Gövde Enkesitinde Çap Doğrultusunda Pim Delikli Bulonların Çekme Taşıma Yükünün Deneysel İrdelenmesi

Abstract

Uzay kafes sistemler, 1950'li yıllardan beri özellikle hareketli yükleri az olan çatı ve benzeri bina bölümlerinde, büyük açıklıkları aşmak için yaygın olarak kullanılmaktadır. Türkiye'de de, özellikle 1985'ten itibaren, uzay kafes sistemler yaygın olarak kulla- mlmaktadır. Türkiye'de bu konuda imalat yapan şirketlerin, düğüm noktası teşkilin de büyük bir oranda, Mero tipi benzeri düğüm noktası detayını tercih ettikleri görülmektedir. Bu tip düğüm noktalan, eksenleri birbirine dik, üç doğrultuda ve bunların uzay açı ortaylarında olan, diş açılmış en çok onsekiz deliğe sahip masif kürelerden oluşmaktadır. Uzay kafes sistem elemanı bu kürelere, gövdesindeki bir pim yardımıyla döndürülerek sıkıştırılan yüksek mukavemetli bidonlarla monte edilmektedir. Bu tür birleşimlerde, mukavemet yönünden zayıf elemanlardan en önemlisi, gövdesinde bulunan pim deliği nedeni ile, bu yüksek mukavemetli bulonlardır. Bu doktora çalışmasında, bulon gövdesindeki pim deliğinin, bulon üzerindeki gerilme yayılışına olan etkisi ve eksenel çekme yüküne maruz bulonun taşıma gücü, deneysel olarak incelenmiştir. Bu amaçla, İ.T.Ü. İnşaat Fakültesi Yapı Laboratuarla rında sayısı ikiyüzü aşan deney yapılmıştır. Yapılan bu deneyler sonucunda, bulon gövdesindeki pim deliğinin, bulon üzerindeki gerilme yayılışına ve dolayısıyla çekme yükü etkisi altındaki bulonun taşıma gücüne olan etkisi, bir diyagram şeklinde elde edilmiştir. Daha önce buna benzer konularda çalışma yapan araştırmacılara ait, bağıntı ve diyagramlar, bu çalışmada elde edilen diyagram ile karşılaştırılmış ve deneysel sonuçlara uygunlukları incelenmiştir. Bu karşılaştırmanın sonucunda, de neysel sonuçlara en yakın değerlerin, bu çalışmada elde edilen diyagramdan alındığı görülmüştür. Bu çalışmada elde edilen diyagram, gövde enkesitinde çap doğrultusunda pim delikli yüksek mukavemetli bulonlarm çekme taşıma yükünün hesabı için gerekli, gerilme yığılması faktörlerinin belirlenmesi, amacıyla önerilmektedir.

