminating in thickened hammerhead walls. These corridor walls and hammerhead walls behave similarly to the webs and flanges of a beam to resist the wind shears and moments. Perimeter columns and flat plate floor construction complete the system. At mechanical floors, outrigger walls are provided to link the perimeter columns to the interior wall system, allowing the perimeter columns to participate in the lateral load resistance of the structure; hence, all of the vertical concrete is utilized to support both gravity and lateral loads. The result is a tower that is extremely stiff laterally and torsionally. It is also a very efficient structure in that the system resisting gravity loads has been utilized so as to maximize its use in resisting lateral loads. The tower is founded upon a 3.7 m thick pile-supported raft, poured in four segments totalling 12,500 m3 of C50 (cube strength), self-consolidating concrete. Supporting the raft are 194 bored cast-in-place piles, 1.5 m in diameter and approximately 43 m in length.
High-performance concrete was utilized for the tower, with wall and column concrete strengths ranging from C80 to C60 cube strength. Additionally, in order to provide increased stiffness to the system, C80 wall and column concrete was specified as a high-modulus concrete. The concrete mix was designed to provide low permeability and high durability. One of the most challenging concrete design issues was ensuring the pumpability of this concrete to reach the record height of the tower, which necessitated that concrete be pumped well over 600 m in a single stage. Four separate basic mixes were developed to enable reduced pumping pressure as the building gets higher. The maximum allowable aggregate size decreased with building height, requiring no more than 10 mm aggregate above level 127.
A horizontal pumping trial was conducted prior to the start of the superstructure construction in order to ensure the pumpability of the concrete mixes. This trial consisted of a long length of pipe with several 180-degree bends to simulate the pressure loss in pumping to heights over 600 m in a single stage. The final pumping system utilized on site employs Putzmeister pumps, including two of the largest in the world, capable of concrete pumping pressure up to a massive 350 bars through a high-pressure pipeline (150 mm).
The Burj Dubai Tower utilizes the latest advancements in construction techniques and material technology. The walls are formed using Doka’s SKE 100 automatic, self-climbing formwork system. The circular nose columns are formed with circular steel forms, and the floor slabs are poured on MevaDec panel formwork. Wall reinforcement is prefabricated on the ground in 8 m sections to allow for fast placement. Three primary tower cranes are located adjacent to the central core, with each continuing to various heights as required. High-speed, high-capacity construction hoists are utilized to transport workers and materials to the required heights. A specialized GPS monitoring system has been developed to monitor the verticality of the structure, due to the limitations of conventional surveying techniques. In order to organize the construction, the floor plate was divided into zones. In each zone, the contractor, Samsung, utilized an “up-up” approach with the walls leading the way and the slabs following. The construction sequence for the structure has the central core and slabs being cast first, in three sections; the wing walls and slabs come next, followed by the wing nose columns and slabs. Concrete booms attached to the jump-form system distribute concrete to each wing.
Utilizing concrete construction for the Burj Dubai was a natural choice. Concrete offers higher stiffness, mass, and damping for controlling building motions and accelerations, which was critical in designing the world’s tallest building. In fact, due to the stiffness of the system, SOM was able to design the tower to satisfy motion and acceleration criteria without the use of supplemental damping devices. Additionally, the tower’s flat-plate floor construction offers increased flexibility in shaping the building, as well as provides for the minimum possible floor thickness in order to maximize the ceiling height. Finally, the design of the structural system and the selection of its construction methods also allowed the tower to be constructed quickly and efficiently.
The Trump International Hotel and Tower in Chicago recently reached its full height of 353.8 m (423.2 m including the spire). Trump Tower is the tallest concrete building in the United States, and the tallest building to be built in North America since the completion of Sears Tower in 1974. Retail space (9,300 m2), parking for 1000 cars, 472 condo units, 286 hotel units, and a health club are incorporated in the tower’s 241,550 m2. The building features setbacks at levels 16, 29, and 51 that correspond to the top elevations of prominent neighbouring buildings, providing visual continuity with the building’s surroundings. These buildings include the historic Wrigley Building on the east, Bertrand Goldberg’s
Marina City to the west, and Mies van der Rohe’s IBM Building located directly across Wabash Avenue. Based upon the phased-occupancy plan, the hotel has been in operation since early 2008, well before the topping out of the structure in the fall of 2008. Rigorous engineering, combined with state-of-the-art materials, has provided the owner with a world-class building. Project innovations came in the realms of advanced materials, scale and complexity of the structural systems, and scheduling benefits for the owner.
