GPS and Building Monitoring Case Study: Republic Plaza Building, Singapore

An RTK-GPS system has been installed for the purpose of monitoring the behaviour of a high rise building in Singapore. This system will contribute to a project of monitoring that commenced in 1995 with the installation of two pairs of accelerometers and two UVW anemometers. The aim of this project is to capture the building loading and dynamic response during strong winds and remote earthquakes to aid local design code development. The GPS monitoring system installed on the Republic Plaza building (at 280m, the maximum height of any Singaporean building) generates on-line antenna coordinate measurements. These will complement and corroborate the acceleration data to provide the complete picture of building displacement across the full spectrum of loading frequencies, allowing for direct estimation of lateral loads. The system design and installation is described, as well as the method of time series analysis that is being investigated. Introduction Today, there are many more large and/or tall engineering structures than in the past. These structures are being designed to be more flexible, and to resist extensive damage from changes in temperature, severe wind gusts and earthquake tremors. Structural engineers require precise, reliable instruments to resolve their concerns about angular movements, displacements and structural vibrations. Such instrumentation, and associated analysis tools, also aid in the development of building design codes. For many years, monitoring the dynamic behaviour of engineering structures has relied on measurements made by instruments such as accelerometers and anemometers, installed on the structure of interest. The response data provided by such instruments normally require an integration process to arrive at the relative displacements. In contrast, the Global Positioning System (GPS) technology can measure directly the position coordinates, and nowadays relative displacements can be measured at rates of 10Hz and higher. This provides a great opportunity to monitor, in real-time, the displacement or deflection behaviour of engineering structures under different loading conditions, through automated 'change detection' and alarm notification procedures. Recent studies have demonstrated the feasibility of deploying GPS instruments as 'smart sensors' for the dynamic monitoring of structures. Reports of experiments, and projects, related to such studies can be found in, for example, Ashkenazi et al (1997); Guo & Ge (1997); Cooper (1998); Brown et al (1999); Celebi et al (1998); Roberts et al (1999, 2000); and Duth & Hyzak (1997). In this paper, the authors describe a project in which a high precision RTK-GPS system has been installed to complement existing structural monitoring instrumentation at the Republic Plaza Building (RPB), Singapore (Figure 1). The purpose of the GPS system is to provide, to subcentimetre accuracy, and at a rate of up to 10 samples per second, position vectors with respect to a fixed base station, of two antennas installed on the building parapet. The system will be operated in parallel with, and linked to, an existing logging system that records signals from accelerometers and anemometers. The system is intended to be 'open' to future software-based improvements in positional accuracy as well as online data analysis. The System Design and Installation The aim of the GPS-based measurement system is to determine, by direct measurements, the absolute structural deflections arising from ambient effects (such as temperature differentials, and static and dynamic components of wind). This system will permit the analysis of data downloaded automatically from the RPB, and to occasionally adjust, by remote control, the system parameters. The existing monitoring instrumentation (Figure 2) has been upgraded to Figure 1. Republic Plaza Building (RPB), one of the tallest buildings in Singapore. incorporate the new GPS component, as shown in Figure 3. The GPS component is composed of three main subsystems: i) One base station comprising a dual-frequency, geodetic-grade GPS receiver installed at a nearby location, complete with mounting brackets, accessories and battery back up for operation for up to one day without power. ii) Two 'rover' stations, comprising a pair of dualfrequency GPS receivers installed on existing masts attached to the 66th level parapet of the Republic Plaza Building. iii) Control centre PC running the Real-Time Monitoring software. The communication link between the control centre PC and the GPS stations is provided via a UHF radio link to the base station and RS-232 connections to the 'rover' receivers. The UHF radio link transmits the base station GPS data to the rover receivers continuously, allowing for Real-Time Kinematic (RTK) solutions to be generated. Instantaneous time and position results for the rovers' antennas can then be output via the RS-232 serial cables. The system is designed to provide accurate position information of up to 10 samples/second in order to detect translational and torsional responses of the building to load. It will run in parallel (and synchronously) with the existing system of two roof-mounted UVW anemometers and B1 accelerometers. The two UVW anemometers supply six analogue signals, while the accelerometers supply four analogue signals, to a data logger installed in level 65 via multi-core cables. The existing system generates samples at 7.5Hz into consecutive records of 4096 samples, and stores the records to disk on event trigger.