Application of GPS in the Simulation Study of Dynamic Characteristics of Tall Buildings

Dynamic characteristics including relative displacements and vibrating frquencies,which are key to assessing drift and stress conditions of large structures such as tall buildings, longspan suspension and cable-stayed bridges and tall chimneys,are difficult to measure directly,especially under the condition of earthquakes and strong winds or typhoons.In general,laser collimator,total station and accelerometers are the most common instruments used to monitor the response of structural systems,but can not work properly during earthquakes and strong winds or typhoons,and in most cases do not take place in real-time or near real-time yet.Until recently,there are no efficient or feasible methods to measure displacements during an earthquake or severe wind.Therefore there is a great need for developing new method or technology to monitor the dynamic characteristics of large structures precisely in real-time or near real-time during very bad conditions. Fortunately,recent advances in Global Positioning System (GPS) technology allow reliable monitoring of above-mentioned long-period structures.GPS units with a capability of resolving motion at the centimeter or sub-centimeter level with sampling frequency 10Hz or even 20Hz are now available from several manufacturers.This provides a great opportunity to monitor long-period structures reliably.To the authors' best knowledge,the capability of identifying vibration characteristics from GPS observations has not been widely verified.For the scientific justification and feasibility of using kinematic GPS technology to identify the dynamic characteristics of tall buildings,some vibrating experiments are designed to simulate the movements of tall buildings in the paper,especially under the condition of earthquakes and strong winds or typhoons.The experimental equipment consists of:①NovAtel-Outrider-DL-RT2 dual frequency GPS receivers with sampling frequency of 10Hz; ②vibrating unit,which is composed of power amplifier,signal generator,signal analyzer,regulator,exciter,displacement sensor,mass block and rigid beam or elastic beam,and which can supply simple harmonic vibration environment or damped simple harmonic vibration environment,and corresponding precise basis of vibrating frequency and amplitude.The intrinsic frequency supplied by vibrating unit is jointly determined by the mass of GPS antenna,the material and geometrical dimensions (length,width and height) of rigid or elastic beam.The paper also discusses in detail how GPS data are recorded,processed and analyzed.With post-process version of NovAtel's Softsurv software and auto-regressive (AR) spectral analysis method,relative displacements and corresponding vibrating frequencies can be derived from GPS observations.The results indicate that clear and accurate dynamic characteristics such as relative displacements and at least low frequencies can be identified by kinematic GPS technology.Moreover,with future advances in GPS technology and improvements in sampling capability,it will be possible to monitor short-period structures as well.This capability can be very useful to be used for the health monitoring purposes of large structures such as tall buildings and long-span or cable-stayed bridges.Considering the practical purposes,some suggestion is also given in the paper.Requisite procedures and software should be developed to permanently deploy GPS units on tall buildings,and to assess and mitigate the natural hazards such as typhoon,flood and earthquake affecting the structures,and methodologies on how the findings can be incorporated into useful practical design procedures.