Smallest GPS Network for Tallest Building
Core Wall Survey Control System for High Rise Buildings.
2.3Core Wall Survey System
The movement of the structure creates several problems for precise survey; at a particular instant in time, theoretically, you need to know exactly how much the design centre line of the building is offset from the vertical axis and at that same instant you need to know the precise coordinates of the instrument. However a 'mean' position taken over a short period for both elements can provide a suitable solution.
Instrument Position Determination
GPS operating in static mode are being used to establish survey control at the upper levels. The system comprises a minimum of 3 GPS antenna/ receivers mounted on tall fixed poles at the top level of the formwork.
A tiltable circular prism is placed below each antenna and a Total Station instrument (TPS) is set up on the concrete visible to all GPS stations. The GPS plus TPS comprises a "measurement system".
In static GPS mode, satellite signal data is received and recorded for a period of up to 1 hour. During this same period of time, the TPS instrument is used to measure a series of angles and distances to the prisms mounted below the GPS antennas. The TPS then measures to the reference marks placed on fresh concrete which are the reference points for control of the formwork.
After completion of observations, data is returned to the office for processing. Computation of GPS antenna positions is carried out, processed against data from a Continuously Operating GPS Reference Station Leica GPS GRX1200 Pro with AT504 chokering antenna and Leica GPS Spider software, using Leica Geo Office software (LGO).
Computation of TPS position is then carried out actually as a least squares resection. Finally transformation is performed of the 3 WGS84 antenna coordinates and resected TPS coordinates into the local coordinate system and from this a determination of coordinates of all measured reference marks is made. These steps yield coordinates of survey instrumentation and reference marks in the site project coordinates.
The coordinates computed by using the observations (directions and distance) are internally consistent but must be transformed into the reference frame defined by the set of GPS antennas. In our case as we use a single total station, the problem is simply a 3D transformation also known as similarity transformation or Helmert transformation.
Building Alignment Determination
The Core Wall Survey System (CWSS) uses NIVEL200 dual-axis precise clinometers to accurately determine displacement of the tower alignment from vertical. Clinometers measure absolute tilt to +/- 0.2” arc. This angular measurement can be applied to the vertical distance of the clinometers sensor above the foundation raft to provide a computed plan displacement in X and Y at that elevation due to the tilt of the structure. A total of 8 precise clinometers are to be networked at approximately every 20 floors up the tower as construction proceeds. When the clinometers are installed initially they will be calibrated in relation to the survey control at that level by verticality observations from the raft foundation. A series of observations will provide a mean displacement in X and Y for that tilt meter at that time and will then be applied to all future readings so that the output will reflect the displacement of the tower alignment at that level in relation to the vertical axis.
Clinometers will be connected through an RS-485 single bus cable to the LAN port of a dedicated PC located at the survey office running Leica GeoMoS software.
Continuous, real-time measurements of structure tilt can be logged for each instrument floor, and data output as X and Y components of building alignment from the vertical. Amplitude peaks of smoothed data represent structure oscillations.
The mean displacement of the regression line represents total mean displacement of the structure. A block of data corresponding to the GPS observation data will be used for this purpose. Differentiation of the tiltmeter data at different heights will allow correction for nonlinear structure tilt.
PRECISION
An examination of the likely errors in the CWSS indicates that it will be possible to continue to set out the formwork along the vertical alignment of the structure to a precision of ± 15mm. It should also be possible to identify any long term movement of the tower that has a value of >20mm in any given direction.
ANALYSIS
Monitoring surveys will provide information on raft foundation settlement and deformation and this can be used to accurately determine the offset of the tower at a particular level due to the influence of these factors. Similarly surveys to measure the differential shrinkage and creep in the core walls and columns can be used to derive this possible component of tower movement.
A dynamic model of the building has been developed and from this it has been possible to derive values at any given level for the effects of construction sequence, building design and solar effects. For the period of the control survey if the tower cranes are shut down then the only remaining unknown component of building movement is that due to wind. Weather stations are to be established at three locations on the tower and these will stream continuous data on temperature, wind speed and direction. This can be correlated with the tilt meter data to determine a relationship. It is anticipated that this analysis will reveal any long term movements in a given direction and if necessary corrective action can be taken.The Nivel200 Network segment of this system can be used for tower monitoring, both during construction and after completion of the structure. If this is integrated with other monitoring information it will provide a complete system of structure monitoring.
CONCLUSIONS
This concept uses a combination of control and common points located in the deformation area. A combination of GPS survey techniques, Automatic Total Station, clinometers readings and mathematical modelling will provide a means to drive the construction of the world's tallest building as a straight structural element and provided a wealth of data on building movement. It's only the start of a long journey up to the final completion of the Burj Dubai tower and the authors know that they will have to complement the existing data fusion system with other elements for the time being.