An Experimental Study on the Way Bottom Widening of Pier Foundations Affects Seismic Resistance

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In a seismic design of embedded foundations, the vertical Subgrade Reaction (SR) acting on a foundation bottom surface and the Rotational Resistance Moment (RRM) generated by the SR are calculated using an SR Modulus (SRM). The SRM and RRM depend on both ground rigidity and Foundation Width (FW). However, the SRM and RRM calculation methods adopted in design codes might not properly consider their FW dependency. In this study, SRM and RRM evaluation methods for embedded foundations subjected to a seismic load were examined by conducting a two-dimensional finite element analysis under the condition where ground rigidity and FW were changed considering the nonlinearity of the ground. The results show that when the seismic load is large and the nonlinearity of the ground appears, the SR distribution is different from the assumption in the design code. The FW dependency of the SRM was lower than the assumption of the design code. Furthermore, methods to calculate the SRM and RRM in accordance with the FW and ground rigidity are proposed.

Section: Imentioning

confidence: 99%

Evaluation Methods of Vertical Subgrade Reaction Modulus and Rotational Resistance Moment for Seismic Design of Embedded Foundations

Nagao

Tsutaba

2021

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“…Therefore, if the seismic load is very large, FW should be very wide. Conversely, an experimental study on the seismic resistance of piers with various FWs has shown that the piers with wider FWs are extremely seismic resistant due to the increase in vertical subgrade reaction [9]. The results have shown that the calculation equation of the design practice could underestimate SRM for wide FW conditions.…”

Section: Introductionmentioning

confidence: 97%

Effect of Foundation Width on Subgrade Reaction Modulus

Nagao

2020

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Structures are subjected to vertical and horizontal loads. Vertical subgrade reaction acts on the foundation bottom surface, and in the case of an embedded structure, horizontal subgrade reaction acts on the embedded part. The subgrade reaction is obtained by multiplying the displacement of the foundation by the Subgrade Reaction Modulus (SRM). In practice, SRM is calculated using an equation that incorporates the negative power relationship of the Foundation Width (FW). If the structure is evaluated to be poorly seismic resistant, it is necessary to widen FW. However, when the FW is widened, the design value of SRM decreases. In this case, it is not possible to expect an increase in the subgrade reaction proportional to the increase of FW. Therefore, when the inertia force is very high, the FW has to be very wide. However, underestimating SRM can lead to structural overdesign. In this study, the relationship between SRM and FW, for a structure in which vertical and horizontal load act simultaneously, was analyzed. Compared with the design practice assumptions, the horizontal SRM was found to be highly dependent on FW while the vertical SRM was shown to be less dependent on FW.

“…All ground motions were imposed on the NPP models in horizontal direction. Normally, the lateral acceleration or displacement responses of structures are monitored during earthquakes [20][21][22]. The seismic response of both non-and base-isolated NPP structures was obtained in terms of the FRS.…”

Section: Numerical Modelingmentioning

confidence: 99%

Seismic Responses of NPP Structures Considering the Effects of Lead Rubber Bearing

Nguyen¹,

Nguyen²

2020

Abstract-This study investigates the effects of Lead Rubber Bearings (LRBs) on Floor Response Spectra (FRS) of Nuclear Power Plant (NPP) structures. Three main structures in the Advanced Power Reactor 1400 (APR1400) NPP including the reactor containment building, an internal structure, and an auxiliary building were numerically developed in SAP2000. The structures were modeled using beam stick elements, and lumped masses were assigned to beam element nodes. All equivalent section properties of beam elements were calculated based on the designed cross-sections of the structures. A series of 40 ground motions with response spectra scaled to match the NRC 1.60 spectrum were utilized in numerical time-history analyses. Finally, a thorough comparison of FRS was conducted at different elevations of the structures, considering both with and without LRB. Numerical results showed that the FRS of base-isolated structures at higher elevations was significantly reduced compared to non-isolated structures. However, at lower elevations, the FRS was higher for the base-isolated structures compared to the non-isolated ones. Additionally, at a low-frequency range, roughly smaller than 3 Hz, the FRS of base-isolated structures was always greater than that of the non-isolated ones.