Since 1950, space trass structures have been used widely, especially at roofs or similar parts of constructions on which there is no effective live loads. The cause of this choice, using space truss structures, can be defined as; -lightness of weight, -stiffness, -high degree of indeterminancy, -almost no bending element so, using minimum quantity of materials, -freedom of drawing and forming, -great facility in erection, in disassemble and in changing the truss members. Also in Turkey, space truss structures have been used widely since 1985. The using ratio of space truss structures in Turkey, has increased continuously, every year. At Table 1, the condition of usage of the capacity of space truss structures in Turkey, has been given. Table 1. The using ratio of space truss structures in Turkey Approximately, fifteen construction companies, manufacture products on this subject in Turkey and, %85 of these companies, choose a joint type similar to Mero joint type, in their manufactures. This kind of joint type is made up of a sphere in which there are eighteen screwed holes at three directions perpendicular to each other and at their bisectors, and members of space truss structures have been connected to this sphere, with high-strength bolts, at the joints. In Figure 1, a member of space truss structures with its end detail has been given. The high-strength bolts used to connect the members to the joints, or spheres, include a transverse brad hole, on their shanks. Bolts, which are used to connect the members to the joints, screwed into the sphere with a brad. Under the tensile load, the stress distribution on the high-strength bolts, icluded a transverse brad hole on their shanks, has been examined in this study. At the laboratories of Faculty of Civil Engineering of Istanbul Technical University, 210 experiments had been done to determine the stress distribution on the high-strength bolts, included a transverse brad hole on their shanks. xiv rı ıı ıı m mm imi ıırı m ırıır-rr Vdm'iiii r i i i i i r r.rrrrr nınnıım: Figure 1. The member of space truss structures The high-strength bolts were subjected to axial tensial loads and at 51 of 210 experiments, the bolts collapsed over the cross section in which includes the brad hole. First of all, it can be suitable to be interested in stress, stress distribution and stress concentration concepts. The actual magnitude of the normal stress at any point of a cross section cannot be calculated until some assumption has been made about the nature of the stress distri bution. The formulas for determining stresses in simple structural elements are based on the assumption that, the distribution of stress on any section of a member can be expressed by a mathematical law or equation of relatively simple form. For example, in a tension member subjected to axial tensile load, the stress is assumed to be distributed uniformly over each cross section and, is obtained by dividing the load by the corresponding area of transverse cross section. The equation which is used to calculate the uniform stress is, o = (D in which, a = uniform normal stress (N / mm2 ) P = axial tensial load (kN) F = cross section area (mm2). The distribution of elastic stress across the section of a member may be nominally uniform or may be vary in some regular manner. When the variation is abrupt within a very short distance, the intensity of stress increases greatly and the condition is described as stress concentration. It is usually due to local irregularities of form such as small holes, screw threads, scratches and similiar stress raisers. The assumption that the distribution of stress on a section of a simple member may be expressed by simple laws, may be in error in many cases. The effects such as -abrupt changes in section such as occur at the roots of threads of a bolt at the bottom of a tooth on a gear, at a section of a plate or beam containing a hole, at the corner of a keyway in a shaft, -pressure at the points of application of the external forces as, for example, at bearing blocks near the ends of a beam, at the points of contact of the wheels of a locomative and the rail, at points of contact of gear teeth or of ball bearings on the races, -discontinuities in the material itself, such as, nonmetallic inclusions in steel, air holes in concrete, pitch pockets and knots in timber, or variations in the strength and stiffness of the component elements of which the member is made, such as, crystalline grains in steel, fibers in wood, ingredients in concrete, xv -initial stresses in a member that result, for example, from over-straining and cold working of metals during erection or fabrication, to heat treatment of metals, to shrinkage in castings and in concrete, or to residual stress resulting from welding operations, -cracks that exist in the member, which may be the result of fabrication, such as, welding, cold working, grinding or of other causes, may cause the stress at a point in a simple member, such as a bar, to be radically different from the value calculated from ordinary formulas. The conditions that cause the stresses to be greater than those given by the ordinary stress equations of mechanics of materials are called discontinuities and stress raisers. These conditions destroy the assumed regularity of stress distribution by sudden increases in the stress, called stress peaks, at points near the stress raisers. Often, large stresses due to stress concentrations are developed in only a small portion of a member. These stresses are called localized stresses or simply, stress concentrations. Whether the significant stress in a metal member under a given type of loading is the localized stress at a point, or a smaller value representing the average stress over a small area including the point, depends on the internal state of the metal such as, grain type and size, state of stress, temperature and rate of straining. All of these factors may influence the ability of the material to make local adjustments in reducing the damaging effect of the stress concentration at the point. The- maximum intensity of elastic stress produced by many kinds of stress raisers can be ascertained by mathematical analysis, photo-elastic analysis or direct strain measurment and is usually expressed by the factor of stress concentration. The stress concentration factor can be defined as, o (2) in which, k : stress concentration factor, Cjnax : maximum stress, (T : average stress at minimum section. Figure 2. Examples of nonuniform stress distribution in members subjected to axial loads xvi Values of this ratio for some important cases can be found easily in the literatu re. However, for stress concentration factors for the high-strength bolts with transverse holes, this situation is not exactly valid. Till now, stress concentration factors of cylindirical bars with tranverse hole, instead of high-strength bolts with tranverse hole, have been examined by researchers. Because of being so, any knowledge about the stress concentration factors for high- strength bolts with tranverse hole does not exist in literature. At the Structural Laboratories of Civil Engineering Faculty of Istanbul Technical University, it has been tried to obtain the values of that stress concentra tion factors. Fifty-one of high-strength bolts which have five different diameters, indicated by "d" symbol, were tested. These bolts have brad holes on their shanks and the diameters of brad holes are indicated by "a" symbol. The aim of the tests was to determine the ultimate tensial loads of high-strength bolts which were subjected to axial tensial loads. At the end of the tests, it was seen that the results, determined from the tests, were different from the values calculated from the equation 1. The cause of this difference between experimental ultimate loads and theoretical values, calculated from the equation 1, is stress concentration around the brad holes on the shanks. To find the actual stress value on the bolts, it was required to determine the stress concentration factors. For this purpose, firstly, the values of experimental stresses, ^max' were calculated then, theoretical stress values, a, were calculated according the quality of bolts and then, experimental stress concentration factors were obtained by the help of equation 2. By taking into consideration values, mentioned above, a diagram on which experimental stress concentration factors put on the Y axis, and the values of the ratio, a/d, put on the X axis, was drawn, and called as experimental diagram. The aim of this study was to form a diagram, with its equation, which gives the similar results according to tests. This diagram is called as (k-a/d) diagram. It was determined in this study and, it was compared to diagrams and equations, obtained for cylindirical bars with transverse holes. This comparison has shown that the most actual and nearest results to experimental values can be obtained from that (k-a/d) diagram. The maximum difference between the values obtained from (k-a/d) diagram and the experimental values is %0.1 of experimental values. The Bravais-Pearson corellation coefficient of (k-a/d) diagram is 0.998. This coefficient shows that the corellation is very strong and emphasizes once more that the values obtained from this diagram are reliable. This (k-a/d) diagram is the union of two different lines. The equations of these lines are, for the line indicated by " 1 *', and for (a / d < 0. 1 575), k!=-9.50(a/d) + 3.922 (3) and, for the line indicated by "2", and for (a/ d > 0.1575), k2 =-0.447(a/d) + 2.496 (4) There are two more equations that give the nearest values to the experimental results, except the mentioned diagram, in this study. One of them is, xvn k = 3d a + d (5) The maximum difference between the values of this equation and the experimental values is %6.4 of the experimental values. The other equation is, k = 3-3.13(a/d) + 3.76(a/d)2-1.71(a/d)3 (6) The maximum difference between the values of this equation and the experimental values is %6.5 of the experimental values. It has seen that the values of these two equations are too far from the experimental values, when they have compared to the results of (k-a/d) diagram, which have the maximum difference of %0.1. 2.50 -, 2.48 - 2.46 - 2.44 - 2.42 - 2.40 2.38 2.36 | I I I I I I I I I | I I I I I I I I I | I I I I I I I I I | I I I I I I I I I | I I I I I I I I I | I I I I I i I I I | 0.14 0.16 0.18 0.20 0.22 0.24 0.26 a/d Figure 3. The (k-a/d) diagram The (k-a/d) diagram, defined in this study, has been recommended to calculate the stress concentration factors for high-strength bolts with transverse hole.