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An aerial photo of the space-constrained site.

An aerial photo of the space-constrained site.

In total harmony

SOM directors Aybars Asci and Mark Sarkisian detail the tower’s design concept

Al Hamra Tower

Al Hamra Tower, rising 414 m, is a new super-tall landmark in Kuwait that expresses a remarkable visual dynamism, with concrete shear walls spiralling around a central void. A parametric design process was used for this iconic structure from conception to construction, where the genesis of its sculpted form, its environmental response to a harsh desert climate, and integration of architecture and structure of the tower to its individual components were explored.

The hyperbolic paraboloid shear walls of the tower were designed and coordinated using building information modelling (BIM), and early discussions with formwork contractors influenced the curvature of the form, enabling it to be built at the same pace as the planar core walls.

Clad with trencadis, a type of mosaic, the custom stone cladding system allows a seamless definition of the complex curvature of the surface. An innovative lamella concrete structure of the lobby was developed from the desire of creating a larger entry space sloping outwardly from the tower above. A complex three-dimensional finite element analysis of the structure along with parametric modelling of form was developed to create the design and ultimately the formwork system of the structure.

Figures showing the geometry and the overall structure of the tower.

The curving form required innovative curtain-wall and facade maintenance systems. Rather than approximating the curvature with facets, 30 per cent of the glass on the tower is bent to smoothly define the geometry of the tower. Extensive glass research with multiple mock-ups was required to find an optimal solution to meet both the visual qualities and the stringent performance requirements for the harsh desert climate. Custom solutions were developed for the façade maintenance systems occupying the complex three-dimensional roof geometry of the tower.

OVERVIEW
Located on a space-constrained site at a prominent intersection in the centre of Kuwait City with a total gross area of 186,000 sq m of commercial office space along with a 34,000-sq-m podium of retail/entertainment outlets as well as an associated parking structure, the Al Hamra Tower sets itself apart from other super high-rise buildings with its unique sculpted form.

An example of architectural expression through structural form on a grand scale, the structural system and exterior form developed together in a process of symbiotic evolution. The building geometry is generated by a spiralling slice subtracted from a simple prismatic volume. The two resultant surfaces are hyperbolic paraboloid reinforced-concrete walls, which extend the full height of the tower and participate in the lateral and gravity force-resisting systems.

The design of tower required consideration of challenging engineering issues complicated by both the height and form of the structure.

The spiralling hyperbolic paraboloid ‘flared walls’, which were required for gravity load support of the cantilevered wing of the building, apply a torsional gravity load to the building core that necessitates consideration of both the long-term vertical and torsional deformations of the building structure.

Plan ... Outer transparent envelope - Inner solid envelope

Building a super-tall tower was not the initial plan of the owners, a joint venture of a local developer and a general contractor. They had started the construction of a 50-storey tower with a four-storey podium designed by a local architect, when Kuwait authorities changed the zoning to allow for a much taller structure on the site with the roof height limit increased from 200 m to 400 m.

FORM
During the planning process, the lease span was tested along the entire 60 m square perimeter of the site and resulted in a 25 per cent reduction of the floor plate to be taken to meet the area requirements.

To maximise on the sea views, a recommendation to remove a floor plate on the southern edge of the square, facing the city, was put forward for evaluation. In parallel with this study, the design team analysed solar and wind conditions to test the performance of different cut-out options. The solar analysis results favoured a southwest corner cut, while the wind studies illustrated that an uneven cut was beneficial to disrupting organised vortex shedding, resulting in a confusion of applied wind loads. The resultant form is one that removes one quarter of the floor plate starting from the southwest corner at the base transitions, creating a spiralling geometry by subtracting a quadrant of a typical filleted square floor plan and incrementally rotating the subtracted portion at each higher level until the southeast corner at the top.

The surface generated by the cut slab edges is articulated as a stone-clad continuous ribbon, which connects the hyperbolic paraboloid shear walls extending from the southwest and southeast corners of the central core (termed the ‘flared’ walls) and the roof of the tower.

