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Yüzeysel temeller

Shallows foundations

  1. Tez No: 66450
  2. Yazar: HAYDAR TANER KARAKEÇELİ
  3. Danışmanlar: DOÇ. DR. TUĞRUL ÖZKAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1997
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 99

Özet

ÖZET YÜZEYSEL TEMELLER Temeller, üst yapıdan gelen yükü ve kendi ağırlığını, altında bulunan sağlam zemine aktaran mühendislik yapılarıdır. Üst yapıdan yükler kolonlar aracılığıyla temele aktarılırlar. Temel taşanım deneme-yanılma yönteminden ibaret olup, ilkönce temel tipi ve boyutları seçilir. Seçilen temelin uygunluğu yönünden gerekli analizler yapılır. Seçilen temel uygun görülmez ise temel boyutları arttırılır. Bazı durumlarda, sözkonusu zemin için uygun bir yüzeysel temel tasannu mümkün olmayabilir. Böylesi bir durumda ya derin temel inşaası yoluna gidirilir yada zemin iyileştirme yöntemlerinden birisi uygulanır. Bir temelin projelendirilmesinde, zeminin üst yapıdan aktarılan yükleri kayma göçmesine sebep olmaksızın taşıyabilmesi, bunun yanında temelin yapacağı maksimum oturma değerinin belirli sınır değerlerini aşmaması ve daha önemli olarak temeller arasındaki farklı oturmaların üst yapıda ek zorlamalar meydana getirmemesi, temel tasarımını etkileyen ana hususlardır. Özet olarak her temel birbirinden bağımsız olarak şu iki ana şartı sağlamalıdır: i. Temelin göçmeye karşı yeter bir güvenliği bulunmalıdır. (Taşıma Gücü Şarü) ii. Doğması muhtemel maksimum toplam ve farklı oturmalar kabul edilebilecek şuurlar dahilinde olmalıdır. (Oturma Şartı) Temel tasannu deneme yanılma yönteminden ibaret olup, bunun yanında karmaşık hesaplar gerektiren bir işlemdir. Bu nedenle hesaplamalar sırasında hata yapılması olasılığı oldukça yüksektir. Hatayı ortadan kaldırmak ve hesaplar sırasında harcanan zamanı en aza indirip, tasarımı çok sayıda kritere bağlamayı sağlayabilmek amacıyla bilgisayar programlarından yararlanmak faydalı olacaktır. Bu nedenle bu çalışmanın sonunda örnek bir yaklaşım olması amacıyla taşıma gücü, gerilme dağılışı ve oturma hesabına yönelik olarak Visual Basic progralama dili kullanılarak yazılmış bir program verilmiştir.

Özet (Çeviri)

