Scaled shaking table testing of higher‐mode effects on the seismic response of tall and slender structures

Abstract

As structures are built taller and more slender, their response becomes increasingly sensitive to dynamic loads that are induced by the buildings' higherfrequency vibration modes. This leads to a more complex seismic response of high-rise structures when compared to the seismic response of low-rise structures that are primarily governed by their fundamental modes of vibration. This paper presents the results of extensive shake table testing and finite element analyses of a small-scale, 1.5-meter-tall shaking table specimen that was designed and scaled to experimentally capture the higher-mode effects of a 125-meter-tall reference tall building. The test specimen is comprised of a simplified superstructure with uniform mass and stiffness representing the main structure of the full-scale reference tall building, and a base-rocking mechanism to represent the moment-limiting effect of the inelastic plastic-hinging mechanism at the base of the reference building. Using the methodology proposed in this paper, the scaled specimen was shown to exhibit similar seismic response characteristics, including the corresponding higher-mode effects, along the height of the structure as those predicted numerically on the full-scale reference building. Results of the shaking table tests experimentally provided further evidence that relying only on a moment-limiting mechanism at the base of a cantilever structure is insufficient in limiting peak seismic loads due to higher-mode effects. In addition, by comparing test results with predictions obtained using several previously proposed analytical methods, the paper demonstrated that predicting shear force amplifications due to higher-mode effects is still challenging. The methodology developed in this study can be used to design other similar small-scale shaking table tests for the development of new analytical methods to predict higher mode effects and experimentally validate the efficiency of new high-performance systems developed to mitigate higher-mode effects on tall and slender structures and more generally to assess the ability of these systems to achieve enhanced seismic resilience.

Seismic response of slender MDOF structures with self‐centering base shear and moment mechanisms

“…Shear force demands at the base and mid‐height were less well‐predicted since shear force demands along the height of the structure were not limited by the base plastic hinging mechanism of the rocking base 2 and thus were more sensitive than the OTM demands. The maximum Interstory Drift Ratio (IDR) showed a good correlation between the experimental and numerical results, as IDR for this structure was mostly influenced by the elastic stiffness of the superstructure that was designed to experience relatively large elastic deformations under the scaled GM excitations in order to better emulate the effects caused by higher mode vibrations 26 . The peak roof accelerations and the maximum rocking uplifts were both more dispersed than the other parameters, probably owing to rocking‐impact and friction‐sliding responses during rocking action that are not accounted for with the rotational spring model.…”

“…The maximum Interstory Drift Ratio (IDR) showed a good correlation between the experimental and numerical results, as IDR for this structure was mostly influenced by the elastic stiffness of the superstructure that was designed to experience relatively large elastic deformations under the scaled GM excitations in order to better emulate the effects caused by higher mode vibrations. 26 The peak roof accelerations and the maximum rocking uplifts were both more dispersed than the other parameters, probably owing to rocking-impact and friction-sliding responses during rocking action that are not accounted for with the rotational spring model. Similar observations were made based on the r values, except that a low r value was reported for the base OTM demands, as a result of the discrepancies between the experimental and numerical rocking and non-rocking cases (cases where rocking occurred during tests but no-rocking was predicted numerically, and vice versa).…”

“…A detailed description of the experimental setup and test procedure is discussed in Zhong and Christopoulos. 24,26 This dataset was selected for numerical validation as it provided shaking table test results of a slender, higher-mode sensitive MDOF specimen with a scaled shear and flexural dual base system (i.e., the SCRIP system) under a large number of spectrally compatible ground motions, enabling both deterministic and statistical validations of the numerical modeling scheme. The scaled shaking table specimen (shown in Figure 2) was designed based on the 42-story benchmark building (shown in Figure 1) studied as part of the case study for the development and numerical validation of the SCRIP system, 27 following F I G U R E 3 Numerical modeling scheme of the shaking table specimen a scaling procedure that maintained the dynamic response characteristics of the slender superstructure including highermode effects.…”

“…The scaled shaking table specimen (shown in Figure 2) was designed based on the 42-story benchmark building (shown in Figure 1) studied as part of the case study for the development and numerical validation of the SCRIP system, 27 following F I G U R E 3 Numerical modeling scheme of the shaking table specimen a scaling procedure that maintained the dynamic response characteristics of the slender superstructure including highermode effects. 24,26 A total of 100 shaking table tests using synthesized ground motions were performed for the specimen with the scaled SCRIP base system shown in Figure 2 and were repeated for the specimen with a free-standing rocking base mechanism for comparison purposes. Both test series were used in this section for the numerical model validations.…”

Effect of ground motion processing and filtering on the response of rocking structures

“…In fact, this approach is consistent with the fundamental seismic design problem, which involves computing the response to a set of ground motions, rather than to an individual one. [95][96][97][98][99][100][101][102][103] For an ensemble of ground motions to be useful, in this paper all ground motions were scaled to the same PGV. PGV was used as an intensity measure, as it performs better than PGA or PGD for rocking structures.…”