Görüntüleme (gezinme ile): 33 -- Görüntüleme (arama ile): 1 -- IP: 216.73.216.108 -- Ziyaretçi Sayısı:

Özgün Başlık
Zemin Büyütmesi ve Yerel Koşulların Spektral İvmeye Etkisi

Yazarlar
Recep İyisan, M. Emre Haşal

Dergi Adı
İTÜ Dergisi D : Mühendislik

Cilt
Ağustos 2011, Cilt 10, Sayı 4, ss. 47-56

Anahtar Kelimeler
Zemin büyütmesi ; zemin koşulları ; 2D dinamik analiz ; spektral ivme oranı

Özet
Bir sahada yerel koşullara bağlı olarak oluşabilecek zemin büyütmesinin belirlenmesi, geoteknik deprem mühendisliğinin önemli konularından birini oluşturmaktadır. Anakaya seviyesinde bir kay-naktan açığa çıkan enerjinin oluşturduğu sismik dalgalar yayılımları sırasında, içinden geçtikleri ortamların özelliklerinden etkilenmekte ; süre, frekans ve genliklerinde değişimler olabilmektedir. Deprem dalgalarının yüzeye yakın zemin tabakaları içinden geçerken genliklerinde meydana gelen artış zemin büyütmesi olarak tanımlanmaktadır. Zemin büyütmesi anakaya derinliği, zemin tabaka-larının kalınlığı, cinsi ve dinamik özellikleri, tabakaların yanal süreksizliği ve topografik özellikler gibi yerel koşullardan etkilenmektedir. Bu çalışmada, yerel koşulların zemin büyütmesine etkisini incelemek amacıyla tepe ve ovadan oluşan bir model seçilmiş, ova bölgesinde zemin kesitinin en üstünde yüksek plastisiteli kil ve siltli kum tabakasının olması durumu için, altı anakaya depremi kullanılarak bir (1D) ve iki boyutlu(2D) dinamik analizler yapılmıştır. Yüzeyde 19 farklı noktada farklı depremler için elde edilmiş ivme zaman geçmişleri ve ivme spektrumlarının mesafeye bağlı değişimleri incelenmiştir. Anakaya ivmelerinin, üst tabakanın kum olması durumunda zemin yüze-yinde ortalama iki kat, üst tabakanın kil olması durumunda ise daha fazla arttığı belirlenmiştir. Üst tabakanın kil olması hali için özellikle anakaya dalımının ova tabanına ulaştığı bölgenin izdüşü-münde yer alan kesimlerde, spektral ivme oranlarının yüksek frekanslarda en büyük değerini aldığı, ova ortasına doğru 1D ve 2D analiz sonuçları arasındaki farkın azaldığı belirlenmiştir. 1D analiz-lerde sonlu farklar yöntemine dayanan, eşdeğer lineer malzeme modeli ile çalışan EERA yazılımı, 2D analizlerde ise frekans ortamında çalışan Flushplus sonlu elemanlar programı kullanılmıştır.

Başlık (Yabancı Dil)
Soil Amplification and Effect of Local Site Condition to Spectral Acceleration

Anahtar Kelimeler (Yabancı Dil)
Soil amplification ; local conditions ; 2D dynamic analysis ; spectral acceleration ratio

Özet (Yabancı Dil)
One of the most commonly encountered problems in geotechnical earthquake engineering is the evalua-tion of soil amplification. Soil amplification can be defined as the increase in the amplitudes of the earthquake waves. Soil amplification is mostly af-fected by the local soil conditions such as bedrock depth, the thickness and dynamic characteristics of the soil layers, the limited width of the soil layers and the topographical properties. The variation of ground motion is denoted as an amplification or de-amplification of amplitudes at all frequencies, which is dependent on many parameters. Some of them are inherent of the dynamic soil behavior and its physi-cal properties such as shearing modulus, damping ratio, shear wave velocity, plasticity index etc., and others are related to the characteristics and the in-tensity of the incoming wave-field, geometrical fea-tures like surface / bedrock topography, lateral geo-logical discontinuities etc. Site effects play an im-portant role in the variation of ground motion inten-sity distribution during earthquakes and therefore they have to be evaluated correctly for the design of earthquake resistant buildings. In this study, in order to investigate the effects of the local soil conditions to the soil amplification, an idealized simple model consisting of a crest and ba-sin geometry was selected. One (1D) and two-dimensional (2D) dynamic analyses were performed for the two different cases of the top basin layer as high plasticity clay and silty sand, and the obtained results were compared. Six bedrock acceleration records with different frequency content were used in the dynamic analyses. Within the scope of this study, EERA and Flushplus softwares were used in the 1D and 2D dynamic nonlinear dynamic anal-yses, respectively. Flushplus is a finite element dy-namic analysis software that is based on equivalent linear method and it works in the frequency domain. Viscous dashpots that were calculated proportional to the shear and pressure waves of the relevant lay-ers were set at the boundaries of vertical and hori-zontal layers in the 2D model. The variations of the soil amplifications with the surface geometry were investigated by the 1D and 2D dynamic analyses. The 2D / 1D spectral acceleration ratios were calcu-lated for different period values at the lowland and highland regions of the 2D model in order to under-stand the difference between 1D and 2D dynamic behaviors. It was understood that the peak acceleration value of the bedrock ground motion was amplified by twice in case the top basin layer was selected as silty sand and the soil amplification value reached to 4.5 for the case of top basin layer as high plasticity clay. When the top soil layer was high plasticity clay, the 2D / 1D spectral acceleration ratios reached their maximum values for high frequencies at the basin edge section where X / D is between 0.5 and 0.75, while X indicates the horizontal distance and D is the depth. As approaching to the mid part of the ba-sin, 2D / 1D spectral acceleration ratios reached their maximum values at lower frequencies because of the wave transformations that occur at basin edg-es. The difference between the results of 1D and 2D dynamic analyses decreased for higher frequencies with the increasing distance from the edge of the basin. When the top layer was selected as sand, the 2D / 1D spectral acceleration ratios showed similar behavior for the lower frequencies, but an increase in the 2D / 1D values was noticed at higher frequen-cies mainly for the sections 0.75<X / D<1. Because of the limited width of the soil layers in ba-sin and vertical boundary conditions, the 2D pre-dominant period of the model took a lower value when compared with the results of 1D dynamic analysis. As a result the spectral acceleration values obtained by the 2D dynamic analyses were different from the spectral acceleration values calculated by 1D dynamic analysis especially at higher frequen-cies. The peak surface acceleration values that were calculated by the 2D analyses approached to each other regardless of the basin geometry when the non-linear behavior became dominant at the top layers. Topographical effects were negligible for very low frequencies, while the effects became sig-nificant for high frequencies or low wavelengths comparable to the geometric characteristics of the model. The intensity of the bedrock acceleration and the variation of dynamic properties with soil type play an important role in the development of non-linear dynamic behavior.