r - The Hong Kong Polytechnic University

flexural tower modes, (vi) longitudinal flexural tower modes and (vii) torsional flexural tower modes. From
these types of vibration mode shapes and their modal participation factors, the major modes and frequencies of
vibration under different types of dynamic loads such as wind and seismic can be identified for
observation/monitoring. The customized software tools are also required to compare the measured results with
the analyzed results. The flow diagram of the software tools for global dynamic features monitoring is shown in
Figure 23.
J. Customized Software Tools for Stay Forces Monitoring [Ref.15 and 29]
The stay forces monitoring refers to the monitoring of the dynamic features of stay cables, hence the tensile
forces and stability against wind-rain-induced vibrations. This monitoring requires the execution of field
vibration measurement works on the stay cables. The equipment and facilities required are the PDAS-1, the
removal accelerometers and the associated portable power and signal cables. During measurement, two
accelerometers (one for measurement and the other for control/calibration) should be used and in order to
minimize the measurement errors induced by the stay cable deck anchorage, the first accelerometer should be
fixed as a distance of no less than six meters away from the edge of the stay cable deck anchorage. Customized
software tools are required to: (i) extract the in-plane flexural frequencies, out-of-plane frequencies and
longitudinal frequencies of the stay cable, hence determine the tensile force in stay cable under ambient
vibration measurement, (ii) determine the logarithmic decrement, hence determine the Scruton number basing
on manual-excited forced-free vibration measurement, and (iii) plot the analyzed results and together with the
corresponding analysis/design/as-built values. The flow diagram of the software tools for stay forces monitoring
is shown in Figure 24.
K. Customized Software Tools for Tendon Forces Monitoring [Ref. 15]
The tendon forces monitoring refers to the monitoring of the forces in the eight tendons per deck-girder-box,
which are designed for enhancing the deck strength at the interfacing regions between steel/concrete in the
side-span. A total of 32 tendons (each of 37 strands) are used and therefore 32 nos. of tensio magnetic gauges
are deployed, i.e. one for each tendon. The tensio magnetic gauge used is EM sensor (EM-T-160) with
dimension 400mm length x 220 mm overall diameter for 160 mm strand bundle at a measuring stress range of
1770 MPa and the data logger equipped is PowerStress P-500-08, which is a 8-channel data logger with a
sampling rate of 0.1 Hz. The customized software tools for tendon forces monitoring are comparatively simply
as solely time-average statistics analysis is required. The flow diagram of the software tools for tendon forces
monitoring is shown in Figure 25.
L. Customized Software Tools for Displacements Monitoring [Ref. 15 and 19]
The displacement monitoring of Stonecutters Bridge is based on GPS (rover stations and reference stations) and
tiltmeters. The GPS (rover stations – for measurement of bridge motions) are installed at tower-tops and on two
edges of the bridge-deck section at 1/4, 1/2 and 3/4 of main span; whereas the tiltmeters are installed in both
towers at H (or tower-top), 2/3H, 1/3H and 0/3H (or deck-level), where H is the tower-height above the deck
level. The tiltmeters are also installed on the internal walls of the upper side-span piers. All tiltmeters are
bi-axial tiltmeters. The GPS (reference stations – 2 nos., one for master reference and one for backup reference)
are installed at roof-top of East Portal Building which is around 1 km from Stonecutters Bridge. All time-series
data from different types of sensors are synchronized and time-tagged to the GPS (reference stations) with a
minimum accuracy of ±0.1 second. Each GPS is installed at a stiff GPS post so that the GPS installed at the
post-top should have the same motion as the measurement point at the post-base. In order to minimize the
multi-path effects at ground/base level and/or avoidance of obstruction induced by the passage of high-side
vehicles, the heights of the GPS posts at deck-levels, tower-tops and roof-top are around 4 meters, 3 meters and
1.5 meters respectively. Both GPS (digital sensors) and tiltmeters (analogue sensors) are configured to have the
same sampling rate of 20 Hz. Figure 26 illustrates the arrangement of GPS (rover stations) at deck-levels.
All GPS are Leica GPS, in which the GPS (rover station) is GX1230 GG GNSS receiver with AX1202 GG
GNSS antenna; whereas the GPS (reference station) is GX1230 GG GNSS receiver with AT504 GG
Choke-Ring GNSS antenna. The acronym of GNSS (or Global Navigation Satellite System) is a technique that
supports the satellite-signals from both systems of American GPS and Russian GLONASS. The horizontal
measurement accuracy of GPS is 3mm±0.5ppm (for static mode) and 10mm±1ppm (for real-time kinematic
mode); whereas the vertical measurement accuracy of GPS is 6m±1ppm (for static mode) and 20mm±1ppm (for
real-time kinematic mode). The tiltmeter used is Model 716-2A Wall Mount Tiltmeter from Applied
Geomechanics, which has a resolution of 0.1 μ-radian.
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