The investigation on The Burj Dubai tower and the Taipei 101 building
The Burj Dubai tower
The Burj Dubai tower is one of the world tallest buildings to have ever been constructed by engineers. This type of a building wind forces and the resulting motions in the upper chamber of the building become some of the factors that call for a major concern in the construction of the building. Engineers had to make an extensive research programs in wind tunnels in the Rowan, Williams, Davies and Erwin (RWDI) 2.4 ×1.9 meter, and 4.9 × 2.4 meter boundary layer wind tunnels in Guelph, Ontario. In the wind tunnel program, it includes pedestrian wind environment studies, local pressure measurements, full aerolastic model study and a rigid body model force balance test. These studies were conducted in the scale of 1:500 but for the pedestrian wind studies a larger scale of 1:2500 was employed in order to develop full aerodynamic solutions focused on the reduction of the wind speeds. The engineers observed some Reynolds number dependency in the aerolastic model and force balance results. For the upper tower high Reynolds number test were carried out in a much larger rigid model at a scale of 1:50 (fig. A) in the 9 by 9 meter wind tunnel, speeds of up to 55 meters per second were attained. Wind statistics played a crucial role in the prediction of the expected levels of response to return period. To establish the wind regime in the upper levels, extensive use of the ground based wind data, computer simulations and balloon data came in handy.
The burl reaches a staggering height of around 509 metr that is to mean 1671 feet tall. Designing engineers purposed to shape the structural concrete Burj ion a Y shape model. This shape reduces the wind forces on the tower and also keeps the whole building simple and easy to construct. The structaral system is buttressed core as shown in the figure 2 below. On each wing ther exists a high perffomance concrete core and perimeter columns reinforced each other with buttresses through a six sided central core or rathe the hub. This results into an extremely stiff torsionally tower. Skidmoe, Owings and Merrlipurposed to allign all the common core elemnts to form a structure without structural transfer.
The floor systems of the building carries a great deal of weight in terms loads after and also during construction. Fro this reason the engineers ensured that then floors should be in a position to accommodate air conditioning systems heating and the built in fire resistance properrties. The floor is a two way systems. One way systems and beam and slab systems. Two way systems comprises of flate plates which are supported by the erected columns. The slabs on the other hand are supported by columns with drop panel or the capitals. Slabs with waffles are also used in th floor system. Braced frames with a single diagonal x-braces and k-braces are used. Lattice bracing technique is employed in pre-cast panel construction. Interior core of the building required steel braced frames because of the easy conection with the connection with the wall pannels is made easier. The composite frames required steel bracing in composite braced frames in concrete frames. Composite floor beams and concrete encasement columns are also used to ensure a rigid building.
The spiral stepping pattern make the tiers of the building to step back. As a result the tower’s width change on each set back. This in turn makes the wind current to be changed in direction as the engineers say that the wind is confused. That is one of the advantages of stepping and shaping the building in that pattern. At each of the vortexes of the building the wind never gets organized due to the fact that at each tier the wind current experiences a different shape of the building.
Wind loading on the main structure
Using the high frequency force balance technique, wind tunnel test were carried out in order to determine the wind loading on the structure. The collected wind tunnel data was then combined with the dynamic properties of the tower for the purposes of calculating the tower’s dynamic response and the total full scale wind force distributions. The Burj has essentially six vital directions of the wind. Three of the directions are when the wind blows directly in to the building which are Nose A Nose B and Nose C. The rest have the wind blowing between Tail A Tail B and Tail C. the force spectra of these wind directions showed less excitation in the crucial wind frequency range impacting on the pointed or the nose end of the wing as shown in the figure below from the opposite direction of the wind. Fig B.
With the orientation of the direction of wind direction in mind the tower was erected relatively to the most frequent strong wind directions. These directions of wind include east northwest and south west. To refine the whole structure architecturally several rounds of force balance were undertaken as the geometry of the tower took another new shape altogether. Wing A sets back first in a clockwise manner. From the wind tunnel testing the data was revised and the necessary adjustments were made to address the issue of the wind effect. This called for the reshaping of the building. These minor but crucial adjustments on the building resulted to a substantial reduction on the overall wind effect on the building.
Fig F illustrates the original plan of the building versus the new adjusted form of the building as a result of architectural refinements. From the figure observe that the wind tunnel frequency is found on the x-axis in relation to the recurrence interval for wind and the y-axis represents the ration of resonant dynamic forces to the square of the of the velocity of the wind squared. More precise and accurate aerolastic model were tests were carried away. Aerolastic model represents a real building in a real life experience. Aerolastic model carries the same properties as a real building like damping, mass and mechanical properties like stiffness. From the aerolastic model the first six modes were simulated on how the tower would look like. The bending moments from the model were calculated and recorded.
