Aerodynamic performance

Hyder started work on the Burj Dubai in 2004. (SUPPLIED)

Monitoring the long-term behaviour of Burj Dubai will be a continuing process, says the project's supervising engineering firm, Hyder Consulting Middle East.

"We have done lots of extensive analysis to try and predict the real behaviour of the building, not just for the spire but for the entire structure," said Jonathan Huang, Principal Structural Engineer - Building Structures.

"Long-term behaviour monitoring is still going on. It was first implemented after the building reached level 50, when we could actually measure the degree of movement in the structure.

"It will be supplemented by a more extensive system which will be put in place on the completion of the building for further long-term monitoring. The system used in the construction phase has been developed and enhanced and this will be put in place once the building is finished for the life of the building.

"The horizontal movement of level 164 should be approximately 200mm over the next 10 years."

Hyder started work on the Burj Dubai in 2004. The firm's role comprised acting as Emaar's supervising engineer and undertaking responsibility for certifying the design of the building. It was also responsible for the peer review, certification of the SOM design, supervision of the construction and is the architect/engineer of record for the UAE authorities. Hyder also designed the geotechnical and surrounding infrastructure of the Downtown Burj Dubai development.

"At the beginning of the project an environmental impact study was done on the brownfield site and certain criteria were decided and then taken through from the design of the site to the services in the project," said Brian Lacey, Director of Property at Hyder.

Principal Mechanical Engineer Alastair Mitchell said: "The environmental impact of a modern building consists of its carbon footprint and the amount of energy it consumes. The Burj has been designed to very high standards in terms of both the envelope of the building and air-conditioning, which consumes the greatest amount of energy in this part of the world. Reducing cooling loads is one of the most significant things that you can do in the Middle East.

"The façade has very high-specification glazing with low U-value and very high shading co-efficient to cut down the internal heat gains, which in turn reduces the size of the air-conditioning plant that is required.

"We have numerous control systems within the building to make sure we have the minimum energy requirements depending on the time of day and seasons while meeting the requirements of the occupants. A centralised district cooling plant services the project and also feeds the adjacent Dubai Mall. We take one-third of the district cooling capacity and it must come to around 45MW of district cooling."

The primary design for the Burj Dubai was completed six years ago when sustainability was not seen as a major requirement, but things have changed over the years.

"In today's marketplace it is different," added Lacey. "However, Hyder has introduced a whole range of sustainability engineering initiatives into the building, which achieves a very high sustainability rating.

"Many of the pumps and fans have variable speed drives, which reduces the capacity on those systems on a seasonal basis so we are only producing the air and water that we need for the building rather than pumping on full volume. Light control systems optimise the use of lighting in the building to reduce the energy demand. Automatic solar shades are provided in all entrance pavilions, which further reduce the energy demand.

"Furthermore, the largest solar array in the Middle East is located on the roof of the office annexe. This provides around 600KW of domestic hot water capacity, which is around 20 to 30 per cent of the actual hot water demand in Burj Dubai.

"These systems are all part of the proactive renewable energy strategy implemented by Hyder for the whole building. You cannot have passively designed buildings in this part of the world."

Mitchell said several energy-saving technologies have been implemented throughout the MEP services design, including the collection of air handling unit condensate water for use in the pre-cooling of Dubai Electricity and Water Authority (Dewa) mains water, plus irrigation.

"In addition there is an extensive use of variable speed drives wherever possible throughout the tower, in particular on the main chilled water pumps, plumbing pumps and supply and exhaust fans," he added. "Heat energy is reclaimed as much as possible and a sophisticated two-port valve control system has been added to the chilled water, air handling units and fan coil units. This includes pressure independent control valves. The building management system has been set up to optimise energy savings between the variable speed drives and the two-port valve control system."

Further measures include the introduction of a system to measure supply and exhaust air rates and pressures to ensure they are balanced throughout the various building zones and air leakage out of and into the building is minimised.

The high-specification facade system specified for the project will reduce solar heat gains, hence cooling loads, thanks to a low-e coating, low U-value and low glazing shading coefficients.

Within the electrical systems, a lighting control system has been used throughout and energy-efficient lamps are being installed for areas such as the car park, plantrooms and back-of-house areas. Electronic control gear will be used to provide lighting control.

An extensive power monitoring and metering system is being installed, plus a power correction factor of 0.95 has been applied to electrical equipment in lieu of the required 0.90, again reducing the energy usage. High efficiency, low loss resin block power transformers have been installed as standard.

Lacey said: "This is a building of considerable size. There are extensive air quality sensors around the building to optimise the air quality of the internal environment. Condensate is reclaimed from the chiller systems to provide irrigation water.

"This is not used much in other parts of the world but it is a significant feature here due to the arid climate and expense of irrigation water. It also reduces the carbon footprint in terms of the condensate that we recover from the air-conditioning systems. We also reclaim the energy from the condensate water to precool the incoming Dewa water by running it through the heat exchangers.

"We are also reusing sewage by using a system called treated sewage effluent (TSE). The TSE is used around the site for the irrigation of the landscaping. The TSE is also used in the district cooling system and is given further treatment for use in the lake fountains and other water features."

Huang said creep and shrinkage were material characteristics of concrete.

