Performance of Rocking Systems on Shallow Improved Sand: Shaking Table Testing

Abstract

Recent studies have highlighted the potential benefits of inelastic foundation response during seismic shaking. According to an emerging seismic design scheme, termed rocking isolation, the foundation is intentionally under-designed to promote rocking and limit the inertia transmitted to the structure. Such reversal of capacity design may improve the seismic performance, drastically increasing the safety margins. However, the benefit comes at the expense of permanent settlement and rotation, which may threaten postearthquake functionality. Such undesired deformation can be maintained within tolerable limits, provided that the safety factor against vertical loading FS V is adequately large. In such a case, the response is uplifting dominated and the accumulation of settlement can be limited. However, this is not always feasible as the soil properties may not be ideal. Shallow soil improvement may offer a viable solution and is therefore worth investigating. Its efficiency is related to the nature of rocking, which tends to mobilize a shallow stress bulb. To this end, a series of shaking table tests are conducted, using an idealized slender bridge pier as conceptual prototype. Two systems are studied, both lying on a square foundation of width B. The first corresponds to a lightly loaded and the second to a heavily loaded structure. The two systems are first tested on poor and ideal soil conditions to demonstrate the necessity for soil improvement. Then, the efficiency of shallow soil improvement is studied by investigating their performance on soil crusts of depth z/B = 0.5 and 1. It is shown that a z/B = 1 dense sand crust is enough to achieve practically the same performance with the ideal case of dense sand. A shallower z/B = 0.5 improvement layer may also be considered, depending on design requirements. The efficiency of the soil improvement is ameliorated with the increase of rotation amplitude, and with the number of the cycles of the seismic motion.

Modeling Techniques, Seismic Performance, and the Application of Rocking Shallow Foundations: A Review

“…Considering this, and recognizing that slender structures exposed to intense seismic shaking encounter greater overturning moments than vertical loads, investigating rotational mode vibration becomes crucial. In Figure 3 (adapted from [38]), it is evident that in the case of rocking isolation, the soil-foundation undergoes plastic deformation, a stage referred to as the "recentering overturning moment". This is in contrast with conventional design, where columns support loads and experience plastic deformation, a scenario termed the "destructive bending moment".…”

Modeling of Seismic Energy Dissipation of Rocking Foundations Using Nonparametric Machine Learning Algorithms

“…The results obtained from five series of centrifuge experiments and four series of shake table experiments (altogether 140 individual experiments on rocking foundations) are utilized in this study. The centrifuge experiments were conducted in the Center for Geotechnical Modeling at University of California at Davis [2,4,60,61] and the shake table experiments were conducted in University of California at San Diego [8] and the National Technical University of Athens in Greece [5,7,62]. Details of these experiments, including types of soils, foundations, structures, and ground motions, number of shaking events, raw data, and metadata, are available in a globally available database in Digital Environment for Enabling Data-Driven Science (DEEDS) website [12] (https://datacenterhub.org/deedsdv/ publications/view/529, accessed on 1 September 2021).…”

Database of rocking shallow foundation performance: Dynamic shaking

“…This approach exploits the nature of foundation rocking that tends to mobilize a shallow stress bulb within the soil. To assess its effectiveness, Tsatsis and Anastasopoulos (2015) conducted several small-scale 1- g shaking table tests of stiff bridge pier superstructures on footings supported on top of uniform or two-layered dry sand profiles. Finally, a series of large-scale tests on flexible, reinforced concrete bridge columns with square rocking footings embedded into dense to very dense sand was conducted at the outdoor shake table at UCSD by Antonellis et al (2015).…”

“…California Department of Transportation (Caltrans), 2010; Comité Européen de Normalisation (CEN), 2005). With the advent of performance-based seismic design, however, the dynamic response of rocking shallow foundations has attracted the attention of several researchers conducting experimental studies (Allmond and Kutter, 2014; Anastasopoulos et al, 2013, 2015; Antonellis et al, 2015; Chiou et al, 2015; Deng and Kutter, 2012; Deng et al, 2012a; Gajan et al, 2005; Gajan and Kutter, 2008; Hakhamaneshi et al, 2012; Kim et al, 2015; Liu et al, 2013, 2015; Loli et al, 2014, 2015; Shirato et al, 2008; Storie et al, 2015; Tsatsis and Anastasopoulos, 2015; Ugalde et al, 2007). These studies have produced compelling evidence that rocking foundations can be designed to offer superior seismic performance features compared to traditional structural hinging design, notably, recentering and energy dissipation with small permanent deformations.…”