Gulf Construction Online - Iconic towers boost Ahmadiah portfolio

Kuwait

Al Hamra Tower ... Kuwait’s tallest structure.

Iconic towers boost Ahmadiah portfolio

01 December 2011

TWO new gleaming towers now stand proud in Al Sharq, Kuwait City’s financial hub, as they are being readied for their commissioning and handover shortly. Apart from being highly-distinctive glass-clad, commercial towers that share the same vicinity, the Al Hamra Tower and the United Tower have one other important aspect in common: they are being built using the expertise of local contractor Ahmadiah Contracting and Trading Company, which has well over five decades of experience behind it in the Kuwait market.

The 414-m Al Hamra Tower is now Kuwait’s tallest structure having reached its full height, with work on the crown of the building currently under way, while the 256-m-high United Tower is poised for completion this month (see boxed article).

The Al Hamra Tower has a unique asymmetrical shape with twisting external concrete walls not often seen in high-rise buildings around the world. Three elevations of the building are glass-clad, while the fourth, southern façade, overlooking the capital, is clad in natural stone. It was designed by Skidmore Owings and Merrill (SOM) of the US in line with the owner’s desire to create an iconic one-of-a-kind high-rise.

Considered to be world’s tallest sculpted skyscraper, the tower offers the largest office area in Kuwait City – at approximately 2,400 sq m of built-up of which 1,800 sq m is leasable area – and boasts a unique luxurious column-free lobby that spreads over an area of 900 sq m and stands 24 m high.

Sky lobbies on the 28th and 52nd levels, serve as transfer floors and also accommodate a fully-equipped business centre and a spacious employee lounge with a 7-m-high ceiling. Sky floors on the 71st and 72nd levels provide the highest business address in Kuwait and the Sky Lounge on the 74th level at a height of 351 m provides a unique dining experience from the highest point in Kuwait.

While the project threw up numerous challenges since the launch of works in August 2006, these were overcome by the brilliance and special techniques applied by the contractor.

"Executing such a sophisticated super-tall structure in Kuwait is a major achievement, given the difficulties in procuring the specified materials and recruiting properly skilled labour from the local and regional markets. Team effort by all concerned parties – owner, project manager, international designers, local consultants, specialised subcontractors and main contractor – positively contributed to building this iconic monument," remarks a spokesman for Ahmadiah.

The first of these challenges were encountered during the foundation works. The soil on which the Al Hamra sits consists of sand that increases in density and cementation with depth and there was no previous practical data on bearing and settlement that the tower would exert on the foundations. This apart, there was little previous experience on foundation design on pure sandy conditions that extended to depths below 80 m as those encountered on the site, the spokesman says, adding that the only available guiding data comes from the Kuwait Telecommunication Tower.

A pile-assisted raft foundation consisting of 289 piles capped with a 4-m-thick raft foundation was chosen for Al Hamra. The bored cast-in-place piles were 1.2 m in diameter and ranging from 22 to 27 m deep. Bentonite was used to stabilise the soil during construction of the piles, and an active cathodic protection system was incorporated to protect against rusting of the pile reinforcement and provide a life expectancy of 150 years for the piling system. A pile testing programme was implemented on site and compression tests were done for up to 3,000 tonnes.

The raft foundation presented challenges starting with the arrangement of reinforcement, which in some areas consisted of 13 layers of 40-mm-diameter bars both on the bottom and top of the raft. No reinforcement splicing was allowed, and all reinforcing bars had to be mechanically connected. Adding to the complexity of the raft was the active cathodic protection system, which had to be introduced to the bottom reinforcement layers and also to the raft sides and surrounding retaining walls. Plastic zip ropes were used to tie the reinforcing bars in lieu of galvanised tie wire to eliminate short circuiting of the cathodic system. The raft was approximately 25,000 cu m of concrete cast in 15 segments in the full 4-m thickness.

Raft core temperatures had to be kept at a particular level calling for radically changing the concrete mix and installing a system of temperature sensors in each cast to monitor the core, top, bottom and sides of each cast.

Commenting on the foundation settlement, the spokesman indicates that following the completion of the structure the site settlement is measured at 30 mm, while the predicted values ranged from 40 mm to 90 mm respectively.

