Evidence of beneficial role of inclined piles: observations and summary of numerical analyses

The behaviour under seismic loading of inclined piles embedded in two idealized soil profiles, a homogeneous and a non-homogenous “Gibson” soil, is analysed with 3D finite elements. Two structures, modeled as single-degree-of-freedom oscillators, are studied: (1) a tall slender superstructure (H _st = 12 m) whose crucial loading is the overturning moment, and (2) a short structure (H _st = 1 m) whose crucial loading is the shear force. Three simple two-pile group are studied: (a) one comprising a vertical pile and a pile inclined at 25°, (b) one consisting of two piles symmetrically inclined at 25°, and (c) a group of two vertical piles. The influence of key parameters is analysed and non-dimensional diagrams are presented to illustrate the role of raked piles on pile and structure response. It is shown that this role can be beneficial or detrimental depending on a number of factors, including the slenderness of the superstructure and the type of pile-to-cap connection.     

On the seismic response of under-designed caisson foundations

The seismic behaviour of caisson foundations supporting typical bridge piers is analysed with 3D finite elements, with due consideration to soil and interface nonlinearities. Single-degree-of freedom oscillators of varying mass and height, simulating heavily and lightly loaded bridge piers, founded on similar caissons are studied. Four different combinations of the static ( $\text{ FS }_\mathrm{V}$ FS V ) and seismic ( $\text{ FS }_\mathrm{E}$ FS E ) factors of safety are examined: (1) a lightly loaded ( $\text{ FS }_\mathrm{V}= 5$ FS V = 5 ) seismically under-designed ( $\text{ FS }_\mathrm{E} < 1$ FS E < 1 ) caisson, (2) a lightly loaded seismically over-designed ( $\text{ FS }_\mathrm{E} >1$ FS E > 1 ) caisson, (3) a heavily loaded ( $\text{ FS }_\mathrm{V} = 2.5$ FS V = 2.5 ) seismically under-designed ( $\text{ FS }_\mathrm{E} < 1$ FS E < 1 ) caisson and (4) a heavily loaded seismically over-designed caisson. The analysis is performed with use of seismic records appropriately modified so that the effective response periods (due to soil-structure-interaction effects) of the studied systems correspond to the same spectral acceleration, thus allowing their inelastic seismic performance to be compared on a fair basis. Key performance measures of the systems are then contrasted, such as: accelerations, displacements, rotations and settlements. It is shown that the performance of the lightly loaded seismically under-designed caisson is advantageous: not only does it reduce significantly the seismic load to the superstructure, but it also produces minimal residual displacements of the foundation. For heavily loaded foundations, however ( $\text{ FS }_{V} = 2.5$ FS V = 2.5 ), the performance of the two systems (over and under designed) is similar.     

S-wave spectral analysis of the 1995 Kozani-Grevena (NW Greece) aftershock sequence

S-wave spectral analysis is applied to 174 strong motion accelerationrecords to obtain the source parameters of 27 aftershocks(3.1 M_L 4.3) of the May 13, 1995, M_w 6.6,Kozani-Grevena (NW Greece) earthquake. The data are derived from atemporary network, of three-component digital accelerographs, deployedwithin the strongly affected area some days after the mainshock occurrence.Site effects were evident in the strong motion records at 3 out of the 4stations used, and a correction was applied to account for theoverestimation of seismic moment due to amplification of thelow-frequency part of the spectrum. The data from this analysis arecomplimented with previously obtained source parameters for earthquakesin Greece, in order to study the applicability of the empirical scalingrelations used so far, towards smaller magnitudes. In general, a goodcorrelation was observed in most cases, validating the use of empiricalrelations that are applicable to the Aegean area. Empirical relations aredetermined between seismic moment and seismic slip, as well as, betweenseismic moment and stress drop, applicable to small magnitude earthquakes(M_L < 4.3). Stress drop values were found to be relatively small,ranging from 2 to 41 bars, indicative of inter-plate environments. Thevalues of f_c and of f_max were found in good agreement withrelations based on observations from larger worldwide earthquakes.     

Storm graded sand at 200 m water depth,Scotian Shelf,Eastern Canada

Graded sand-to-mud beds are a common shelf facies in the rock record. Similar beds were studied in nine cores from the sandmud transition at the edge of Emerald Basin to determine their frequency and mode of origin. Graded sand-to-mud beds 5 to 20 cm thick can be correlated between cores and thicken basinwards, with average sand content decreasing from 60% to 5% over 7 km. European weed pollen distribution indicates only the top bed is less than 200 years old. The graded beds were deposited from suspension during storms with a several hundred year recurrence interval.     

