The Building that Put Dubai on the World Map (Part 2)

 

Image by: Richard MacLennan (Pinterest)

Challenges and Solutions

1.  The Island Construction

Burj Al-Arab was to be built on an artificial island over 6ft water, about 280 meter off the coast of Dubai. Building a 9000 tons structure over an artificial island in the sea presented an unprecedented challenge, especially in the harsh weather conditions of Dubai. To make matters worse, the architect wanted the island to be low rising above the sea level to make it look like the building is rising directly from water like a boat. This would expose the structure to the damaging forces of sea waves.

Initially, a rock Island was proposed but a rock island had to rise to a considerable height above the sea level to ensure the structure won’t be damaged by the sea waves. Thus, the engineer Mike McNicholas suggested that they use hollow hexagonal concrete blocks on the sides of the island to dissipate the energy of the waves. The problem with that solution was that it had never been used. Thus, it had to be tested first by building a scaled island and exposing it to the proportionate forces of the sea waves. Thankfully, the results of the tests came back positive and work on the implementation of the solution was initiated. After two years of work on the island, it was finally ready; rising only 6 meter above the sea level.

Image by: (Shane McGinley, title: Burj Al-Arab at 20, Arabian Business, Travel and hospitality)

2.  Sealing off the sea water from seeping into to structure

Another major obstacle faced by the construction team was the threat of water from the surrounding sea seeping into the structure area from underneath and flooding the whole structure. This situation, if happened, could have ended in the death of hundreds of workers. Engineer Mike McNicholas sealed off the sea water by installing a 20m deep steel wall around the area of construction. This steel wall made a coffer dam and was to work as the external walls of the building basement after completion. The coffer dam had solved the threat of sea water seepage from the sides, however, issue still remained of the area being flooded from underneath, especially after the excavation of sand for the construction of foundation. To solve this problem, liquid cement was injected into the ground to make a cement blanket. The cement blanket effectively resisted the pressure of sea water and helped in the removal of sand from the construction site and the construction of the foundation.

Image by: (Jim Anderson; engineeringmechanicalsystems.com/dubai-engineering/)

3.  Lateral Stability of the Structure

The building is located in an earthquake prone region. However, a bigger challenge is the wind. Being a coastal area and keeping in view the harsh winds of Dubai, the design wind speed was taken at 45m/s (162 km/hour). With the unique design of the building, a wind of this magnitude could prove to be catastrophic for the building. In order to give lateral stability to the building, an external steel exoskeleton was designed. This skeleton consisted of two huge H-shaped steel frames embedded in the concrete substructure and going above the height of the concrete superstructure towards the back of the building to support the top mast. The exoskeleton was further braced by providing huge steel trusses that were longer than a football field and 10 times as heavy as a double decker buss. The exoskeleton provided front to back lateral support. For side-to-side lateral support X-shaped braces were provided between the two concrete wings inside the Teflon coated fabric wall.

 

       Image by: Dimitar Dimitrov (Diagnol Truss)

                               Image by: Martin J. Halford and Paul J. Walters (Cross Brace)

4.  Neutralizing Vortex Shedding

Before the implementation of the exoskeleton, a scaled model of the building with the exoskeleton had to undergo wind tunnel testing to confirm the adequacy of the design. The wind tunnel tests revealed another major issue with the exoskeleton. This issue was of Vortex Shedding (Phenomenon where when wind passes through a narrow structure, a series of alternate low pressures on the sides of the structure leads to an oscillating force that oscillates the structure at right angles to the direction of the wind). Vortex shedding could have damaged the exoskeleton. To counter the impact of Vortex shedding, tuned mass dampers were hung from the vulnerable points inside the exoskeletal frames.

Image by: (Michael Gaspar; The mechanism and applications of a Tuned Mass Damper (TMD), bsbgroup.com)