Following completion of the foundation, Ahmadiah took on the task of creating the complex superstructure with special formwork. "The complex geometry of the tower walls, especially the flaring walls, which have a hyperbolic paraboloid surface, presented a challenge. We needed a system that could be easily lifted from floor to floor and which had the capability of adjusting to the continuously changing geometry of the walls at each floor while accommodating the eight-day floor-to-floor cycle," says the spokesman.

The South punched wall with its various slanted window openings, which have a checkerboard-like geometry, also needed special attention. "So we designed a custom-made lightweight metal formwork, which was easy to install and remove to create the complex window geometry in cast-in-situ concrete," he points out.

For the slab, a formwork system was needed that was easy and quick to install and dismantle. For this purpose, a lightweight aluminium formwork was selected.

To create the column-free main lobby as per the client’s specifications, five columns had to be moved 8 m away out of the lobby area by curving the columns outwards starting from the 12th floor, he says. "In order to keep the columns stable, a complex system of 3D elements (lamella) was introduced to laterally brace the entrance columns against buckling," he adds.

Composite steel construction was used to strengthen the structural capacity of these columns and their bracings.

"The formwork for the entrance lamella was another big challenge. We chose to make it out of fibreglass moulds built using the exact geometry defined by the 3D model from the architect for the various elements," says the spokesman.

This work, originally estimated to be completed in 250 days, turned out to be more complicated and eventually took 400 days to complete. This meant an unacceptable 150-day delay to the entire project.

"In order to mitigate this problem, we had to introduce new structural steel temporary bracing to the five curved columns where they were tied back to the core without blocking the lamella and then complete the columns all the way to the roof of the lobby. This allowed us to proceed with casting the slabs above the entrance while continuing to work on the lamella, removing its criticality," explains the spokesman.

The overall effect on the project’s progress was minimal as the slab excluding the zone above the entrance was proceeding at an eight-day cycle, while the slab zone above the entrance was reduced to a four-day cycle. "We were able to merge both at the 52nd floor," he says.

Ensuring that the super-tall building went up straight was also crucial. For this, a special GPS surveying system was employed. "While regular GPS provides accuracy in the range of 150 mm, this is not suitable for a building surveying system, which requires an accuracy level of 2 to 3 mm. In order to achieve our objective, a GPS system was utilised along with two ground fixed stations. Tilt meters were cast into the core to obtain an accurate 3D location of all building points," the spokesman says.

A vertical correction programme for elastic shortening, creep and shrinkage volume change in the concrete was implemented through the structure both in walls and columns. Moreover, adjustment in slab rotation was also required to correct any building twist resulting from the tower’s asymmetric shape. To exercise better control over this aspect, Ahmadiah enlisted the services of a US laboratory. Eventually, a total of 160 mm horizontal correction of opposite twist was gradually built into the slabs till the top – 140 mm out of which was actually recovered.

The tower’s top 70 m, which will house a restaurant and observation area, is 10 m wide and rises from 8 m up to 40 m following the building shape, which spirals to a single point at 412 m elevation. The whole area is column free with the core on one side and the glazed curtain-wall supported on steel hanging mullion on the other.

"In order to structurally frame and cover this space, heavy structural steel trusses cantilever 10 m from the core walls supporting at their tip steel hanging mullions holding up the curtain-wall. The truss spacing is 6 m and the mullions range from 8 to 40 m high," says the spokesman.

The structural steel is estimated at more than 1,000 tonnes. Work is currently under way on this crown area, where structural steel will be lifted in components with a maximum weight of seven tonnes and welded together on site.

The Al Hamra Tower has 40 elevators, 13 of which are shuttle, VIP and service elevators ranging in speed between six and 10 m per second. These elevators have heavy motors, weighing 15 to 18 tonnes each. During the initial planning stages, it was decided to have a special winch to lift these motors to place, reducing the maximum weight to be lifted by the luffing boom tower cranes to seven tonnes, thus significantly increasing the size of the tower crane.

"A special 30-tonne electrical winch was installed in the basement with a pulley assembly inside the elevator shaft. The motors were moved to the shaft door on a special motorised trolley on the ground floor level to the lifting point. The winch then lifted the heavy motors to the correct level. Large openings were temporarily kept in the shaft walls to move the motors into the adjacent machine rooms," the spokesman states.

Ahmadiah expected all the construction to be completed in six months.