PSI - Issue 44

Masoud Pourmasoud et al. / Procedia Structural Integrity 44 (2023) 590–597

595

6

M. Pourmasoud et al. / Structural Integrity Procedia 00 (2022) 000–000

Table 4 and Fig. 4 summarize the yield strength results under the maximum displacements and maximum seismic loads which usually are the target during the design and isolators’ prototype tests. The yield strength ratios (   ) show the accuracy of the predicted yield strength in comparison with the tests results. The fifth case study shows the tests’ yield strength relatively less than the equation outcome. It is because this test was carried out after 36 cycles of loading, which caused a partial rounding in the circumference of the lead core and a correspondingly decrease in the lead area. Generally, the test results show that the lead cores’ yield strength have varied between 6 MPa to 10 MPa depends on their confinement conditions and proposed equation has predicted that with about 3% difference in average.

σy − equation σy − result 92.28 %

Table 4 Yield strength comparison under maximum seismic loads and maximum displacements.

Shear Strain (      

σ y result (MPa)

σ y equation (MPa)

Pressure (MPa)

Isolator

1

11.73

118.18%

0.72

9.78

9.02

2

6.49

80.00%

0.88

6.42

7.05

109.73 %

6.85

7.10

103.70 %

3

9.73

80.00%

0.92

7.29

7.31

100.36 %

4

19.86

80.00%

0.90

9.34

10.79

115.50 %

5

14.95

91.42%

0.57

9.59

9.87

102.91 %

6

20.06

91.42%

0.48

7.49

7.36

98.32 %

7

5.67

68.40%

0.29

Average

103.3 %

12.00

10.00

8.00

6.00

4.00

2.00

0.00

Yield strength (MPa)

Isolator 1

Isolator 2

Isolator 3

Isolator 4

Isolator 5

Isolator 6

Isolator 7

Isolator number

σ y - result

σ y - Equation

Fig. 4. Yield strength comparison under maximum seismic loads and maximum displacements.

4. Summary and Conclusion

Lead core plays an important role in the performance of Lead Rubber Bearings (LRB) as it provides the damping effect of the system. Underestimation of the lead core’s yield strength causes insufficient damping and extra travel on the isolation level, while overestimation produces a stiff system that transmits unfavorable base shear to the super-structure. The yield strength of the lead core is suggested to be between 6 to 10 MPa in current codes/guidelines depending on loading conditions. Yield strength can be calculated from the hysteresis loops that are obtained from prototype tests. However, there can be incompatibilities between the design targets and test results. This study investigated more than 300 tests to develop a new approach to estimate the yield strength of lead cores with high accuracy prior to prototype testing. The following conclusions were made based on the new approach: • Lead rubber bearings are energy dissipative even under zero/tension and shear actions. While the hysteresis loops of isolators under tension/zero axial loadings are not as inflated as an isolator under heavy axial loading (particularly those with      0.5 ), they still provide a reasonable amount of damping. Currently this effect is neglected in the design process of lead rubber bearings.