SITE
The Al Hamra site is typical for Kuwait City and consists essentially of silty sand with density varying from medium dense to very dense with cementation increasing with depth. Heavily cemented sandstone and siltstone is encountered approximately 75 m below grade. The local hydrogeology consists of a phreatic water level at 2 m below grade, requiring temporary site dewatering during excavation and basement construction. The water level is predominated by rainfall that percolates into the ground with very little runoff to the sea. Evaporation leads to high concentrations of soluble salts, resulting in an aggressive chemical environment for below-grade concrete construction.

Form of the tower showing: 1 – Typical office space 2 – High-rise Sky Lobby 3 – Mechanical floor 4 – Mid-rise Sky Lobby 5 – Mechanical floor

Kuwait City is known to be located in a region of low seismicity. However, given the low incidence of seismic activity in the area and the short history of significant urban developments, there is little published information about the level of seismicity. Seismic parameters established in TI809-04, Seismic Design for Buildings, published by the US Army Corps of Engineers, was used along with the seismic provisions of ASCE 7-02 (referenced from IBC 2003) for design.

The synoptic wind patterns in the Gulf region are the result of the large-scale movement of air channelled along the northwesterly/southeasterly axis of the Arabian Gulf. Very localised and short-term wind phenomena are known to exist in the Gulf region due to thunderstorms, producing strong downbursts close to the ground. The project wind engineer BMT Fluid Mechanics (BMT) established a basic mean hourly wind speed of 23 m per sec at 10 m height in open terrain, representing the 50-year return period synoptic wind event consistent with the methodology of ASCE 7-02. Non-synoptic thunderstorm wind events were also extensively studied and it was concluded that while the thunderstorm wind profile could prove to be the critical wind event for the structural system of a tower lower than 200 m in height, the gross effect of the synoptic wind profile over the full height of the Al Hamra Tower, controlled the design in all aspects other than localised cladding pressures on the lower storeys.

In the diagram above, A shows the shear walls and B the perimeter columns.

FOUNDATIONS
Based on initial calculations and the local knowledge of the geotechnical engineer, CGC Consultants, it was determined that a raft foundation supported on cast-in-place bored piles would be needed. Local construction techniques dictated the maximum pile diameter (1,200 mm) and soil conditions dictated the closest allowable spacing (3,600 mm centre-to-centre), allowing for calculation of the expected pile load demands and for the commencement of a pile load test programme.

Considering fast-track construction, 289 piles were released and constructed in seven phases working inwards towards the piles beneath the southwest flared wall. The duration of piling works allowed the design of the superstructure to mature and the final mat foundation construction drawings to be completed as work progressed on site.

Because of the complexity of the project, the San Francisco office of URS Corporation (URS) was retained to perform a peer review of the recommendations of the project’s geotechnical engineer in a process including a full three-dimensional non-linear analysis of the soil strata under and around the foundations of the tower. Both analysis approaches were separately used to generate effective soil spring stiffnesses, accounting for the combined effect of mat and pile in each of the zones under the mat.

The final design of the raft foundation was for a 4.0-m-thick raft approximately 70 m by 60 m in plan, with an additional 1.6-m-thick triangular section of raft approximately 24 m by 12 m in a region to the north, beyond the footprint of the tower. The tower raft is supported by 289 piles, each 1,200 mm in diameter and varying in length from 20.0 m to 27.0 m measured from the bottom of raft. The design concrete compressive strength of the raft was 50 MPa (cube compressive strength), and varied in the piles from 55 MPa to 80 MPa (56-day cube compressive strength).

To provide an appropriate level of durability to the sub-grade concrete construction, the effect of the corrosive environment on both the concrete and concrete reinforcement was considered. Moderate heat of hydration and moderate sulphate-resistant cement (Type II) was specified for the subgrade construction. This cement was determined to be the most appropriate compromise between the corrosion-resistance requirements and the need to control the curing temperature of the 4.0-m-thick mat in the hot desert environment in Kuwait.

