Validating structural behaviour and response of Burj Khalifa

Validating structural behaviour and response of Burj Khalifa
A key note paper to be presented at the “R. N. Raikar Memorial International Conference” in December 2013Burj Khalifa, the tallest man-made structure (828 metres); composed of more than 162 floors above ground and three basement levels. The total floor area of 460,000 sq. metres includes residential, hotel, commercial, office, entertainment, shopping, leisure and parking facilities. The tower is designed as a reinforced concrete building with HPC from the foundation level to level 156 and topped with a structural steel braced frame from level 156 to the highest point.
The residential and hotel-flooring system consists of two-way R.C. flat plate. The foundation system comprises of 3,700-mm thick high-performance R.C. raft over 192-1,500 mm dia. bore piles extending approximately 45 metres below the base of raft. Raft foundation bottom and sides are protected with waterproofing membrane. A complete cathodic protection system is also installed in the tower foundation system.
Structural systemThe tower massing is organised around a central core with three wings. The structural system was designed to behave like a giant column with a cross sectional shape reflecting building massing and profile. Managing the gravity load flow to building extremities was a significant consideration in the development of structural concept to overcome the overturning moments caused by extreme lateral loads (wind, seismic and stability). Most of the tower overturning resistance to lateral loads is managed by the tower’s own gravity loads. The design concept additionally required that all columns and walls were sized to resist gravity loads on equal stress basis, and tied rigidly by multi-storey walls at approximately every 21 floors to overcome the differential column shortening issues.
The strategies for selection of structural are expounded by the author in his paper; which include optimising the system, cost-effectiveness, redundancies, speed of construction, utilising latest technical advances in structural materials, incorporating latest innovations in analysis, design and construction methods, limiting the building movements to within internationally accepted standards, controlling the dynamic response of the tower under wind loading and so forth.
The tower’s lateral load resisting system consists of R.C. ductile walls linked to exterior R.C. columns through a series of R.C. shear wall panels at mechanical levels. The core walls are typically linked through a series of R.C. or composite link beams at every level. At the top of the R.C. core wall, a tall spire tops the building, the lateral load resisting of which consists of a diagonal structural steel bracing system from top level 156 to top of spire at approximately 750 metres above ground. The pinnacle consists of structural steel pipes.The author’s presentation of the design criteria for wind behaviour of such super tall buildings along with the critical gravity load management having direct impact on the overall efficiency and performance of the tower is interesting, as also his views on the art of structural engineering based on the knowledge of structural system behaviour and the materials.
The wind management of the tower was achieved by varying the building shape along the height. It was achieved by reducing the floor plan along the height effectively tapering the building profile, using building shapes to introduce spoiler type of effects along the entire height to reduce the dynamic wind excitations, changing the orientation of the building along its stiffest direction in response to the most severe wind direction and tuning the building natural frequencies and mode shapes for optimizing the dynamic response.
Challenges for placing towerTower construction was monitored through several programmes utilising the latest development in geodetic electro-optical total stations referring to fixed reference points; but the constantly increasing height of the tower made it difficult to use ground-level fixed points. The author has elaborated further complications due to increasing height, slenderness and movement of tower during construction due to wind excitations, large crane loads at uppermost constructed level, foundation settlement, column shortening due to elastic, creep and shrinkage effects, daily temperature fluctuations (resulting to possibly over 150-mm change in building height at top of concrete), uneven solar effects, lateral drift of building under gravity loads, building construction sequence and mix of concrete.
Full understanding of building movements and behaviour during construction and the development of extensive monitoring programmes and use of latest development in GPS technology in combination with precision inclination sensors and clinometers to provide relative position of building at highest level almost immediately, has been well documented by the author in his paper; as also the measurement system developed for use at every level to track immediately the tower’s lateral movements and to make the necessary corrections to bring the ACS formwork system to its geometric centre at every level.
The programme measures foundation settlement, column and wall total shortening, overall lateral displacement at every setback level and lateral displacement of spire/pinnacle structure during construction.
Finite Element Analysis model (FEAM) A detailed 3D-finite element analysis model was developed to predict the above building movement to the actual measured movements; taking into account the actual material properties, the foundation system flexibility (sub-grade modulus) and the actual construction sequence of the tower (as a function of time).
Correlation between predicted and actual valuesFor measuring the tower foundation settlement, 16 survey points at top of the raft foundation were installed and monthly measurements were taken till completion of work. The measured settlements were significantly lower than those predicted.
An extensive survey monitoring programme concept was developed to monitor the total column shortening at very setback level and reported monthly. Evaluation of the measurements indicated that the column differential shortening was within the predicted range. The tower lateral movement was monitored daily and a detailed optical survey programme was also performed monthly at every setback level. Comparison between the measured and predicted movement indicated excellent correlation.
An extensive strain measurement programme was also developed during construction and for permanent building condition. The strain gauges were located to measure column and core wall strains, strain distribution along the pile length and bending strain at bottom of raft. Good correlations between predicted strains and measured strains were found. Load cells at raft foundation to measure direct load transfer from raft to upper stiff sandstone layer by bearing were also installed.
Temporary real-time monitoring programmemeA temporary real-time monitoring programme was developed and installed at the tower in cooperation with Notre Dame University to monitor the building acceleration level during construction. Also, a complete GPS system to measure building real time displacement with time and a weather station to measure the temperature, humidity, wind speed and direction.Incidentally, the building movement from wind load remained relatively small during the construction.
The author also describes in his paper, the measured motion of the tower and the peak accelerations observed (in x and y directions) etc. due to a remote earthquake in Iran on September 9, 2008 during the construction of the tower.
Permanent full-scale real time structural health monitoring (SHM) programmeFinally, a comprehensive permanent full-scale real-time SHM programme was developed and installed. This was an extension of the already developed temporary SHM system for monitoring building behaviour during construction. The programme measured the building acceleration, movement, dynamic characteristics (frequencies, mode shapes), acceleration time history records, wind velocity and direction along the entire height and fatigue behaviour of the spire/pinnacle.
The author makes a very interesting presentation of the data measured in real time at Burj Khalifa, during an earthquake in southern Iran on July 20, 2010. Though the magnitude of this earthquake was diminished when it reached Dubai, the earthquake had frequency content that matched pinnacle frequencies, thus setting the pinnacle in resonance. The acceleration time history record captured at lower basement level was used to perform the time history analysis of the tower, and measured acceleration and predicted displacement were summarised by the author in the paper.
Author’s conclusions    Historically, design and construction of tall buildings relied solely on minimum building code requirements, fundamental mechanics, scaled models, research and experience. Research and monitoring programmes carried out in many tall buildings, which had very limited research and scope and are yet to be systematically validated and holistically integrated. The comprehensive SHM programmes at Burj Khalifa provided immediate, direct feedback on actual structural performance of the tower. The data collected were found to be in good agreement with predicted structural behaviour and validated the design assumptions and parameters. The programmes also provided real-time information of structural system responses, identified anomalies and allowed corrections etc. apart from generating a large data and full feedback.
The survey and SHM programmes developed for Burj Khalifa will pioneer the use of survey and SHM programmes as part of the fundamental design concept of building structures. Also, this will be benchmarked as a model for future monitoring programmes for all critical and essential facilities.“Validating the Structural Behavior and Response of Burj Khalifa” is tipped to be one of the star technical papers to be presented at the R. N. Raikar Memorial International Conference. The paper is written by Mr. Ahmad Abderazaq, Senior Vice-President, Samsung C & T. The abstract is written by M. A. Jacob, President ICACI.
Abstract by -M. A. Jacob, President, India Chapter of American Concrete Institute

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