Ioannis Anastasopoulos

Shaking Table Testing of Rocking—Isolated Bridge Pier on Sand

This article studies the seismic performance of a rocking-isolated bridge pier on surface foundation, resting on sand. A series of reduced-scale shaking table tests are conducted, comparing the performance of a rocking-isolated system to a pier founded on conventionally designed foundation. The two design alternatives are subjected to a variety of shaking events, comprising real records and artificial motions of varying intensity. In an effort to explore system performance in successive seismic events, three different shaking sequences are performed. Rocking isolation is proven quite effective in reducing the inertia forces transmitted onto the superstructure. The rocking-isolated pier is effectively protected, surviving all seismic excitations without structural damage, at the cost of increased foundation settlement. In contrast, a certain degree of structural damage would be unavoidable for the same system founded on a conventionally designed foundation. The rocking-isolated system is proven remarkably resistant to cumulative cyclic loading, exhibiting limited strength degradation even when subjected to cyclic drift ratio in excess of 5.5%. Due to soil densification, the rate of settlement accumulation is found to decrease with repeating seismic excitations. The rotational response is practically insensitive to the shaking history when the preceding seismic motions are symmetric (such as sinusoidal motions). In stark contrast, when the preceding seismic motions are non-symmetric (such as the directivity-affected records of this study), the system tends to accumulate rotation after each event, progressively worsening its performance. Nevertheless, the rocking-isolated system survives toppling collapse, even when subjected to a highly improbable, unrealistically harsh, sequence of seismic events.

Seismic Wave Propagation in a Very Soft Alluvial Valley: Sensitivity to Ground-Motion Details and Soil Nonlinearity, and Generation of a Parasitic Vertical Component

This paper explores the sensitivity of 2D wave effects to crucial problem parameters, such as the frequency content of the base motion, its details, and soil nonlinearity. A numerical study is conducted, utilizing a shallow soft valley as a test case. It is shown that wave focusing effects near valley edges and surface waves generated at valley corners are responsible for substantial aggravation (AG) of the seismic motion. With high-frequency seismic excitation, 1D soil amplification is pre- vailing at the central part of the valley, while 2D phenomena are localized near the edges. For low-frequency seismic excitation, wave focusing effects are overshadowed by laterally propagating surface waves, leading to a shift in the location of maximum AG toward the valley center. If the response is elastic, the details of the seismic excitation do not seem to play any role on the focusing effects at valley edges, but make a substantial difference at the valley center, where surface waves are dominant. The increase of damping mainly affects the propagation of surface waves, reducing AG at the valley center, but does not appear to have any appreciable effect at the valley edges. Soil nonlinearity may modify the 2D valley response significantly. For ideal- ized single-pulse seismic excitations, AG at the valley center is reduced with increas- ing nonlinearity. Quite remarkably, for real multicycle seismic excitations AG at the valley edges may increase with soil nonlinearity. In contrast to the vertical component of an incident seismic motion, which is largely the result of P waves and is usually of too high frequency to pose a serious threat to structures, the valley-generated parasitic vertical component could be detrimental to structures: being a direct result of 2D wave reflections/refractions, it is well correlated and with essentially the same dominant periods as the horizontal component.

Shallow and Deep Foundations under Fault Rupture Or Strong Seismic Shaking

Two topics of interest in soil-foundation-structure interaction are presented: the first refers to the consequences on shallow and deep foundations and their superstructures from a seismic fault rupture emerging directly underneath them; the second topic addresses the seismic response of tall structures resting on shallow foundations that experience uplifting and inducing large inelastic deformations in the soil. The numerical and analytical methodologies developed for each topic have been calibrated with centrifuge experiments. The outlined parametric results provide valuable insight to the respective soil-foundation interplay, and could explain qualitatively the observed behaviour in a number of case histories from recent earthquakes.

