Zayıf zemin üzerine inşa edilen sismik taban yalıtımlı binaların performanslarının incelenmesi
Investigation of the performance  of seismic base-isolated  buildings built on weak soil
Yazar: GÖKAY DENİZ
Danışman: PROF. DR. YASİN FAHJAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Sismik izolasyonda genel prensip üstyapı ve temelin birbirinden ayrılmasıdır. Bu tasarımda zeminden yapıya aktarılan deprem kuvvetlerinin azaltılması amaçlanmaktadır. Bu azaltma binanın periyodu arttırılarak ya da yapı etkin sönümü arttırılarak yapılır. Zayıf zeminlerde genellikle sismik izolasyon tercih edilmez. Bu tarz zeminlerde zemin hakim periyodu çok uzundur. İzolatörler de binanın periyodunu arttıracağından dolayı yapı periyodu ile zemin hakim periyodu çakışarak rezonansa sebep olabilir. Bu da yapının alması gerekenden çok daha fazla deprem kuvveti almasına yol açar. Yapısal hasar deprem ve yapı özelliklerinin yanı sıra zemin koşullarına da bağlıdır. Deprem dalgaları anakayadan zemine doğru ilerlerken değişime uğrar. Zeminin deprem dalgaları üzerindeki bu etkisine zemin büyütme etkisi denir. Zemin büyütmesindeki en önemli etkenlerden birisi zemin kayma dalgası hızıdır. Kayma dalgası hızı düştükçe zemin daha yumuşak olmaktadır. Yumuşak zeminlerdeki zemin büyütmesi, sert zeminlerin aksine binanın uzun periyotlarda daha fazla taban kesme kuvveti almasına sebep olabilir.
Bu çalışmada bir hastane yapısının farklı kayma dalgası hızlarındaki kurşun çekirdekli kauçuk izolatör (LRB) ve sürtünmeli sarkaç sistem (FPS) tipi yalıtım birimleri ile  analizleri yapılmıştır. Zayıf zeminlerdeki değişimi daha net görebilmek için ZD zemin sınıfı için 4 farklı kayma dalgası hızı seçilmiştir. ZC zemin sınıfı için birbirinden farklı 2, ZB zemin sınıfı için ise 1 kayma dalgası hızı kullanılmıştır. Öncelikle LRB ve FPS tipi yalıtım birimleri hakkında bilgi verilmiş ve sismik izolasyonlu yapılarla ilgili Türkiye Bina Deprem Yönetmeliği şartları açıklanmıştır. Daha sonra bu şartlar doğrultusunda izolatör boyutlandırılması yapılmıştır ve ilgili kontrolleri gösterilmiştir. Uygulama bölümünde ise 4 katlı bir hastane binası ankastre mesnetli ve izolatörlü olacak şekilde modellenmiştir. Yapı modellerinin zaman tanım alanlı analizleri ETABS programı kullanılarak yapılmıştır. ETABS programında modellemenin nasıl yapılacağı detaylı bir şekilde açıklanmıştır. Analizler sonucunda elde edilen izolatör yer değiştirmeleri, maksimum kat yer değiştirmeleri, göreli kat ötelemeleri, kat kesme kuvvetleri, taban kesme kuvveti oranları, kat ivmeleri ve kolon kapasiteleri karşılaştırılmış ve zemin kayma dalgası hızının sonuçları nasıl etkilediği araştırılmıştır.

Abstract:
Seismic isolation is a design technique used to minimize damage and loss of life caused by earthquakes. The main purpose of seismic isolation is to reduce the earthquake loads on the structure by increasing the dominant vibration period of the structure or by increasing the damping with the elements with low rigidity in the horizontal and high in the vertical placed between the infrastructure and the superstructure.
In buildings with fixed support foundations, earthquake vibrations are transmitted directly to the structure. This situation creates significant effects on building elements. However, in structures with seismic isolation, these vibrations are reduced by intermediate elements and only some of them pass to the superstructure. In seismic isolation, the design is achieved by making an optimization between the displacement of the isolators and the acceleration transferred to the superstructure.
In principle, seismic isolation differs from other design methods. In classical design methods, the increasing demand is met by increasing the capacities of the elements, while in seismic isolation, it is aimed to reduce the earthquake demand coming to the structure. In an isolated structure, the forces transferred to the superstructure are expected to be between 10-15%. If the soil is weak, it is very difficult and costly to achieve this ratio.
Basic principle of seismic isolation is to prevent the acceleration due to resonance by keeping the soil dominant period and the dominant period of the building away from each other. If the dominant period of the soil and the building are the same, since the forces vibrating the building will be in the same direction, the sum of these two forces will increase the vibration amplitude of the building and thus its acceleration. The damage that will occur in a resonating building is directly proportional to the increase in its amplitude.
One of the most common problems in civil engineering is the evaluation of soil amplification. It has been determined as a result of the examination of acceleration records taken from different soil classes in regions close to each other in large earthquakes that local ground conditions seriously affect the characteristics of strong ground motions. The first important information was obtained with the data obtained after the San Francisco Earthquake in 1957. Earthquake waves change as they move from bedrock to the ground. If the soil is weak, the amplitudes of earthquake waves increase. Soil amplification can be defined as this increase in the amplitudes of earthquake waves. It has been observed that very small accelerations in the bedrock increase several times with the effect of local ground conditions and cause serious damage.
The most important factors affecting soil amplification are the relative density, stiffness, thickness, damping ratio and shear wave velocity of the soil layers. As the intensity of the incoming earthquake waves increases, the effect of the soil on the earthquake wave increases depending on these parameters.
