Comparative Assessment of Two Rocking Isolation Techniques for a Motorway Overpass Bridge

102

Summary This paper characterizes the ability of natural ground motions to induce rocking demands on rigid structures. In particular, focusing on rocking blocks of different size and slenderness subjected to a large number of historic earthquake records, the study unveils the predominant importance of the strong‐motion duration to rocking amplification (ie, peak rocking response without overturning). It proposes original dimensionless intensity measures (IMs), which capture the total duration (or total impulse accordingly) of the time intervals during which the ground motion is capable of triggering rocking motion. The results show that the proposed duration‐based IMs outperform all other examined (intensity, frequency, duration, and/or energy‐based) scalar IMs in terms of both “efficiency” and “sufficiency.” Further, the pertinent probabilistic seismic demand models offer a prediction of the peak rocking demand, which is adequately “universal” and of satisfactory accuracy. Lastly, the analysis shows that an IM that “efficiently” captures rocking amplification is not necessarily an “efficient” IM for predicting rocking overturning, which is dominated by the velocity characteristics (eg, peak velocity) of the ground motion.

Section: Introductionmentioning

confidence: 99%

Rocking amplification and strong‐motion duration

Giouvanidis

Dimitrakopoulos

2018

102

“…Since then, a plethora of papers on rocking structures has been published studying these structures theoretically [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and experimentally. [17][18][19][20][21][22] The rocking oscillator has been used in bridge engineering, [23][24][25][26][27][28][29][30][31][32][33][34] as well as to model the behavior of masonry walls, [35][36][37][38][39][40] of ancient temples, [41][42][43] of unanchored equipment, and of museum artifacts. [44][45][46][47][48][49] What has received less attention in the literature published in English is that rocking has been used in the USSR since 1960s (and is still used in former ...…”

Section: Introductionmentioning

confidence: 99%

Rolling and rocking of rigid uplifting structures

Bachmann

Vassiliou

Stojadinović

2019

Self Cite

Summary Allowing structures to uplift modifies their seismic response; uplifting works as a mechanical fuse and limits the forces transmitted to the superstructure. However, engineers are generally reluctant to construct an unanchored structure because the system could overturn due to lacking redundancy. Using a safety factor for the design of a flat rocking foundation, ie, designing it wider, goes against the main idea of this seismic modification method as the force demand for the structure increases. We propose to extend the flat base of a rocking block with curved extensions to better protect the block from overturning, yet not prevent its uplifting. After investigating the seismic response of such rocking blocks, we extend the study to investigate the seismic response of rolling and rocking frames comprising columns with curved base extensions. The equations of motion are derived, time history analyses are performed, and rocking spectra are constructed. We draw two important conclusions: (a) the response of a class of rocking oscillators with curved base extensions is equivalent to the response of a flat‐base rocking oscillators of the same slenderness, yet larger size; (b) the rotation demand on two negative stiffness rocking and rolling oscillators with the same uplifting acceleration and the same size is roughly the same as long as the rocking oscillators are not close to overturning. The above findings can serve as a basis for the rational seismic design of structures supported on rocking columns with curved bases, a system that has been used since the 1960s.

“…The rocking behavior of bridge structures on shallow footings when excited by earthquake motions has been recently considered as a potentially beneficial seismic design concept . The rocking motion in such systems involves the sliding and uplifting of footings, with significant nonlinear behavior expected from the supporting soils.…”

Section: Introductionmentioning

confidence: 99%

Seismic fragilities of single‐column highway bridges with rocking column‐footing

Xie

Zhang

DesRoches

et al. 2019

Summary Rocking isolation has been increasingly studied as a promising design concept to limit the earthquake damage of civil structures. Despite the difficulties and uncertainties of predicting the rocking response under individual earthquake excitations (due to negative rotational stiffness and complex impact energy loss), in a statistical sense, the seismic performance of rocking structures has been shown to be generally consistent with the experimental outcomes. To this end, this study assesses, in a probabilistic manner, the effectiveness of using rocking isolation as a retrofit strategy for single‐column concrete box‐girder highway bridges in California. Under earthquake excitation, the rocking bridge could experience multi‐class responses (eg, full contacted or uplifting foundation) and multi‐mode damage (eg, overturning, uplift impact, and column nonlinearity). A multi‐step machine learning framework is developed to estimate the damage probability associated with each damage scenario. The framework consists of the dimensionally consistent generalized linear model for regression of seismic demand, the logistic regression for classification of distinct response classes, and the stepwise regression for feature selection of significant ground motion and structural parameters. Fragility curves are derived to predict the response class probabilities of rocking uplift and overturning, and the conditional damage probabilities such as column vibrational damage and rocking uplift impact damage. The fragility estimates of rocking bridges are compared with those for as‐built bridges, indicating that rocking isolation is capable of reducing column damage potential. Additionally, there exists an optimal slenderness angle range that enables the studied bridges to experience much lower overturning tendencies and significantly reduced column damage probabilities at the same time.