The subgrade construction was further protected by a complete waterproof membrane on the external surfaces of the raft and the foundation walls. The piles, the bottom layers in the raft, and the outer curtains in the foundation walls were all designed to be reinforced using corrosion-resistant reinforcement manufactured by MMFX Technologies Corporation. Clear cover requirements of ACI-318M were also increased to 100 mm to further protect the pile reinforcement. As a contractor substitution, the corrosion-resistant reinforcement was ultimately eliminated from the project, and replaced by an engineered cathodic protection system.

The raft foundation was poured in 15 separate pours over the total period of four months because of limited batch plant capabilities. This segmented approach to the raft pour was beneficial in limiting the peak concrete curing temperature with insulation preventing damaging temperature differentials building up near the concrete surface. Concrete curing temperatures were further minimised with the use of a high volume fly-ash cement replacement concrete mix.

SUPERSTRUCTURE
The lateral system for resisting the controlling wind and gravity load combinations consists of a cast-in-place reinforced concrete shear wall core supplemented by a perimeter moment resisting frame. The shear wall core was designed with thicker walls on the perimeter of the core, optimising the placement of material to maximise the resistance of the core to the gravity-load-induced torsion. The flared walls, which connect back to the core, also participate in the lateral force resisting system. As the shear wall core was resisting the majority of the wind induced forces, it was determined that the most efficient approach to the seismic design of the tower would be to designate only the reinforced concrete shear walls to be the seismic force resisting system. This allowed a full seismic design of the tower to be performed without needing to increase the use of materials anywhere in the structure. The reinforced concrete shear walls in the Al Hamra Tower vary from 1,200 mm to 300 mm in thickness, and from 80 MPa to 50 MPa in compressive strength (cube compressive strength).

The moment resisting frame beams are typically 800 mm wide by 600 mm deep and are poured with the floor framing using 40 MPa concrete (cube compressive strength).

Air flow studies considering the tower’s form.

Shear walls on the south façade were engineered to incorporate complex, unsymmetrical cuts that were placed to control heat gain and light into interior spaces. Custom reinforcing solutions were incorporated around each opening with specific geometry defined. Occupants looking directly outward from the elevator lobby will experience a series of small openings around the floor that allow views of the city and the desert beyond, and when walking along the interior of the south wall, the only light entering into the space will be through “cracks” in the wall.

To produce a gravity force resisting system, a 160-mm slab spanning between beams at 6.0 m on centre was chosen, which uses only slightly more material than a solution with a thin slab spanning 3.0 m on centre, but this contributed to the diaphragm shear capacity of the slabs. The 700-mm-deep reinforced concrete gravity beams span 10.6 m between core and perimeter frame. The perimeter columns vary from 1,200 mm square to 700 mm square. Composite columns are used from mat foundation level to level 29, with embedded W360 steel column sections of varying weights, allowing 1,100 mm square columns to be used in all typical office floors from level 40 down to level 5. However, 1,200 mm square columns are required below level 5 due to the increased storey heights within mechanical floors and double-height podium levels. Reinforced concrete in the perimeter frame columns varies from 80 MPa to 50 MPa and beam and slab floor framing is all constructed using 40 MPa concrete.

LOBBY LAMELLA STRUCTURE
 At the north side of the building is the main lobby, which is a 24-m-high space that extends from the building core to the perimeter frame. To increase the area of the lobby, the north columns of the tower, which are vertical from level 12 to the tower roof, slope away from the building core following a circular arch. The result of this movement is that the main tower columns passing through the lobby are 24-m tall and curved, developing large bending moments in the columns.
A complete three-dimensional finite element analysis model of the lobby lamella structure was built to study the effectiveness of the bracing scheme that had been developed and to guide the architectural design of these elements. A series of non-linear buckling analyses were performed on the lamella scheme, each model adding the next layer of bracing elements.

Shear walls on the south façade were engineered to incorporate complex, unsymmetrical cuts that were placed to control heat gain and light into interior spaces.

