Applications on Bridges

5.2.1 Applications as bridge lock-up devices

According to the European Standard (EN15129) ^[56], about the anti-Seismic devices, the shock-transmission units (STU), which are usually used on bridge structures, are also called lock-up devices, which are defined as following:

Device whose output is an axial force that depends on the imposed velocity; its principle of functioning consists of exploiting the reaction force of a viscous fluid forced to flow through an orifice in order to provide a very stiff dynamic connection whilst for low velocity applied loads the reaction is negligible.

The STUs are installed between the pier walls and girders of bridge moving ends ^[58], as shown in Fig. 5-22, and are used for keeping the moving condition under normal operations, such as thermal, creep and shrinkage, but locking structures under large excitations, such as earthquakes. The conventional designs for STU focus on the mechanisms of piston heads; the design of small orifices on the piston head allows silicone compound flow through and prevents much damper forces be generated, as shown in Fig. 5-23 ^[58].

The design concept for STUs are similar to nanofluid dampers; these two kinds of devices both provide very small force behavior under low velocity and operate as general viscous dampers under earthquake excitations. As suggested in the European Standard (EN15129) ^[56], the STUs should be tested with the low velocity tests under triangular waves with velocity less than 0.1mm/sec, and the force responses should be smaller than 10% of damper design forces. According to Fig. 4-74, the force responses of specimen one under low speed tests with velocity less than 0.1mm/sec are about 1KN; by contrast,

the force responses under high speed tests, about 150mm/sec to 200mm/sec, which are usually the design velocity range of buildings, are about 200KN. Similarly, for the testing results of specimen two, shown in Fig. 4-75, the force responses under low velocity tests must be much smaller than 10% of design forces. Therefore, the nanofluid dampers are suitable for the STU uses.

5.2.2 Applications as bridge dampers with long durability

To further verify the long durability properties of nanofluid dampers used on bridge structures, the time history response data of a practical bridge experiment ^[59] is used to comparison the difference of dissipated energy between nanofluid dampers and conventional viscous dampers. The purpose of this experiment is to verify the effectiveness of retrofit on the damaged highway bridge after a strong earthquake. The testing bridge and piers are made of reinforced concrete, and the testing unit comprises seven continuous spans and eight piers, named A1, and P1 to P7.

The tests are conducted with one loaded truck driving at three different constant speeds passing through P4 to P7; the constant speeds are set as 20, 40, and 60KM/hr, named case one, case two, and case three respectively, as drawn in Fig 5-26. The truck is weight about 14tf, and will be loaded with concrete blocks until the total weight being 24tf, as shown in Fig. 5-24. The bridge deck is attached by three speedometers in the middle of every two piers from P4 to P7, as shown in Fig. 5-25. Assume the same nanofluid dampers tested in Chapter 4 are adopted, and the specimen one (filled with nanofluid PPG3000-R972-10%) and specimen two (filled with nanofluid PPG1000-R972-10%) are respectively installed between the pier P7 and the girder of bridge. For comparison, conventional viscous dampers merely possess the same force property of 1 part of these nanofluid dampers are adopted; for example, if the

specimen one possesses F _D 11V_D³ for velocity smaller than 0.93mm/sec and

59 .

25 0

.

9 _D

D V

F  for velocity larger than 0.93mm/sec, the conventional viscous damper used for comparison merely possesses F _D 11V_D³ for any velocity. Owing to the installation position of nanofluid damper, the longitudinal measured records of the speedometer between P6 and P7 (channel 7) are used, which are plotted in Fig. 5-27 to Fig. 5-29. Assume the piers are rigid under the tests; hence the vibration responses measured on the bridge deck could be regarded as the relative motions between the both ends of nanofluid dampers. Hence, according to the velocity responses of the three tests, the hysteresis loops for nanofluid damper and conventional damper are compared in Fig.

5-30 to Fig. 5-32 for specimen one and Fig. 5-33 to Fig. 5-35 for specimen two. From these comparisons, it could easily be discovered that nanofluid dampers exhibit much smaller hysteresis loops under normal operations. To further compare the dissipated energies, the comparisons of accumulative damping energy for each specimen and case are plotted in Fig. 5-36 to Fig. 5-41. It could be seen that the nanofluid dampers produce much less damping energy under low speed motions; the accumulative damping energies for specimen one are only 2.96%, 2.29% and 1.62% of corresponding conventional damper according to the three tests from low velocity to high velocity respectively, and such for specimen two are also merely 11.81%, 9.74% and 9.47% of corresponding conventional damper. Although no former study confirms that the accumulative damping energy will directly affect the durability of one damper (or the seal system), the less energy implies that the seal system will receive smaller pressure and wear.

CHAPTER 6 Summary