PSI - Issue 44

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

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

2019). An optimized seismic isolation system should diminish floor accelerations and drifts to less than the damage thresholds which are specified by seismic codes (Pal et al. 2019). It is particularly important for the case of seismic isolators that are subjected to high peak ground motions in near field zones (Özuygur et al. 2021; Bhagat et al. 2018; Alhan and Öncü-Davas 2016). It should be noted that the vertical component of ground motions in a near field zone can also increase considerably the structural axial loads as well as horizontal responses (Wei et al. 2018). Seismic isolated structures are generally designed according to the specifications that are assumed for isolator bearings rather than their accurate characteristics (Kelly 2001; Skinner et al. 1993; BS EN 15129 2018; Du et al. 2021). Seismic design codes such as FEMA 451 (2006) simplify the complexity of seismic isolator behaviour to facilitate an easier designing process (Constantinou et al. 2011). The adequacy of a seismic isolator’s specifications is assessed during three distinctive stages (BS EN 15129 2018). The first stage is to determine the components specification of the isolator (JIS K6410-1 2011), where the resilience of each component of isolator bearings shall comply with relevant codes under recommended actions and be approved by the seismic isolation designer. In second stage, the isolator units are designed under the load combinations and lateral displacements to pass the prototype/production tests criteria (BS EN 1337-3 2005). The final stage is to assess the performance of the group of isolators to meet the design targets and structural expectations, using key performance parameters such as the maximum base shear and displacement. In addition, the durability and maintenance of isolators need to be addressed during the building lifetime. In a conventional lead rubber bearing, the lead core is the key component that provides damping. Lead plugs can provide high damping capacity due to their relatively low yield strength of about 10MPa and a perfect elastic-plastic behavior (Skinner et al. 1993). Although 10 MPa is known as a nominal number for lead yield strength ( σ y ), parameters such as the magnitude of applied axial load, dimension of lead core, lateral shear strain of bearing, and the adopted cyclic loads (degradation) can also considerably affect the lead yield strength (McVitty et al. 2015). Kelly (2001) proposed 7 to 8.5 MPa as the pragmatic yield level of the lead core depending on the axial load and level of confinement, with 10.5 MPa as the theoretical yield level. Furthermore, it was reported that more shim plates and thinner rubber layers restrain the lead core from bulging into the rubber layers, which help enhance yield strength along with utilizing confining plates at the top and bottom of the lead core. The multilinear behavior of a lead rubber bearing tends to be bilinear, providing appropriate confinement for the lead core, so a yield strength closer to the nominal value will be achieved. Pourmasoud et al. (2021) investigated the influential parameters on the lead core confinement and the lead yield strength of lead rubber bearings regarding the prototype tests’ results of more than 300 tests. For each test, the characteristic strength was calculated from data extracted from the third hysteresis loops and then divided to the characteristic strength under zero axial load (Q 0 ) to achieve the normalised characteristic strength (      . The applied axial loads were also normalised by the buckling load of each bearing (      to obtain the characteristic strength trend versus the applied axial load. Fig. 1 shows the trends extracted from all the selected tests. Generally, three different trends are recognised. The green line represents Low Confinement (LC) and low yield strength. This characteristic strength, even under high axial loads (      0.5 , is approximately 20% more than the characteristic strength under zero axial load (      1.2 . It should be noted that for all the case studies, the axial loads under seismic load combinations were more than 50% of the buckling load. The orange dots show the High Confinement (HC) trend with normalised characteristic strength of more than 1.6. The yield strength of this category is quite close to the theoretical yield strength (10 MPa). The outcomes between the two aforementioned categories (green and orange) are designated Medium Confinement (MC) and their normalised characteristic strength values lie between 1.2 and 1.6.

0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20

Q/Q0

0.00

0.20

0.40

0.60

0.80

1.00

P/Pcr

Fig. 1. Confinement categories and their effect on the normalized characteristic strength

Based on the results presented in Fig. 1, for each confinement category the characteristic strength of a lead rubber bearing is achievable through Eq. 1.          (1)