GPS Structural Deformation Monitoring: the Mid-Height Problem
GPS has been used to monitor engineering structures for a number of reasons. One important reason for monitoring high rise buildings (and other engineering structures) is their safety assessment in events of extreme loading, such as earthquakes and storms. Decisions must be made as soon as possible, whether to allow re-occupation of such buildings, or to assess them for further damage. The time required to reach such decisions is cost-critical, both for the building owner or manager and for the agency doing the assessment. Peak inter-storey drift ratio and detection of permanent damage are some of the damage assessment parameters recommended by assessment agencies. Traditionally, accelerometers have been used to monitor these parameters. Accelerometers measure accelerations which are double-integrated to get displacements. These double integrated displacements are then used for computing the inter-storey drift ratios and locating permanent damage. Displacements obtained by double-integration and inter-storey drift ratios by subtraction of these displacements, are often erroneous and unreliable and direct measurement of displacement is preferred. Direct measurement of displacement is required at a number of points along the height of the building. For example, for computing inter-storey drift ratios, measurements of displacement at both the floor level and roof level are required. Such points on buildings and other engineering structures of vertical profile are termed as mid-height points in this thesis. While GPS has been used for deformation monitoring of engineering structures and to assist in damage assessment during and after extreme loading events, its use has been limited to roof top installations. This research is an attempt to measure displacements at mid-height locations of engineering structures of vertical profile using GPS.
A novel technique based on the combination of GPS observations from two GPS receivers, installed on opposite sides of a high rise building, is developed in this research. GPS observations from one of these two receivers are shifted geometrically to portray observations received at the other receiver. Such a geometrical shift and subsequent combination of GPS observations makes a complete set of GPS observations available at one GPS receiver. Although GPS observations are recorded at two GPS receivers installed at different points, they are equally affected by any displacement of the building, provided there is no relative movement between the two GPS receivers. If such combined observations are processed using standard GPS processing software, any change in position can be determined with sufficient accuracy.
The technique developed in this thesis is tested under conditions of variable complexity for proof of concept. The results show that by using this technique, mid-height points on engineering structures can be monitored with centimetre level accuracy. The technique developed in this thesis extends the use of GPS to monitor midheight points on engineering structures of vertical profile. Currently GPS can be used to monitor only locations with a clear view of the sky. The objective of this thesis is to prove the concept of combining GPS observations from two GPS receivers to monitor mid-height points using the available technology and standard processing software. With improvements in technology and by further development, the performance of the technique developed in this thesis can be further refined.