Core Wall Survey Control System for High Rise Buildings. 2.CORE 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. 2.1Instrument 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”.  Figure 3. GPS active control points 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 as described in 1.4.1. 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).  Figure 2: Continuous Operating Reference Station Computation of TPS position is then carried out actually as a least squares resection. Finally transformation is performed of the 3 no 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. A total station, or more generally any theodolite, can be considered as a dual axis system supporting the line of sight of a transit/telescope. For reducing the effect of the mechanical misalignments on the observations, classical operational procedures have been applied since the first use of such instruments. Today, a total station can take these axis misalignments into account using an inbuilt dual axis compensator and special firmware to correct the resulting error in the measurements. However, the operational range of the compensators is restricted, typically to about six minutes of arc. The operator aligns the main axis coarsely by keeping the bubble of the station inside the graduation. In case of a compensator “out of range” signal, the station must be realigned manually. This procedure known by experienced operators as simply inappropriate when operating a total station in this case when we expect dynamic behaviour and overal as we the building main axis will not be aligned with the direction of local gravity To remove that restriction it will be necessary to consider this instrument as a local 3D axis system. 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.  Figure 4: Tiltmeter to PC Connection 2.2Building 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 measure 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. Each instrument will be mounted in the center core wall in a boxout within the wall where casual disturbance is unlikely. 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. |