Recent code provisions for buildings and other structures (1994 and 1997 NEHRP Provisions, 1997 UBC) have adopted new site amplification factors and a new procedure for site classification. Two amplitude-dependent site amplification factors are specified: Fa for short periods and Fv for longer periods. Previous codes included only a long period factor S and did not provide for a short period amplification factor. The new site classification system is based on definitions of five site classes in terms of a representative average shear wave velocity to a depth of 30 m (V¯s). This definition permits sites to be classified unambiguously. When the shear wave velocity is not available, other soil properties such as standard penetration resistance or undrained shear strength can be used. The new site classes denoted by letters A - E, replace site classes in previous codes denoted by S1 - S4. Site classes A and B correspond to hard rock and rock, Site Class C corresponds to soft rock and very stiff / very dense soil, and Site Classes D and E correspond to stiff soil and soft soil. A sixth site class, F, is defined for soils requiring site-specific evaluations. Both Fa and Fv are functions of the site class, and also of the level of seismic hazard on rock, defined by parameters such as Aa and Av ( 1994 NEHRP Provisions), Ss and Sl ( 1997 NEHRP Provisions) or Z ( 1997 UBC). The values of Fa and Fv decrease as the seismic hazard on rock increases due to soil nonlinearity. The greatest impact of the new factors Fa and Fv as compared with the old S factors occurs in areas of low-to-medium seismic hazard. This paper summarizes the new site provisions, explains the basis for them, and discusses ongoing studies of site amplification in recent earthquakes that may influence future code developments.
A simple analytical solution is developed for computing the dynamic impedances of floating rigidly-capped pile groups with due consideration to pile-soil-pile interaction. The method introduces some sound physical approximations and considers the interference of cylindrical wave fields originating along each pile shaft and spreading radially outward. Axial, lateral, and rocking oscillations of rigidly-capped pile groups are studied parametri-cally. Results are presented for the dynamic stiffness and damping of the whole group, and for the distribution of dynamic loads amongst the individual piles. The predictions of the simple method for vertical and rocking oscillations compare extremely well with rigorous numerical solutions, thereby offering a valuable insight into the nature of pile-soil-pile interaction. It is demonstrated and explained how the dynamic efficiency may far exceed unity at certain resonant frequencies due to destructive wave interference. The proposed method can be readily applied by engineers already familiar with the use of static interaction factors in the design of pile groups. Une solution analytique très simple est développée pour calculer les impédances dynamiques de groupes de pieux flottants à têtes rigides dans le cadre de à l'interaction pieux/sol/pieux. La méth-ode introduit des approximations physiques val-ables et prend en considération l'interference de champs d'ondes cylindriques qui ont leur origine le long de chaque fût de pieux et se propagent radi-alement vers l'exterieur. L'article étudie sous l'aspect paramétrique les sollicitations axiales, latérales et oscillants de groupes de pieux à casques rigides. Des résultats sont présentés pour la rigidité dynamique et 1'amortissement du groupe entier et pour la distribution des charges dynamiques entre les pieux individuels. Les prédictions par cette méthode très simple des oscillations verti-cales et basculantes supportent extrêmement bien la comparaison avec celles des solutions numér-iques rigoureuses, ce qui donne un aperçu valable de la nature de l'interaction pieux-sol-pieux. On démontre et explique comment le groupe dynamique représentant l'effet utile peut dépasser de beaucoup l'unite à certaines fréquences de résonance à cause de l'interférence destructive des ondes. La méthode proposée peut être facilement appliquée par des ingénieurs qui connaissent déjà bien I'emploi de facteurs statiques d'interaction dans la construction de groupes de pieux.
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