Gamma rays from cosmic radioactivities

Abstract— Gamma rays from radioactive byproducts of cosmic nucleosynthesis are direct messengers from nuclear processes taking place in various cosmic sites, and can be measured with telescopes operated in space. Due to low detector sensitivity, up until now, only a handful of sources have been detected in that electromagnetic window. Cobalt lines from SN1987A and ⁴⁴Ti lines from the Cassiopeia A (Cas A) supernova remnant offer unique constraints on the properties of the innermost regions of core collapse supernovae. Diffuse gamma‐ray lines from the decay of radioactive ²⁶Al and the annihilation of positrons are bright enough for mapping the Milky Way in the MeV regime, and are both measured by recent spaceborne spectrometers with unprecedented precision. This constrains the sources of Al production and the state of interstellar gas in the vicinity of these sites: the total mass of ²⁶Al produced by stellar sources throughout the Galaxy is estimated to be ～3 M_⊙ per Myr, and the interstellar medium near those sources appears to be characterized by velocities of ～100 km s^?1. Positron annihilation must occur in a modestly ionized, warm phase of the interstellar medium, but at present the major positron production site(s) remain unknown. The spatial distribution of the annihilation gamma‐ray emission constrains positron production sites and positron propagation in the Galaxy. ⁶⁰Fe radioactivity has been clearly detected recently; the flux ratio relative to ²⁶Al of about 15% is on the lower side of predictions from massive star and supernova nucleosynthesis models. Those views at nuclear and astrophysical processes in and around cosmic sources by space‐based gamma‐ray telescopes offer invaluable information on cosmic nucleosynthesis.     

Development of Winkler model for static and dynamic response of caisson foundations with soil and interface nonlinearities

As an extension of the elastic multi-spring model developed by the authors in a companion paper [Gerolymos N, Gazetas G. Winkler model for lateral response of rigid caisson foundations in linear soil. Soil Dyn Earthq Eng; 2005 (submitted companion paper).], this paper develops a nonlinear Winkler-spring method for the static, cyclic, and dynamic response of caisson foundations. The nonlinear soil reactions along the circumference and on the base of the caisson are modeled realistically by using suitable couple translational and rotational nonlinear interaction springs and dashpots, which can realistically (even if approximately) model such effects as separation and slippage at the caisson–soil interface, uplift of the caisson base, radiation damping, stiffness and strength degradation with large number of cycles. The method is implemented in a new finite difference time-domain code, NL-CAISSON. An efficient numerical methodology is also developed for calibrating the model parameters using a variety of experimental and analytical data. The necessity for the proposed model arises from the difficulty to predict the large-amplitude dynamic response of caissons up to failure, statically or dynamically. In a subsequent companion paper [Gerolymos N, Gazetas G. Static and dynamic response of massive caisson foundations with soil and interface nonlinearities—validation and results. Soil Dyn Earthq Eng; 2005 (submitted companion paper).], the model is validated against in situ medium-scale static load tests and results of 3D finite element analysis. It is then used to analyse the dynamic response of a laterally loaded caisson considering soil and interface nonlinearities.     

Seismic design of RC structures: A critical assessment in the framework of multi‐objective optimization

The assessment of seismic design codes has been the subject of intensive research work in an effort to reveal weak points that originated from the limitations in predicting with acceptable precision the response of the structures under moderate or severe earthquakes. The objective of this work is to evaluate the European seismic design code, i.e. the Eurocode 8 (EC8), when used for the design of 3D reinforced concrete buildings, versus a performance‐based design (PBD) procedure, in the framework of a multi‐objective optimization concept. The initial construction cost and the maximum interstorey drift for the 10/50 hazard level are the two objectives considered for the formulation of the multi‐objective optimization problem. The solution of such optimization problems is represented by the Pareto front curve which is the geometric locus of all Pareto optimum solutions. Limit‐state fragility curves for selected designs, taken from the Pareto front curves of the EC8 and PBD formulations, are developed for assessing the two seismic design procedures. Through this comparison it was found that a linear analysis in conjunction with the behaviour factor q of EC8 cannot capture the nonlinear behaviour of an RC structure. Consequently the corrected EC8 Pareto front curve, using the nonlinear static procedure, differs significantly with regard to the corresponding Pareto front obtained according to EC8. Furthermore, similar designs, with respect to the initial construction cost, obtained through the EC8 and PBD formulations were found to exhibit different maximum interstorey drift and limit‐state fragility curves. Copyright © 2007 John Wiley & Sons, Ltd.