Rocking Isolation of Frames on Isolated Footings: Design Insights and Limitations

To date, a significant research effort has been devoted attempting to introduce novel seismic protection schemes, taking advantage of mobilization of inelastic foundation response. According to such an emerging seismic design concept, termed “rocking isolation,” instead of over-designing the footings of a frame (as in conventional capacity design), they are intentionally under-designed to promote uplifting and respond to strong seismic shaking through rocking, thus bounding the inertia forces transmitted to the superstructure. Recent research has demonstrated the potential effectiveness of rocking isolation for the seismic protection of frame structures, using a simple 1-bay frame as an illustrative example. This article: (a) sheds light in the possible limitations of rocking isolation, especially in view of the unavoidable uncertainties regarding the estimation of soil properties; (b) investigates the potential detrimental effects of ground motion characteristics; and (c) assesses the effectiveness of rocking isolation to more complex structures. It is shown that the concept may be generalized to 2-bay frames, and that even when foundation rocking is limited, the positive effect of foundation under-design remains, especially when it comes to very strong seismic shaking. In contrast, its effectiveness may be limited when the frame is subjected to combined horizontal and synchronous vertical acceleration components — a possible scenario on the surface of alluvial basins.

Dimensional Analysis of SDOF Systems Rocking on Inelastic Soil

Aiming to derive results of generalized applicability and provide a generalization framework for future research on the subject, this article performs a dimensional analysis of SDOF systems rocking on compliant soil, taking account of soil inelasticity, foundation uplifting, and P–δ effects. The effectiveness of the proposed formulation, under static and dynamic conditions, is verified through numerical analyses of self-similar “equivalent” systems. Then, a parametric study is conducted to gain further insights on the key factors affecting the performance, with emphasis on metaplastic ductility and toppling rotation. It is shown that P–δ effects may lead to a substantial reduction of (monotonic) moment capacity, especially in the case of slender and heavily loaded structures. Interestingly, this reduction in moment capacity is compensated (to some extent) by an overstrength that develops during cyclic loading. Asymmetric (near-field) seismic excitations tend to produce larger maximum and permanent rotation, compared to symmetric multi-cycle (far-field) excitations, which are critical in terms of settlement. The dimensionless toppling rotation ϑ ult /ϑ c (where ϑ c is the toppling rotation of the equivalent rigid block) is shown to be a function of the factor of safety against vertical loads FS v and the slenderness ratio h/B. In the case of lightly loaded systems (FS v → ∞), soil plastification is limited and the metaplastic response approaches that of the equivalent rigid block : ϑ ult /ϑ c → 1. The toppling rotation ϑ ult /ϑ c is shown to decrease with FS v : ϑ ult /ϑ c → 0 for FS v → 1. The role of the h/B becomes increasingly important when the response is governed by soil nonlinearity (FS v → 1). Finally, an approximate simplified “empirical” equation is proposed, correlating ϑ ult /ϑ c with h/B and FS v .

Numerical and Experimental Assessment of Advanced Concepts to Reduce Noise and Vibration on Urban Railway Turnouts

The short service life of rail turnouts and the related noise and vibration disturbance, are directly related to their dynamic distress. Especially in the case of urban rail systems, such problems are amplified due to the increased train frequency and the proximity to inhabited structures. This paper presents three new concepts for the reduction of noise and vibration produced by railway turnouts in urban railway lines, and provides an assessment of their performance. The effectiveness of the three new concepts is first evaluated analytically, using two different methodologies, earlier validated against line measurements on existing turnouts. Then, the actual perfor- mance of one of the three concepts, along with the effectiveness of the two analysis methodologies, is verified through real-scale measurements. Based on the presented analyses, all three new concepts are shown to provide a substantial enhancement of turnout performance. Furthermore, soil conditions and soil-structure interaction are shown to play an important role in the behavior of the investigated systems.

