Figure 2. Kingdom Tower Y-shaped plan
The Y-shaped plan forming the base of the design offered several advantages. A 120-degree separation between wings allows for views to be expansive but not directed toward adjacent units, which would have created privacy issues. A Y-shaped plan would provide a higher ratio of exterior wall to internal area, which would maximize the residential units’ views and exposure to light (see Figure 2).
The “Y” shape also allowed the spreading of the base of the tower without increasing the depth of the lease-span. This means that the building can still maintain a reasonable 10:1 height to width ratio without having a large amount of unusable deep internal space. The “Y” shape also helps develop a clean and simple structural diagram in which every element participates in both gravity and lateral support. If one considers the shape as being similar to a wide flange beam with its mass of structural material at the end of each wing (the “flanges”) where it is most optimally placed, connected by shear walls (the “webs”) forming the corridor walls and core, the structural efficiency is maximized by placing the material where it will do the most good in resisting wind-induced bending loads. The triangular core thus formed by the shear walls is also an optimized shape, very good at resisting wind induced torsion. Lastly, the unique nature of the Y-shaped plan and structure allows for certain serendipitous results. Having three legs, the plan naturally has at least three stairs, creating an immediate redundancy. With its continuous, reinforced concrete shear walls, those stairs are completely enclosed within a very robust and secure environment, again enhancing that aspect of the life safety system of the tower. Furthermore, these very thick shear walls provide more than the minimum required fire separation and are a very effective sound-control mechanism between public corridors and private residential spaces (see Figure 2).
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Figure 3. Kingdom Tower tapering shape
Early on, it was also decided that the tower could not just be vertically extruded. Had that approach been pursued, wind vortices would have become organized on the sides of the tower and amplified the effect, inducing accelerations of lateral movement well above advisable levels as these amplified vortices approach the resonant frequency of the tower. To counter this natural magnification, one very typical approach is to reduce the plan size of the tower as it rises in height. On Burj Khalifa, the tower has 27 tiers with setbacks/plan shape reductions at each. Similar to Burj Khalifa, Kingdom Tower “confuses the wind” by reducing the cross-sectional size of the tower as it rises. However unlike Burj Khalifa, Kingdom Tower does it by means of a continuous taper to each wing (see Figure 3). A continuous taper was a result of one of the primary lessons learned in the construction of Burj Khalifa. Eliminating the stepped setbacks as on the Burj obviated the need for outrigger transfers and belt trusses at certain locations, which in turn will reduce the concentration of structural loads as well as permit a more efficient, continuous, and uninterrupted construction process. Furthermore, having different rates of taper to each wing results in different termination heights for each, thereby creating the dramatically distinct three part spire of Kingdom Tower. This intuitive concept was confirmed by wind studies performed by RWDI at its facility in Toronto.
The structure for the Kingdom Tower is comprised entirely of cast-in-place reinforced concrete walls, coupling beams, and conventionally reinforced flat plate concrete floor framing (see Figure 4). The structure contains no outriggers or wall/column transfers. Due to the continuous and uninterrupted vertical nature of the walls for the tower, a highly efficient jump form system can be utilized. Formwork for the floor slabs also can be reused due to the highly repetitive geometry of the tower. The faces of the shear walls are all vertical, with the exception of the ends of each wing, which taper to follow the form of the Tower. This can be accomplished by a simple shift of between 200 and 300 millimeters for each 4 meters vertical lift or internal blockouts in formwork that is lifted vertically. Because the ends of the shear walls slope, the taper at the end of each wing is created by a consistent 3.5 meter cantilever extension of the slab edge. All building envelope notching is achieved by simply dropping off slab edge cantilevers.
Figure 4. Structural components
Through this concept of dropping off structural components to create the architectural expression, and allowing formwork to easily jump up on each level of the tower, a highly efficient and constructible system is realized while creating a very distinctive form.
Concrete compressive cube strengths required to realize the tower structure are not expected to exceed 115 MPa, and 60 to 80 MPa at significant heights. Advances in concrete mix designs and concrete pumping technology will be brought to bear in order to place high strength concrete in the upper portions of the tower.
The foundations for the tower are a raft supported on bored circular piles located approximately 2.5 meters below existing grade due to site modifications raising the base of the tower (see Figure 5). This modification minimizes the amount of potential dewatering required on the site during construction of the lower levels.
Figure 5. Kingdom Tower raft section
As part of a performative design process, any solution should reflect a specific culture, location and time:
In the case of Kingdom Tower, the solution reflects the desire of Jeddah Economic Company (JEC) and the leaders of Saudi Arabia to show the world the potential of their vision of the Kingdom as an increasingly important, vital, and thriving member of the international community. It is also rooted in the desert landscape. The sleek, streamlined form of the tower can be interpreted as a reference to the folded fronds of a growing young desert plant. The way the fronds sprout upward from the ground as a single form, then start separating from each other at the top, is an analogy of new growth fused with technology. As AS+GG partner Gordon Gill explains: “the tower evokes a bundle of leaves shooting up from the ground – a burst of new life that heralds more growth all around it. This symbolizes the tower as a catalyst for increased development around it.”
In terms of the project’s location, studies of sun, wind and views, among many others parameters, informed and guided the design process that shaped the tower. The tower orientation in plan was adjusted to point one wing toward Mecca, while another points almost directly north, by which both the overall solar load was reduced while simultaneously improving the views of the majority of residential units (see Figure 6). As it turned out, that adjustment in orientation also helped mitigate the magnitude of pedestrian level winds in several of the more critical locations, although it slightly increased the overall load due to design wind conditions. Regarding “time,” Kingdom Tower uses current technologies and materials, but also incorporates “future-making” designs. In the field of vertical transportation, for example, the height and program requirements will be a challenge that elevator manufacturers have accepted in agreeing to develop what will be the world’s tallest multi-deck elevator. In a similar vein, the location and height of the towers’ mechanical floors will push the technology of the buildings’ chilled water, sprinkler, and domestic systems and equipment with their limits to working pressure and capacity.
Kingdom Tower, Jeddah
