Validating the Structural Behaviour and Response of Burj Khalifa
Ahmad Abderazaq, Senior Vice-President, Samsung C & T
From a key note paper to be presented at the "R. N. Raikar Memorial International Conference"- organized by the India Chapter of ACI will be held in Mumbai on 20th and 21st of December 2013.
What are the odds of having a Designer who worked on Burj Khalifa as a Structural Designer; then later worked on the same project as an Executor with a contractor executing his own designs and fulfilling all the design requirements and stipulated specifications; and then finally the same person working as a Performance Monitor who executed the monitoring program to validate the design? Rather low, one would expect! However, we have such an exceptional presenter, Mr. Ahmad Abderazaq, Senior Vice-President, Samsung C & T, who will make his outstanding presentation on the tallest man-made structure in the world, Burj Khalifa, at the "R. N. Raikar Memorial International Conference".
"Validating the Structural Behaviour and Response of Burj Khalifa" is already generating a lot of buzz within the engineering community and is tipped to be one of the star presentations of the event, involving active interaction between the audience and the speaker.
Brief Details, well known to the students of Tall Structures
Burj Khalifa, at 828 m, is the tallest man-made structure; composed of more than 162 floors above ground and 3 basement levels. The total floor area of 460,000 sq m 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 is 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 3700 mm thick high performance R.C. raft over 192 - 1500 mm dia. bore piles extending approximately 45 m 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 System
The strategies for selection of structural are expounded by the author in his paper; which include optimizing the system, cost effectiveness, redundancies, speed of construction, utilizing 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 m 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, 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 the tower in correct position relative to its vertical axis
Tower construction was monitored through several programs utilizing 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 period and the development of extensive monitoring programs and use of latest development in GPS technology in combination with precision inclination sensors, 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 program 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 3-dimensional 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 values
For 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 program 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 program was also performed monthly at every set back level. Comparison between the measured and predicted movement indicated excellent correlation.
An extensive strain measurement program 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 program
A temporary real time monitoring program was developed and installed at the tower in cooperation with Notre Dame University to monitor the building acceleration level during construction; as 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 throughout the construction period.
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) Program
Finally, a comprehensive permanent full scale real time Structural Health Monitoring (SHM) program was developed and installed; which was an extension of the already developed temporary SHM system for monitoring building behaviour during construction. The program 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 summarized 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 programs carried out in many tall buildings had very limited research and scope and are yet to be systematically validated and holistically integrated. The comprehensive SHM programs at Burj Khalifa provided immediate and 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 programs 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 programs developed for Burj Khalifa will pioneer the use of survey and SHM programs as part of the fundamental design concept of building structures and will be benchmarked as a model for future monitoring programs for all critical and essential facilities.
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Acknowledgement: The above Abstract has been exclusively prepared by Mr. M. A. Jacob, President, India Chapter of American Concrete Institute on the occasion of R. N. Raikar Memorial International Conference