"All concrete will creep and shrink," he added. "It is long-term movement and over time it reduces. In terms of floor to floor compensation, it is between 3mm and 4mm per floor.

"The reason we can do that is because the tower structure distributes the load very evenly. So from early in the project, you know that the stress is very uniform and that the compensation is required is very small."

Mitchell added: "With tall buildings you also have to study the expansion and contraction of all the pipes. With the Burj Dubai we studied the actual movements of the building, which shrinks and compresses over time, and that has to be factored into our analysis of the movement of the main risers.

"Some of the methods that we have used to accommodate the movement result in the use of products that have not been used in the building industry before but have been used in the gas industry. This should ensure very high specifications and reliable operation."

Fatigue behaviour is also very important.

Huang said: "We have actually conducted two designs. One will ensure that the spire has enough capacity strength-wise on a daily basis and the other is for long-term fatigue. Structures can fail at a much lower loading than their actual strength if they are subjected to a large number of loading cycles. This is called fatigue.

"It is analogous to you breaking your credit card by repeatedly flexing it in your hands. The spire has repeated loading cycles induced by wind vibration and it is designed to resist this fatigue loading in a similar manner to an aircraft wing."

A much-touted rumour in the market is that the spire can be raised and lowered.

Huang said: "Technically it can be done, but do we need to? The spire was built within the building and raised from within the building but it was never intended to be lowered again or jacked up from inside the building. There is no intention to modify or change the height of the spire – this would be a mammoth undertaking."

The decision to double the height of the tower's office annexe produced constraints, he added.

"It was quite a challenge to raise the height by so much. Obviously the building shape was defined and we were unable to go beyond the existing building boundary. The foundations had been designed in the early days to accommodate a particular height with a known foundation capacity. However, by using an advanced pile-assisted raft foundation technique, we managed to double the capacity of the foundation.

He said the major breakthrough was using dynamic analysis for the seismic design. "This showed that the earthquake loading was slightly reduced despite nearly doubling the height of the building. This was mainly due to the increase in the period of vibration for the taller structure. The earthquake loading was further reduced by designing the additional portion to be lighter – we changed the floor system so that the new top 10 levels were about a third of the weight of the original office building."

Increasing the height of the actual tower was not a problem, though. "The Burj has a tapered shape and as you add height to the building it doesn't add significantly more weight to the building," said Huang.

"That is one of the key aspects that made the increase in height possible. So when you stretch the building it does not change the building weight or quantities that much.

"The taller the building gets, the more flexible it becomes, thus reducing the seismic loading. At some stage, usually about 30 storeys in Dubai, the wind loading becomes more critical than the seismic loading."

So confusing the wind became a priority. The wind climate in the Arabian Peninsula includes a unique regional flow called the shamal in addition to ordinary synoptic wind flow and thunderstorms. These three wind profiles were part of the Arabian climate studies carried out for the tower by the wind tunnel consultant, RWDI.

Shamal is the Arabian word for northern winds. Shamals tend to occur in mid to late summer as a result of temperature inversions over the Arabian desert. A shamal is similar to a constant thunderstorm that can blow for days. When you have high pressures on the Iranian side and high pressures on the Arabian side of the Gulf, it is a like a mixer that creates a tunnel of air. In summer air becomes hot and rises and at sunset it is cooled very quickly and cannot rise any more, and this creates a dense layer or inversion.

When the wind runs over the top layer it accelerates and blows constantly as long as the high pressures on either side of the Gulf continue, thus generating a shamal.

"The building performs aerodynamically very well. The terraces are cut back at different levels and this disturbs the vortices when the building is subject to wind loading, ie it confuses the wind. Consequently the wind loading cannot lock in and sway the building.

"This lock-in sway is analogous to a child in a playground on a swing getting it to swing, or sway. The Burj Dubai is an excellent example of a building that has good dynamic properties. Consequently it does not need a damper."

Another feature that differentiates the Burj from other projects is the high-voltage network throughout the building, said Mitchell.

"Every 30 floors there is a high-voltage substation and it is a private network that will be owned, operated and maintained by the client," he added. "Another major differentiator is that we have service riser pipes that are 300m high. The plumbing risers operate at 30 bars and the chilled water systems operate at 35 bars.

"The construction techniques for these risers and the size required because of the pressures and volumes of water being pumped into the building meant that we had to be very careful with the selection of the materials and the installation methods. We have done some very extensive non-destructive testing programme on our main chilled water and plumbing risers."

There were certain other challenges throughout the project.

"Many issues were thrashed out between Hyder and the contractor prior to their implementation on site," said Mitchell. "But the floor-to-floor heights were very low at 3.3m. This meant that we had to pack a lot of MEP equipment into a very small ceiling void. This led to many MEP challenges."

Huang added: "Yes, there were challenges. No matter how well you plan, unexpected things do happen. The Burj Dubai is a complex building that took a long time to construct. When we went higher, we learned numerous lessons.

"One thing we learned was not to have a steel spire on a very tall building. When you have steel you have to assemble, not build. You are then faced with challenges such as crane down time, which is 60 per cent when it is windy, and also it is slow.

"You have to lift pieces and the rate of the construction slowed down during the building of the spire. If we had built the spire in reinforced concrete it would have been much quicker. With future buildings we will assess reinforced concrete. The concrete work cycle time is three to four days per floor."

 

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