r - The Hong Kong Polytechnic University

displacement influence lines and stress influence line. The measured displacement range at GPS instrument
point: GP04 is 115 mm (or 1.4 mm per ton), and the corresponding measured stress range at strain instrument
point: SW60X is 16 MPa (or 0.195 MPa per ton).
3.4.4 Global Dynamic Features Monitoring (Figures 38-39)
Four hours of time-series acceleration data obtained from the two nos. of accelerometers installed inside the
steel deck-section at mid main span are used. The time duration for one data frame used is 30 minutes and the
total number of data points per data frame is 180,000 which are computed on a sampling rate of 100 Hz. The
frequency resolution for extraction is therefore 0.000556 Hz. The total number of data frames considered in
average is fifteen and the corresponding wind speed and air temperature references are respective 2.05 m/s
(10-miniute mean) and 16.4~17.5°C. The result of vertical flexural deck frequencies (the first 20 nos.) extracted
from the power spectrum is shown in Figures 38. Of these 20 measured frequencies, 16 of them are
corresponding to the analyzed results. The comparison between the measured results and analyzed results (by
normal mode analysis of a 3D space frame finite element model – built and analyzed by ANSYS/Multiphysics
[Ref. 38]) is shown in Figure 39. The comparison shows good correlation between measured and analyzed
results, and the model is reliable and accuracy for the analysis of highway load-effects.
3.4.5 Stay Forces Monitoring (Figures 40-41)
The example of stay forces monitoring is refers to the ambient vibration works which was requested by the
Engineer for checking the as-built stay forces with their computed stay forces under their reference (completion)
conditions [Ref. 36]. A total of 224 stay cables were measured in the seven nights during the period of 4-12
November 2009. Figure 40 illustrates the installation of accelerometer on stay cable and corresponding
procedures of data processing and analysis. Figure 41 illustrates the results of stay forces in the main span of
East Tower – North side obtained from the Engineer, the Contractor and Highways Department.
3.4.6 Displacements Monitoring (Figures 42-43)
The displacements monitoring refers to the input of the GPS data into the pre-built finite element bridge model
and re-generate the deflected bridge geometry profile/profiles and stress contours with stresses output at
key/instrumented structural sections/locations. Figure 42 illustrates the derived displacement profile (with
colour contours) of Stonecutters Bridge, and Figure 43 shows the corresponding derived stress in deck trough
sections.
3.4.7 Fatigue Damages Monitoring (Figures 44-45)
The example of fatigue damages monitoring is based on the results of dynamic strain gauges installed in the
deck trough section at bottom of north longitudinal box at mid main span. Figure 44 shows the stress demand
ratio plot, which is used to detect whether the measured stress range at deck trough section exceeding the
maximum allowable stress range of 35 MPa (Class F2). If the measured stress range exceeds the maximum
allowable limit, then micro-cracks might be occurred and the Palmgren-Miner rule cannot be applied in fatigue
assessment. As Figure 44 shows that the maximum measured stress range at the deck trough section considered
is 9.99 MPa, which is much less than the allowable stress range of 35 MPa (F2 Class), fatigue assessment can
therefore be proceeded. Figure 45 illustrates the fatigue damage assessment result. The estimate fatigue life at
the fatigue assessment point considered is 713 years, which is greater than the design fatigue life of 200~500
years.
3.4.8 Permanent Loads Monitoring (Figures 46-47)
The results of permanent loads monitoring at Stonecutters tower-base is shown in Figures 46 and 47. The total
measured compressive strains in Locations A and B of Figure 46 are 626 με and 651 με respectively at a
reference concrete temperature of 20°C. The strain analysis shows that the percentages of structural strain, creep
strain and shrinkage strain are respective 44%, 20% and 36% (upon the bridge opening to public traffics). The
strain due to permanent loads is around 44% of the total measured compressive strains or 281 με (=44%
x(626+651)/2). This strain corresponds to 8.486 MPa ≈ 8.392 MPa which is estimated basing on self-weight of
concrete (71250 tons), stainless steel (1400 tons), steel reinforcement (5500 tons), self-weight of steel deck and
stay cables (20100 tons per tower), pavement (2126 tons), street furniture (277 tons) and a tower-area of 119.94
m 2 at 5.8 mPD or the strain instrument level.
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