The models analysed included “A” elements alone, “A” and “B” elements, all elements except “D” elements and finally all the lobby elements. By tying together the “A”, “B”, “C” and “E” elements, buckling failure of any of these elements is effectively prevented and the critical buckling mode became the buckling of the “A” elements at the first conventional storey above the lobby. This study confirmed the concept of the lamella bracing scheme and demonstrated the structural importance of all the elements in the lamella.

To prevent the lobby lamella construction period from having a negative impact on the overall construction schedule, the observations of the beneficial effect of adding bracing elements was incorporated into the design schedule. While the “A” and “B” element must be in place prior to the construction of the floor slab above, construction of the typical floors above was allowed to proceed as work continued on the lobby lamella. There was a limit on the number of floors that may be poured prior to installation of the “C” and “E” elements as well as another limit prior to installation of all the “D” elements. Shop drawings for all the work were generated from three-dimensional models of the lamella structure using Gehry Technologies’ digital software, with fibreglass formwork moulds being fabricated directly from these models.

EXTERIOR WALL SYSTEMS
The outer boundary of the tower is conceived as a ‘tectonic’ glass and metal curtain-wall that offers panoramic views of the Gulf from the office floors, whereas the sculpted inner surfaces of the tower are clad with limestone panels, reflecting the ‘stereotomic mass’ that shield the tower from intense solar exposure.

A rendering of the lobby entrance and buckling analysis of the lamella structure (below).

The unitised curtain-wall is formed by typical panels, 1,500 mm by 4,200 mm in size. The functional objective in selecting the glass was to optimise the visible light transmittance to allow for maximum transparency, while minimising solar loads. Daylight studies are also performed to avoid glare in the office spaces.

The aesthetic objective was to visually create a tower as light in colour as possible. In order to achieve a silver/white appearance for the building, a series of mock-ups tested fritting, inter-layers, coatings and a combination of all these. A key challenge in glass selection was that 30 per cent of all the glass used on the project is bent, which meant that insulated glass unit make-up had to be bendable.

The south wall and the sculptural flare walls are all clad with jura limestone. The south wall has a staggered pattern of deep-set punched windows, which allow for framed views of Kuwait City and the desert beyond. The enclosure is designed to create an open joined wall, following a pressure-equalised rain-screen principle.

The sculpted flare walls are rationalised as a series of hyperbolic paraboloids to enable ease of construction using the self-climbing jump form system. The initial cladding option for these walls was to use 1,400 mm by 600 mm stone slabs on a metal frame attached to the concrete substrate. In order to create an efficient panellisation system, an advanced computational script is used with the resultant panellisation allowing for 94 per cent of stone slabs to be identical in size.

Subsequently, the team has revised this wall type to a trencadis tile system that adheres directly to the concrete substrate. This artisanal approach uses jura limestone in small randomised tiles, resulting in a wall that perfectly follows the doubly curved surfaces of the concrete, and is much lighter than the initial panellised wall system.

CONCLUSION
The concept for the tower was holistic – one that combined an iconic form with practical, regularly occupied spaces and a structure where all primary elements are functional while corresponding directly to the architectural solution. The regular reinforced concrete frame responds directly to organised and regular office spaces; the punched reinforced concrete shear wall on the south façade is used to control heat gain and light on interior spaces while efficiently resisting gravity and lateral loads.

Aybars Asci, AIA, LEED AP, leads an award-winning design group at SOM. He has worked on complex institutional and commercial projects around the world and is currently located in the firm’s New York office. He holds a master’s degree from Columbia University.

Mark P Sarkisian, PE, SE, LEED AP, is the director of seismic and structural engineering in the San Francisco office of SOM. His career has focused on developing innovative structural engineering solutions for over 100 major building projects around the world. Mark holds four US patents for high-performance seismic structural mechanisms and has additional patents pending for seismic and environmentally responsible structural systems. He received his BS degree in civil engineering from University of Connecticut where he is a fellow of the Academy of Distinguished Engineers as well as his MS degree in structural engineering from Lehigh University.




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