The 92-storey tower is structured entirely out of conventional cast-in-place reinforced concrete. Typical residential floors are of 225 mm thick reinforced concrete: the flat-plate construction spans up to 9 m without perimeter spandrel elements. This construction minimizes the structural depth of the floor, allowing higher ceiling heights. At the lowest of three building setbacks (level 16), transfer girders allow for a column-free space at the ten parking levels. The lateral system for the Trump Tower consists of a reinforced-concrete core and outrigger system. Large outrigger elements at the mechanical levels tie the concrete core to perimeter columns, thus engaging the perimeter columns in the building footprint, and increasing the building’s lateral stiffness, as well as its resistance to overturning due to wind. Perimeter belt walls at the roof and the three mechanical levels provide additional torsional stiffness and redundancy, and equalize column loads along the perimeter. The tower foundation consists of a 3 m thick reinforced-concrete mat supported on rock caissons. It is believed that the 3,825 m3 mat foundation pour for the Trump Tower represented the largest single self-consolidating concrete placement in North America to date. Concrete was chosen as the primary structural material for the Trump Tower to take advantage of the lower floor to floor height and construction costs. SOM specified a series of high-performance concrete mixtures for the structure. Concrete cylinder strengths of 80 MPa at 90 days have been specified for all vertical column and wall elements up to level 51. Local areas in the outrigger zones, however, require 110 MPa concrete at 90 days. For the typical gravity framing, 35 MPa concrete was specified. To reduce congestion, all primary reinforcing bars in the outrigger levels are U.S. grade 75 yield strength. The benefits of the utilization of high-strength concrete were twofold. The high strength limited the size of the elements resisting vertical loads, controlling the weight of the building, and resulting in residential units with smaller vertical obstructions. Secondly, the inherent increase in modulus of elasticity of high-strength concrete allowed the designer to utilize the overall stiffness of the structure to control building accelerations and occupant perceptions in wind events, without supplemental damping measures.
In a distinctive application of architectural expression through structural form, the Infinity Tower twists a full 90 degrees from its base to its crown, 305 meters above the ground, through a series of incremental plan rotations at each level. The architects proposed the tower’s twisting geometry as a means to create an iconic tower on a prominent site. Near the base of the tower, the marina is the primary view. Ascending up the tower, views to the gulf take precedence and the building geometry is a response to this topography.
The tower is founded upon a 3 m thick, reinforced-concrete mat foundation that is supported by 99 bored, cast-in-place reinforced-concrete piles (Ø 1.2 m) extending approximately 30 m below the mat foundation. The tower’s lateral load-resisting system consists of a combination of a moment-resisting perimeter tube frame and a circular central-core wall system, connected by the two-way spanning reinforced-concrete flat plates at each level acting as diaphragms. This system maximizes the effective structural footprint of the tower by utilizing a significant amount of the vertical reinforced concrete for lateral load resistance. The design philosophy is based upon the exterior form of the building as a direct expression of the structural framework.
The engineers studied a series of options for the perimeter frame in order to create the unique twisting geometry of the tower. As the perimeter columns ascend from storey to storey, they lean in or out, in a direction perpendicular to the slab edge. At every level, the columns shift in position along the spandrel beams so that each column maintains a consistent position at each floor relative to the tower envelope. The corner columns and the six interior columns twist as they ascend.
In order for the Infinity Tower to be realized as a viable structure, the seemingly complex building form must ultimately be derived from a structure that is easy and efficient to construct, and which leads to practical architectural floor layouts. The system as described above offers significant construction simplification through formwork repetition, which directly impacts the construction cycle time. Also, this system leads to residential floor layouts that are repetitive at each level despite the twisting nature of the building form.
The circular central core walls, which ascend purely vertically, are cast using a slip-forming system operating through incremental but not continuous advancement, in order to remain ahead of, but in phase with the construction of the perimeter frame of the tower. The perimeter concrete tube frame (columns and spandrel beams) and slabs are cast using an interlocking aluminium formwork system by MFE (Mivan) formwork. The perimeter column-stacking configuration means that the forms are identical at each story, simplifying the formwork-erection operation. A construction cycle lasts six to seven days per storey.
Cast-in-place reinforced concrete was selected as the primary construction material due to its ability to accommodate the geometry of the tower and its mass and stiffness characteristics, which aid in the reduction of wind-induced movement of the tower. Particularly for Infinity Tower, cast-in-place concrete offers the geometric flexibility required for construction of the unique building form. In addition, it is a cost-effective and practical option for construction in Dubai. The 73-storey Infinity Tower is currently under construction and scheduled to open in 2011. Infinity Tower will stand as an iconic centrepiece for the Dubai Marina.