SUMMARY SHALLOW FOUNDATIONS All engineered constructions such as buildings, bridges, highways, tunnels and retaining walls resting on earth must be carried by a proper foundation structure in accordance with its appropriate function. The foundation is the part of an engineered system that transmits to, and into, the underlying soil or rock the loads and its own-weight. The loads carried by the superstructure are brought to the foundation by columns. The design of a foundation system is a trial method based on scientific principles, engineering judgment, training and experience so that the type and dimensions of an appropriate foundation are determined. After a field exploration program, all necessary field and laboratory tests are established within a test program to determine the soil parameters. First a typical type of foundation and dimensions are chosen and then analysis based on geotechnical engineering principals are made. If the chosen dimensions do not satisfy the engineering requirements, they are increased till the foundation seems to be acceptable. In some cases it may not be possible to design a proper shallow foundation. If this is the case, then a deep foundation construction such as pilling may be taken into account or a soil improvement program may be applied. The foundation should be economical and be able to be built by the available construction methods and personnel. All risks must be taken into account and kept within acceptable limits. The major factors that effects the foundation design are the bearing capability of the foundation to carry the loads transmitted from the superstructure without causing a shear failure and the maximum settlements should be within acceptable limits but the most important is that the differential settlements should not cause any extra loads on the superstructure. In other words foundation elements must be proportioned to both interface with the soil at a safe stress level and limit settlements to an acceptable amount. XIWe may say that every foundation should meet the following criteria independent from each other: i The foundation needs to be secured against shear failure ii. Maximum total and differential settlements should be in tolerable limits for the structure. Any load that meets these condition is named as allowable bearing capacity. If the bearing capacity is exceeded, this may cause a partial or a total collapse. The bearing capacities of the foundations depend on the density, shearing resistance and the deformation characteristics of the soil, the stress history and hydraulic conditions, the dimensions, depth, shape and flexibility of the foundation, the load carried and the construction methods used. Most of the structural problems correlated to improper foundation design is due to extensive settlements. In foundation engineering, experience shows that few modern buildings collapse from extensive settlements, however it is not uncommon for a partial collapse or a localized failure in a structural member to occur. Major reason behind this is that high safety factors are used in design and settlements are time dependent where proper actions can be taken to recover the situation. More common problems arising from improper design are generally unsightly wall and floor cracks, uneven floors, sticking doors and windows and similar. There are two different methods to determine the bearing capacity of a foundation. One of them is to assure that the foundation can carry the loads from the superstructure with the chosen foundation depth and dimensions by theorical foundation engineering methods. While doing this, firstly the foundation is assured against shear failure. Then total and differential settlement calculations are made and it is assured that they are within acceptable limits. The second approach depends on calculation related to empirical methods tied to tests performed at site. Terzaghi' s bearing capacity equation is the most common application today due to its simplicity and easiness. Terzaghi' s equation is the improved form of PrandtT s work by using the theory of plasticity to analyze the punching of a rigid base into a softer soil.. The equation is in the given form: q^t = c.Nc.sc + q.Nq + 0,5.B.y.Nrsy ( 1 ) xuWe can accept soil as a skeleton made of soil particals surrounded by air and water. In such a structure under general stress conditions, we can accept the soil particals and water do not make any deformation caused by external loading. The volume changes under applied load do occur due to the particle rolling, sliding and crushing. In cases where the soil is saturated, the volume change occurs under applied load as a result of the water coming out through the soil particles. The change of water amount is based on permeability and time. The excess of water takes more time in soils with low permeability. The settlements are usually classified as: i. Immediate settlements, that take place as the load is applied, or within a time period of about 7 days. ii. Consolidation settlements, that are time dependent and taking months to years to develop. The consolidation settlements are more important in clays. The leaning of Pisa Tower in Italy has been settling for over then 700 years. This may be an extreme case but for most projects the principal settlements take place in 1 to 5 years. Immediate settlement analyzes are used for all fine grained soils including silts and clays with a degree of saturation S < 90 percent and for all coarse grained soils with a large coefficient of permeability. Consolidation settlement analyzes are used for all saturated or nearly saturated, fine grained soils where the consolidation theory applies. Both types of settlement analyses are in form of Aq AH = 8.H = - H (2) Ec The consolidation characteristics of soils are determined at laboratories through consolidation experiments and in-situ tests also. The clays can be grouped under two headings based on the state of consolidation of the soil i. Normally consolidated clays ii. Overconsolidated clays. xinIn design of foundations, it is a must to analyze how the loads are transmitted to the underlying soil and the stress distribution in order to determine the settlements. Soils are loaded under their own weight in natural conditions and changes occur with the effect of the external loads. While calculating settlements, it is assumed that the foundation pressure is uniform and linear. It can be accepted that a uniform load applied to a homogeneous soil will result in uniform base pressure. This is an idealistic assumption and it is difficult to find it in reality. As the rigidity of the foundation increases the distribution of the pressure becomes un-uniform. Today the determination of stresses in soil mass due to footing pressure is generally obtained by using Boussinesq equation based on Theory of Elasticity. Boussinesq' s equation considers a point load on the surface of a s'' ni-finite, homogeneous, isotropic, weightless, elastic half space. The equation is in ı form of 3.Q.cos50 (3) 2.71.zz The equation for point load can be adopted to circular and rectangular loading cases and some other numerical solutions may be obtained. One o them is to divide rectangular area into small square areas so a series of point loadr. are produced. Then equation is applied to each small square areas. Another simple aethod is produced by Newmark by integrating the Boussinesq equation over a rectf ngle of dimensions B x L. There are a number of additional considerations w ich have to be taken into account while designing foundations at specific sites. spth must be adequate to avoid lateral squeezing of material from beneath the foi idation. Also the depth of foundation must be below the zone of seasonal volume changes caused by freezing, thawing and plant growth. In additional to compressive strength considerations, the foundation system must be safe against overturning, sliding and uplift. Foundation system must be also safe against corrosion or deterioration due to harmful materials present in the soil. It must be considered that there r/iay be some later changes in site or construction geometry. xivThe variability of soil in combination with unanticipated loads or soil movements such as earthquakes can develop problems for the structure and the current state of art design methods in foundation design may reduce the risks for the construction but it is hard to provide a risk free project. On the other hand the lack of enough knowledge about the characteristics and properties of the soil underlying the foundation and uncertainties in loads may result in an overdesigned system. To summarize these several comments, the proper design is related to the building purpose, probable service life loading, type of framing, soil profile, construction methods and costs, client or owner needs, envoirement and the tolerable risk that may be undertaken with a margin of safety. Since the design of a foundation is a trial method, there are number of calculations to be made, and there is a big risk of making mistakes during these calculations. In order to reduce the risk of error and to gain time, one might use of computers. By using a computer program related to foundation design, one will also be able to try a range of problem variables to obtain a feel for the effect of specifying or using a particular set of parameters. At the end of this study a computer program for the calculation of bearing capacity, stresses in soil mass due to footing pressure and settlement of a shallow foundation is given. The Visual Basic programming language is used for this program. The given program can enlarged for foundation group systems. xv

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