Nonlinear Dimensional Analysis of Trapezoidal Valleys Subjected to Vertically Propagating SV Waves

This paper studies the seismic response of soil basins emphasizing the sensitivity of 2D dynamic response to geometric and material properties. This is accomplished through a formal dimensional analysis accounting for fully inelastic soil response thus augmenting the generalization potential of the results, and provid- ing a novel framework for future research on the subject. It is shown that 2D valley response may be described through the following key dimensionless parameters: (1) the valley shape factor s, expressing the slope inclination; (2) the impedance ratio i, which expresses the stiffness of the soil relative to the bedrock; (3) the wavelength ratio λS, which is a function of soil stiffness and seismic excitation frequency; (4) the rigidity ratio v, expressing the stiffness of the soil relative to its strength; and (5) the resistance ratio r, which expresses the degree of soil nonlinearity. The effectiveness of the dimensional formulation is verified through the numerical analysis of equivalent valleys, assuming elastic and nonlinear soil response. Finally, a parametric study is conducted to gain insight on the effects of the introduced dimensionless parameters on the dynamic response of trapezoidal alleys. It is shown that decreasing thevalley slope or the wavelength ratio promotes wave reflections within the wedge, thus enhancing the possibility of wave interferences and subsequently leading to 2D aggravation on the valley surface. On the other hand, the geometry-dependent parasitic vertical accel- eration increases as the valley slope becomes steeper. As the degree of soil nonlinear- ity increases, 2D phenomena tend to become localized close to the valley edges.

Soil bentonite wall protects foundation from thrust faulting: analyses and experiment

When seismic thrust faults emerge on the ground surface, they are particularly damaging to buildings, bridges and lifelines that lie on the rupture path. To protect a structure founded on a rigid raft, a thick diaphragm-type soil bentonite wall (SBW) is installed in front of and near the foundation, at sufficient depth to intercept the propagating fault rupture. Extensive numerical analyses, verified against reduced-scale (1 g) split box physical model tests, reveal that such a wall, thanks to its high deformability and low shear resistance, “absorbs” the compressive thrust of the fault and forces the rupture to deviate upwards along its length. As a consequence, the foundation is left essentially intact. The effectiveness of SBW is demonstrated to depend on the exact location of the emerging fault and the magnitude of the fault offset. When the latter is large, the unprotected foundation experiences intolerable rigid-body rotation even if the foundation structural distress is not substantial.

Nonlinear analysis of earthquake fault rupture interaction with historic masonry buildings

The response of historic masonry buildings to tectonic ground displacements is studied through analysis of a simple yet representative soil–foundation–masonry wall system. A nonlinear 3D finite element method is developed and employed to reproduce the strong nonlinear response of the rupturing soil, as well as the masonry structure. Following a sensitivity analysis of the effect of the exact location of the structure with respect to the emerging fault, the paper discusses several characteristic mechanisms of soil–structure interaction and evaluates the associated structural distress. The observed failure pattern and the consequent structural damage are shown to depend strongly, varying from minimal to dramatic, on the exact position of the structure relative to the fault. Alleviation of tectonic risk through foundation enhancement/improvement is investigated by considering alternative foundation systems. Results highlight the advantageous performance of rigid embedded and continuous foundations as opposed to more flexible and isolated supports indicating that foundation strengthening may provide important shielding against settlement and structural drift.

The Collapse of the Hanshin Expressway (Fukae) Bridge, Kobe 1995: Soil-Foundation-Structure Interaction, Reconstruction, Seismic Isolation

The collapse of 18 spans (total length 630 m) of the Hanshin Expressway Route 3 elevated highway bridge in Fukae during the 1995 Kobe earthquake is investigated. The overturned concrete deck was monolithically connected (“piltz” form) to 3.1-m-diameter circular-column piers, founded on 17-pile groups in alluvium sand and gravel. The collapse has been attributed by many research engineers to inadequate structural design, stemming from insufficient and prematurely-terminated longitudinal reinforcement, inadequate hoop anchorage, and (for the large intensity of shaking) insufficient shear capacity. The importance of other factors has been largely ignored. This study presents evidence in the form of a parametric study of the inelastic response of the bridge-foundation-soil system, showing that the role of Soil-Foundation-Structure Interaction (SFSI) was significant and decisively detrimental.