Site visit: Pentominium Tower
CW gauges progress on what will be world's tallest residential tower
Home to the world’s tallest tower, the world’s highest restaurant and the world’s largest shopping mall, it comes as no shock that the record-breaking city of Dubai is now on track with the world’s tallest residential building.
The real surprise is that we are not talking about the Princess Tower. At just 107 storeys and a height of 414m on completion, Princess, which made waves thanks to its tall order, will be no match for the soaring penthouse skyscraper of the Pentominium.
Superseded by only the Burj Khalifa, the Trident project will, once finished, stand at a massive 516m with 122 floors, 172 luxury penthouse apartments and a total built up area of 163,720m2.
The project is unique,” says main contractor ACC's project manager Ashraf Salman. On completion, it will be the third-tallest tower in the world. In truth, however, the Pentominium is more than just another tall tower.
Asymmetrical, and extremely slender for a building of its height, it is actually one of the most architecturally significant projects in the city currently under construction.
Designed by Aedas, it features six hanging gardens and several apartments down just one side of the structure at the high levels, and, most importantly, a 604m2 penthouse on every ‘floor’.
The finished height of the Pentominium Tower will be 516m
While in construction terms the building has just 122 levels, technically it could be described as having 172 floors because it has apartments on each side of the building offset or separated by a flight of stairs, with lifts stopping at individual apartments.
“The reason it is called the Pentominium is because it is a residential complex with all penthouses, so rather than a condominium, it is a Pentominium,” says George Larkins, site operations manager from Burj Khalifa consultant Hyder.
The company was engaged by Trident to provide engineering and design expertise to the Pentominium, as well as construction supervision. “Unlike other structures, there will also be a large glass façade on the outside of the building or, rather, a curtain wall made of glass, stainless steel, aluminium and stone.”
As for the amenities, the building will be complete with swimming pools on levels 4 and 99, a sky lounge, a business centre, men and women’s health clubs, a cigar lounge, a banquet hall, a theatre, an observation deck and 26 high-speed elevators. Certainly everything you would expect from a luxury Dubai skyscraper.
Having commenced with construction in 2008, the Pentominium is still in its early stages. The enabling works were completed by Swissboring Overseas Piling Corporation, with one of the deepest excavations in the world, followed by the main contract award to Dubai-based contractor Arabian Construction Company (ACC) for $400million.
"The project was launched in 2007 and scheduled for completion 52 months after the main contract was awarded in 2009,” says project manager Precipio’s CEO Amer Khan. “Before that time we were doing the enabling works, meaning the excavation and piling.”
It was, in fact, not long after ACC was contracted that the Pentominium achieved its first record.
The construction of the raft foundation specifically, was the largest concrete pour in the Middle East, with 13,139m3 of concrete poured (as one pour) in just 44 hours.
Since the raft, Khan says work has continued to progress in a timely fashion, with 700 workers currently on-site and an expected peak of 2,000. According to ACC, the project is actually running nine to 11 days ahead of schedule, with the team working through the night to ensure speedy progress.
We have been working 24-hours a day and it is very hard to gain time on the project, says Salman.
“Trident is a good developer, and Hyder is very professional - we are also working with them on other projects.
At the time of visit, ACC had begun working on the typical or residential floors, with all contracts except the furnishings and interior design fit-out contracts awarded.
“The substructure (six basements) is complete, and the six podium levels are also done,” says Larkins. “We are currently at level eight (level two of the typical floors) and the core structure is at level 13.”
Peak number of workers on site will hit 2,000
To speed up construction, the tower is relying on special jump-form formwork systems for the main core walls which, unlike conventional systems, allow a jump of one-and-a-half floors a time.
“Normal jump-form is done on a floor-by-floor basis, where a single floor is typically 3.5m to 4m. This system, which relies on technology recently developed in Australia, actually allows a one-and-a-half floor jump, so we are doing fewer jumps overall, reducing the floor cycle and total construction time.
At the moment the contractor is achieving about a floor a week, but this is likely to increase as they get used to the system. Separate jump-form systems are also being used for the columns.
The next steps will be the same as those taken during the construction of any other high-rise: building onwards and upwards, beginning the full MEP works, cladding, curtain walling, and interior fit-outs of lower-level apartments after reaching level 20.
In addition, Larkins explains how the lift rails will also need to be installed fairly soon, but with temporary lift cars that can be used for construction purposes.
“Because the building is so high and on such a small plot of land, there is nowhere to put hoists on the outside of the building, so we are going to use the lifts inside the building for construction purposes.
We will put in the permanent lift rails, as these will not get damaged by construction, and then the temporary lift cars, so all we need to do at the end of the job is take them out and replace them.
We will start the rails when the core goes past level 15, but they will not be ready for operation until April or May.”
Unfortunately, not everything on the Pentominium is quite as simple. Due to its architectural significance, the tower is actually one of the most complex construction projects in the city, dependent upon extremely sophisticated engineering techniques, much like those utilised on the Burj Khalifa.
It was for this reason why Trident chose Burj Khalifa builder Hyder as the engineering and design consultant.
“The building is a bit unusual in that at the high levels, there is a large number of offset cantilevered spa gardens and apartments down one side,” explains Larkins.
“That is creating an imbalance for the building and, as a result, some fairly significant building sways which have to be corrected during construction.
Total built-up area of the developement is 163,720m2
So as we go up, we are having to monitor the deflections of the building and compensate for them so that when the project is completed it is in the right place.
We are doing this using very sophisticated surveying techniques that Hyder developed for the Burj Khalifa, and we are taking them to the next level of sophistication for this building. It was very complicated to design a structure that satisfied these architectural constraints.”
Asked exactly how the technique works, Larkins says: “Basically, at critical intervals, we are measuring where the building is exactly, and then with the next jump of the jump-form, we are correcting this movement.
But we also have to make some predictions for the future, because a lot of this movement will take place in the years after we finish construction. So we have had to do some very advanced engineering modelling to predict where the building is going to be.
Linked to the challenge of preventing building movement, Hyder was also chiefly responsible for wind and earthquake engineering.
For the former, the company worked with US-based wind-tunnel consultants to derive a suitable wind climate for Dubai in general and for the Pentominium in particular, which included providing data on Shamals, a type of wind unique to the Arabian Peninsula.
But the challenges were not limited to building movement. Given the slender shape and extremely small plot on which the Pentominium is being built, it was also essential, from a structural engineering point of view, to utilise the highest strength concrete, and to design the building for optimum structural support.
“We used 100MPa concrete for the columns and central core walls, which is among the highest used in the UAE to date (the Burj Khalifa used 80MPa).
This increases the structural stiffness of the tower by 50% compared with normal strength concrete.”
He adds that triple-blend concrete mixes and cathodic protection systems were also used in the substructure, to ensure a minimum design life of 100 years in a very aggressive groundwater environment.
“One of the other things we did to increase support was combine structural steel sections together with concrete and reinforcement bars for the columns, to create high-strength composite columns.
This involved using 460MPa high-strength steel sections that are twice as strong as conventional steel. We also extended the columns into walls to resist lateral loading. So we effectively engaged the columns to work with the core. We then engaged the link beams, which link the column to the core, which again relied on very advanced computer modelling.”
But even in the context of all these challenges, Hyder says the project’s biggest problem is still the building’s small footprint. With three sides of the building inaccessible, all materials for all trades have to be delivered to just one area.
“The logistics and materials movement is the biggest challenge we have got,” says Larkins. “Of course, there is not a lot we can do from a technical point of view.”