Two methods can be used to examine the effect of soil amplification on the spectrum. The first of these is to model the current state of the soil at a depth of 30 meters in a program such as Deepsoil and examine how it changes the amplitude of incoming earthquake waves. Another is the prediction of soil behavior for different shear wave velocities by using the ground motion prediction equations used in seismic hazard analyses. In the NGA (Next Generation Access) method, the effect of soils on seismic waves is calculated over a certain standard deviation depending on the earthquake magnitude, the distance of the structure to the fault line and shear wave velocities. In this thesis, the effect of soil amplification on the spectrum was obtained by NGA method. The earthquake magnitude was 7.6, the distance of the earthquake to the fault line was 11 m and different spectrum were obtained by changing the shear wave velocities. For all shear wave velocities, the median+N.σ standard deviation was taken into account in the deterministic analyzes made during the region-specific earthquake hazard determination study.
In this thesis, the design of a 4-storey hospital building was made using LRB and FPS type isolators designed according to TBDY-2018, and its performances were examined depending on the variation of shear wave velocity.
Lead-core rubber isolators, which were developed and started to be used in the 1970s, are similar to low-damping rubber isolators in terms of models, but differ from them in that they have a lead core in the middle. While this core increases the energy absorbing ability of the isolation unit, the rubber also provides stabilizing properties. When the rubber is subjected to large displacements under the influence of high lateral loads, the lead core plastically deforms and absorbs this energy. As a result of lead flow, the lateral stiffness of the isolator is noticeably reduced, resulting in an increase in the period of the structure. The structure becomes more ductile and the seismic forces on the structure are greatly reduced. It has been determined that when a structure with an effective period of between 0.1 and 1 second is isolated with rubber isolators, its period extends to 2-3 seconds.
Friction pendulum systems consist of an articulated slide that moves on a spherical surface made of stainless steel. In this way, a low friction composite material was formed. There is a protective cylinder element to prevent horizontal displacements. This protective cylinder ensures the safety of the system in very large earthquakes. The basic principle of FPS is to limit the shear forces at the isolation interface and to absorb the energy generated as a result of earthquake action by friction between the spherical surface and the articulated slider. This type of isolators are activated when the incoming horizontal load exceeds the friction force. Buildings isolated with a friction pendulum system behave like a fixed support system in earthquake loads under friction force. If the earthquake forces are large enough to exceed the limit values, the FPS starts the sliding movement and increases the period of the structure. Since the rigidity of the building with an increased period decreases, the structure receives less earthquake force and the earthquake isolation of the building is provided. 
Infrastructure design was made for DD-1 earthquake ground motion level with the nominal value of the parameters of the isolator unit and superstructure was made for DD-2 earthquake ground motion level with upper limit values of the parameters of the isolation unit. As a result of the nonlinear analysis, it has been observed that the hospital building designed with FPS and LRB type isolators is affected approximately the same by the changes in shear wave velocity.
The higher the soil shear wave velocity means the harder the soil type. It is SD1 that determines the acceleration coming to the building at high periods, and the SD1 value decreases as the soil hardens. Therefore, as the shear wave velocity increases, the seismic forces transmitted to the building from the ground will also decrease. As the earthquake forces on the building decrease, the displacement value of the building will also decrease.
As a result of the nonlinear analysis, it was observed that the FPS type isolation unit made 2 times more displacement between the boundary shear wave velocities of the ZD soil class (180-360), while the LRB type isolation unit made 3 times more displacement. 
When the maximum story displacements are examined, the change in shear wave velocity in hard soils did not cause much displacement difference, but the change of shear wave velocity in weak soils can increase the story displacement up to 2 times.
When the story drift ratios are compared, it is seen that the values of all the soil shear wave velocities of the building are below the regulation limit value and are very close to each other. Accordingly, it can be said that the soil shear wave velocity does not have much effect on the story drift ratios in seismically isolated buildings.
When the story shear forces are examined, a similar relationship is observed with storey displacements. The story shear force almost does not change when shear wave velocity increases from 800 to 1200. But when it goes from 180 to 360, the story shear forces decrease by 2 times.
For buildings with seismic isolation to be effective, approximately 10-15% of the base shear forces are expected to transmit to the superstructure. As a result of the analysis, it was seen that this demand was met in shear wave velocities on hard soils, while it was seen that up to 50% of the base shear forces were transmitted to the superstructure in weak soils. This shows that seismic isolation is not effective in weak soils. The structure modeled with isolator at 180 shear wave velocities transmits more earthquake force to the superstructure than the building with fixed support foundations the at 1200 shear wave speed.
When the story accelerations were examined, it was observed that the story accelerations increased 1.5 times when the shear wave velocity decreased from 360 to 180. Story accelerations decreased by 2 times at 1200 shear wave velocity compared to 180 shear wave velocity. The difference between 800 and 1200 shear wave velocities is negligible. Changes in shear wave velocity on hard soils do not affect the accelerations and column capacities of the building.
In order to examine the column capacities, 2 corner columns and 2 columns in the middle of the building were selected. At 180 shear wave speed, almost no column can carry the loads on it. When the shear wave velocity reaches 360, the columns carry the building using 80% of their capacity, and no column exceeds its capacity. At 1200 shear wave speed, the columns use 50% of their capacity. While a 1.5-fold difference was observed in column capacities between shear velocity wave limit values of ZD soil class, a 2.5-fold difference was observed between 180 and 1200 shear wave velocities.
As it can be understood from this, small shear wave velocity changes in weak soils cause large differences, while large shear wave velocity changes in hard soils almost do not affect the results. Therefore, the concept of ground class is insufficient in seismic isolated designs to be made on weak soils. Shear wave velocity should be obtained with ground studies specific to the area to be designed and the effect of this shear wave velocity on the spectrum should be consider.