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Sky Gate: An urban window of structural ingenuity from CCL

Rabih Hajjar, projects manager - Middle East and North Africa (MENA) at CCL, writes about the post-tensioning techniques his team deployed during the construction of Sky Gate, a gravity-defying 42-storey residential tower in east Beirut

Sky Gate residential building in Beirut, Lebanon.
Sky Gate residential building in Beirut, Lebanon.

Rabih Hajjar, projects manager - Middle East and North Africa (MENA) at CCL, writes about the post-tensioning techniques his team deployed during the construction of Sky Gate, a gravity-defying 42-storey residential tower in east Beirut...

In a city where high-rise towers are the norm and high-end residential schemes are redefining the term ‘luxury apartment’, standing out from the crowd can be a tough challenge. With its cantilevered structure, 42-storeys and rooftop infinity pool, however, the Sky Gate development is truly a residential development unlike any other.

Designed by Nabil Gholam Architecture, Sky Gate has already been honoured with an international award at MIPIM (Marché international des professionnels de l’immobilier), the international property convention held in Cannes, France each March. It has also become a breathtaking landmark for Beirut; rising high above the adjacent towers and bringing an oasis of greenery, swimming pools, water features and creative landscaping to the heart of the Achrafieh district.

What really sets the building apart against the Beirut skyline, however, is the unique silhouette of its cantilevered central sections, which look as though they have been pushed to either side of the tower like bricks in a giant game of Jenga. The effect not only gives a striking appearance to the whole tower but has also enabled the developer, MENA Capital, to maximise the footprint of the site without compromising on the levels of natural light within each apartment, the 360° views of the city, the sea and the mountains, or the design possibilities for leisure and landscaping.

The vision for this extraordinary building combined MENA Capital’s ethos of challenging architectural convention and Nabil Gholam’s willingness to embrace those principles.  Making that concept work in practice, however, rested upon the design and engineering expertise of the CCL team to ensure that the cantilevered sections of the building could support the weight of subsequent floors both during construction and for the lifespan of the building.

Creating the ‘Urban Window’

Located in one of the highest areas of Beirut, maximising the panoramic views of the city and the coastline, Sky Gate is positioned on a 4,500sq. m site with residential towers on either side.  Enabling sufficient separation from these neighbours and plentiful natural light was not just a planning consideration for the developer; it was also a design priority.  As a result, the triple height ground floor lobby overlooking a reflecting pool, swimming pools and green landscaped areas to the front elevation, features 11-metre-high glass façades. The concept for the building was to create ‘urban windows’ at each side of the tower as it rises to continue this feeling of space and light.

To achieve these goals, the lower section of the tower is positioned off-centre on the site.  As viewed from the front elevation, the first 10 storeys of the Sky Gate building abut the neighbouring tower to the right, with a substantial vacant space to the left, providing views to the urban landscape behind the building. At level 10, the building cantilevers to the left by eight metres, creating a vacant space between Sky Gate and the neighbouring tower to the right while abutting the building on the left.  At level 17, the Sky Gate Tower cantilevers eight metres in the other direction, switching the ‘urban window’ of vacant space to the opposite side of the building for a further four storeys, before the tower finally rises in a central position for the remaining 20 levels.

The building’s design was still at concept stage when the CCL team first became involved with developing an engineered solution to enable the cantilevered vision to be achievable on site. CCL was already involved in the project for the design, engineering and installation of the post-tensioned floor slabs using its XF bonded system. The CCL team was soon asked to work with the project’s structural engineers to devise a cost-effective solution that could enable construction of the cantilevered storeys without sacrificing floor to floor heights or adding substantial weight to the building. The resulting solution was three post-tensioned cantilever beams at levels 10, 17 and 22.

Complex Calculations

The Sky Gate project is one of the first in the world to use post-tensioned beams to facilitate construction of a cantilevered tower and engineering the beams involved complex calculations from the CCL team to ensure that each beam could support the weight of construction above it.

For the section of the building that cantilevers at level 10, this involved engineering a beam that could support the weight of construction of the following seven storeys, until the level 17 beam was put in place, enabling the shearing forces to be shared between the two beams. The building’s total width at level 10 is 44 metres and the CCL team calculated that the smallest possible beam to support the floor above, both during the construction phase and following the building’s completion, was a 15-metre-wide beam, which is 2.7 metres in depth. Of the 15m width, eight metres extends beyond the width of the building below as the cantilever while the remaining seven metres provides the load bearing capacity to enable construction above the vacant space. On each of the cantilevered sections, the building includes an outdoor swimming pool on the resulting partial rooftop.

The calculations for the post-tensioned beams not only had to take into account the weight of the structure above and the buildability challenges, but also needed to consider the design implications for other elements of the building fabric.  Consequently CCL factored in the deflection limitations stipulated by the project’s cladding supplier, ensuring that natural deflection from the sustained loads does not compromise the integrity of the finished façades.

Engineering Expertise

With design and engineering of the post-tensioned cantilever beams completed, the CCL team was on hand to manage their construction on site.

To build the beams above the vacant space of the ‘urban window’, the construction team had to continuously prop each beam in place until the concrete had cured sufficiently to support its own weight.

The transfer beams were then stressed to increase their structural loading capacity, using concrete’s natural compressive strength to rapidly acquire tensile strength. Each beam had 32 tendons, with five strands per tendon, making a total of 160 post-tensioning strands. Stressing was carried out in phases for each of the beams, which were stressed to a jacking force of 210kN per strand. 

The complexity of the engineering required and the construction challenges of building the post-tensioned cantilever beams at such heights on a restricted site have contributed to the extraordinary achievement of the Sky Gate scheme. From a resident’s point of view, the wow factor may be in the size of the individual apartments - which range from a generous 344m2 to a palatial 1,515m2 penthouse, each with 3.5m floor to ceiling heights – and finishing touches such as the 3000m2 of landscaped gardens incorporating a running track.  However, from a structural perspective, it’s the post-tensioned beams that have enabled such an eye-catching building design to defy gravity thanks to the compressive strength of concrete and the engineering expertise of the CCL Team.

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