Data from each round of wind tunneling test was collected analyzed and the building was reshaped in order to minimize the wind effects. Spacing of the setbacks change the shape of the wing resulting to reduced wind force on the tower. The chart below is a plot of the response after some refinements of the original building and the response after corresponding refinements. The horizontal axis is the wind tunnel model frequency which can be compared with the recurrence interval of the wind. The vertical axis is proportional to the resonant dynamic forces divide by the wind velocity squared. Fig C
The figure below is an illustration of the relative change in the mean base moment coefficient on the aerolastic model as a function of the wind tunnel test for the wind directions. Observe that at the circular cylinder the mean drag coefficient also drops at certain Reynolds number and again climbs as the Reynolds number increases. Fig D. It is important to note that an architectural tool that made this building a reality is the wind tunnel test. By the use of several repeated sets of wind tunnel test and the resultant adjustment made on the building in terms of shape led to reduced accelerations and wind forces of the building. The balance test give a relatively higher overall wind loads and accelerations than the aerolastic model tests. This was so partly because of the Reynolds number effects in the force balance test and also due to aerodynamic damping effects as well as the responses from the purely Gaussian processes. In the scale of 1:50, the Reynolds number tests on the model clocked speeds of 55m/s implying that the Reynolds number on the model and the pressure model tests the results were not greatly influenced by the Reynolds number in general. The upper chambers were supposed to accelerate within the comfort limits without necesarilly using the supplementary damping.
Tapei 101
This building is located at the Hsyinyi district of Tapei in Taiwan. The building consists of 101 floors erected above the ground. The height of the building reaches a staggering 455 meters at the main building and at the spire of the building gives at total height of 508 meters. Before the building was started preliminary design and optimization was carried out. To match the limiting accelerations, displacements and stresses, codes and standards are used effectively. Thorough analysis of risk was carried out with safety and reliability to arrive at suitable factors in overturning and sliding. Tapei 101 building developed an uplift in its foundation which was designed for suitability. The first selection of the structural system involved electrical, mechanical and the architectural requirements. Preliminary design and optimization was carried out in an iterative fashion by addressing the drift and acceleration limits. A simple software like cantilever box beam model was used to simulate this came in handy.
The building has three tuned mass dumpers weighing 660 metric ton ball installed between the 91st and the 87th floor. This mass dumper is observable from the 88th and the 89th floors. The rest tuned mass dumpers weigh 4.5 metric tons each. These two mass dampers provide passive motion dampening effect on the spire of the building. The mass dampers reduce the vibrations caused by the wind turbulence and also the earthquakes. The building consists of four stations: two pairs of force balance accelerometer and R-1 rotational seismometer emplaced respectively. These are located in the southwest (T1S3 and T1S3R) and at north east of the building in the 90th floor (T1S4 and T1S4R). Another pair of force balance accelerometer is located at the southwest corner of the 74th floor of the building and others below the ground and (T1S2 and T1S1) respectively. The accelerometer ah the ability of recording shaking levels of up to 2g and has a frequency range of DC to 200 Heartz. The broadband velocity sensor on the other hand is capable of measuring ground motion of around 20 cm/sec and a broadband width 0.008 Hz to 70Hz.
The wind issues for the structural design of the Tapei towers include structural integrity under ultimate loads, building motions and occupant comfort, resultants under service loads uncertainties from wind loading and climate as well as the structural properties of the building like damping and stiffness. Codes and standards also play a crucial role in the design of the building. The relationship between the wind and height of the building is an exponential graph as shown in the diagram below. Fig D
The vortex shedding frequency is given by the formulae shown below
Shedding frequency N is given by N = S U ÷ b where S is the Strouhal number, U is the wind speed and b is the building width. The peak response due to vortex excitation is given by the relationship between crosswind response and wind velocity shown below. Fig E
Before erecting the building some wind tunnels were carried out and these are the observed results as a result of the shape effect. The results were very vital especially in the adjustment on what the Mother Nature could possibly throw at the building. Fig G
The main reason of aerodynamic forms employed in Tapei 101 reduce the cross wind vibration at the top by interrupting vortex shedding the boundary layer around the façade making a mild turbulence on the top of the building. Irregular shapes generally can be dangerous in building and construction but they can be resourceful in the reduction of the wind load and the building response.
The design technicalities on the preliminary design and optimization are briefly summarized and a rational methodology of design was shown. This enhanced optimization of the initial structural systems for drift and stresses depending on the lateral loads and the force of gravity. Insights concerning these buildings was given by the designers from the past experience in tall building design. The design issues are the efficiency of the systems, member depths, bracings, balance between sizes of columns and beams, struts and girders areas and inertias of the members. Accelerations and drifts of these properties should be kept within the limits. A good preliminary design always ensures a better fabrication construction and the cost of the building in general. Structural weight of the building determines the erection cost of the building. A good foundation laid upon the building leads to an efficient structural design, even in difficult soil conditions.
FigA the Dubai Burj under the wind tunnel test. ( The Burj Dubai tower. Peter Irwin)
Fig B The diagrammatic representation of the Burj from the top view showing the direction of wind from the corresponding angles. (The Burj Dubai Tower. Peter Irwin)
Fig Cthe diagrammatic expression of the vertical axis is proportional to the resonant dynamic forces divide by the wind velocity squared.
Fig D
Fig E
Fig E
Fig F
Fig G
Bibliography
Irwin Peter A. May 7 2010. Wind issues in the designs of the tallest buildings. L.A Tall building structural design council.
Irwin Peter and Baker William. The Burj Dubai Tower. Wind engineering.
