Civil Engineering Reference
In-Depth Information
TEST: -- WSU: 100
2
2000
1000
0
-2
0
0
0.5
1
0
200
Time (s)
400
600
Frequency (Hz)
x 10
4
20
2
0
1
-20
0
0
200
Time (s)
400
600
0
0.5
1
Frequency (Hz)
Fig. 8.3
Sample acceleration time-history data (
left
) and corresponding frequency domain representation (
right
)
a
b
c
South Tower Displacement vs. Temperature
North
Side Displacement vs. South Side Displacement
North Tower Displacement vs. Temperature
1
35
35
0.5
30
30
25
25
0
20
20
15
15
-0.5
10
10
-1
-0.5
0
0.5
1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
1.5
relative displacement (inch)
relative displacement (inch)
north relative displacement (inch)
Fig. 8.4
Relationship between the bridge girder longitudinal displacements and temperature
analysis tools are used to extract modal properties at six different time periods over a day. Figure
8.3
is a typical plot of
vertical acceleration data of the deck in the time and frequency domains; this type of data is used to extract modal properties
using SSI. The relationship between modal frequencies and environmental variables is studied. In detail, the relationship
between modal frequency and temperature, modal frequency and time of the day at different days in the week, and the
relationship between horizontal movement of the bridge deck and temperature are all studied.
8.4.1 Relationship Between Modal Frequencies, Temperature and Traffic
Temperature change in the Bay Area varies in the range of 8-40
C during a typical day in the summer. This large
temperature change will impact the material properties of the steel girder box of the bridge. The orthotropic steel box girder
of the New Carquinez Suspension Bridge is connected to the concrete footings at the wind tongues located at the towers. It is
allowed to move in, longitudinal and vertical directions to compensate for imposed loads. To evaluate and monitor the
performance of the bridge, the behavior of the deck movement is studied for the testbed structure. Data is collected from the
string potentiometers installed longitudinally at the wind tongues (Fig.
8.2c
) of both towers. As the initial position between
the girder and the wind tongues is impossible to know, only the relative deck movement is obtainable as temperature varies
(Fig.
8.4
). This data verifies deck contraction with reducing temperature. The relative displacements from both sides of the
bridge (at north and south towers) are also linearly related with a slope of 1 (Fig.
8.4c
). As the way the string potentiometers
are installed, the positive direction is defined as north for data from the north side and south for data from the south side.
These facts indicate the bridge girder expands in the longitudinal direction when temperature increases and shrinks when the
temperature decreases, which obey thermal expansion rules as expected. The contraction behavior due to temperature
changes is symmetric when looking at the north and south sides of the bridge.
