r - The Hong Kong Polytechnic University

A wind and structural health monitoring system (WASHMS) is deployed by Highways Department to monitor
the structural performance of Stonecutters Bridge and its environment under the designated performance criteria
at serviceability limit state. The performance limits at the serviceability limit state are selected as the base-line
of structural health monitoring. This is because the serviceability load combinations specified in most limit state
design codes for bridgeworks are reasonable estimates of combinations of loads likely to cause maintenance
problem or affect the performance of the structural system and its components, and the probability of
exceedance (POE) for the serviceability load combination is approximately 0.528% per year or 63.36% in
120-year return period basing on 120-year design life (i.e, when the return period is equal to the design life, the
POE = 63.36%). Such value of probability of exceedance is normally considered as the standard for load and
load-effect design at serviceability limit state in most bridge design codes and standards such as BS5400 and
AASHTO.
2. SCOPE OF SCB-WASHMS
The SCB-WASHMS is the acronym of the Wind And Structural Health Monitoring System for Stonecutters
Bridge. It is ultimately composed of four systems, namely, structural health monitoring system (SHMS) [Ref.
15], structural health evaluation system (SHES) [Ref. 32], structural health rating system (SHRS) [Ref. 31] and
structural health data management system (SHDMS) [Ref. 15]. They are devised to carry out the respective
works of monitoring, evaluation, rating and data management. A flow diagram of the SCB-WASHMS is shown
in Figure 1. The SHES and SHRS, which are currently under development, are arranged under separated
contracts other than the SCB-WASHMS, and therefore they will not be presented in this paper.
3. FUNCTIONS AND COMPONENTS OF SHMS
3.1 Functions of SHMS
The SHMS is devised to monitor the structural performance and the environment of the Stonecutter Bridge and
its environment under its in-service condition by collection, processing and analysis of the measured data
obtained from its on-structure instrumentation system. The structural performance and the environment are
referring to the designated performance criteria at the serviceability limit state (SLS). These SLS performance
criteria are expressed in terms of the following four categories of physical and chemical quantities, i.e., (i)
environment loads and status monitoring which includes the measurands of wind and weather, temperatures,
seismic and corrosion status, (ii) operation loads monitoring which includes the measurands of highway traffics,
ship impacting and permanent loads, (iii) bridge features monitoring which includes the measurands of static
and dynamic features of Stonecutters Bridge, and (iv) bridge responses monitoring which includes the
measurands of stay forces, tendon forces, displacements, stress/force distribution, fatigue damage and
articulation responses. The operation flow diagram of SHMS is illustrated in Figure 2.
In SHMS, both direct data acquisition and indirect data acquisition are adopted. The former refers to the usage
of the measured data (such as temperatures and strains/stresses) directly in the interpretation of structural
performance such as temperatures variations and strain/stress status in structural components; whereas the latter
refers to the further processing and analysis of measured data (such as displacements, accelerations and loads)
by analytical models or methods in interpretation of structural performance such as global bridge dynamic
features variations, global bridge geometry profiles variation, etc.
3.2 Hardware of SHMS
The hardware of SHMS is composed of sensory system, portable data acquisition system, data acquisition
system, cabling network system, data processing and control system, and portable inspection and maintenance
system. The schematic hardware layouts for one-dimensional signals and two-dimensional (video) signals are
given in respective Figure 3 and Figure 4. The brief functional description of these six modular systems is
outlined in following paragraphs.
3.2.1 Sensory System (SS)
The sensory system, which refers to the transducers and their interfacing devices for detecting and collecting the
physical/chemical signals, is the most important part of the SHMS as all hardware and software are devised and
deployed as a results of the selected sensory system. The sensory system should be deployed to achieve six
functions, i.e., (i) able to measure bridge responses at both local and global levels, (ii) able to integrate the
measured results with the analyzed/predicted results, (iii) able to acquire data in a consistence and retrievable
-244-