Yapı elemanlarının yangına dayanıklılık testleri

Abstract

Bir binada başlayan yangımın tahrip gücünün somut bir büyüklük haline getirilebilmesi için, yangın şiddeti, kavramı kullanılmaktadır. Bir yangının şiddeti, yanma ortamında bulunan yanıcı malzemelerin ve ortam geometrisinin özelliklerine bağlıdır. Olası bir bina yangınının sıcaklık seviyelerinin zamanla değişimini temsil eden standart yangın eğrileri, farklı şiddetlere sahip yangınlardan elde edilen sonuçlar ile belirlenebilmiştir. Bir yapı elemanının, yangını başladığı ortam içinde sınırlı tutabilme yeteneğinin tesbiti için ortaya atılan, yangın dayanım değerleri, her ülkenin konu ile ilgili standartları uyarınca yürütülen testler sonucunda varılan kararlar ile belirlenir. Bir bina yangınının etkilerini temsil ettiğine inanılan, standart yangın eğirisini takip edecek şekilde ısıtılan bir fırın içindeki numunenin standartlarda belirtilen özelliklerini kaybedene kadar geçirdiği süre, numunenin temsil ettiği yapı elemanının yangın dayanım değerini gösterecektir. Bundan dolayı, kullanılan numunenin her açıdan gerçeğinin tam temsilcisi olması ve elemanın yapı içerisindeki şartlarının numune üzerinde oluşturulması gerekmektedir. Bunun için deneyden önce numunenin nem içeriği, malzeme ve mukavemet özellikleri şartlandır ılmakla birlikte deney sırasında numune üzerine yükleme yapılmakta ve ısıl genleşmesi engellenmektedir. Yapı elemanlarına ait yangına dayanıklılık testleri, her ülkede kendine ait ilgili standartlara göre yapıldığından dolayı sonuçların kıyaslanması için testlerin karşılaştırılması önemlidir. Alman, İngiliz, Amerikan ve ISO standartları karşılaştırıldığında, aralarında çok büyük farklılıklar olmamasına rağmen, özellikle yanma sonucu fırın içindeki atmosferin özellikleri, numune boyutları ve sıcaklık ölçüm teknikleriyle ilgili uygulamaya dayalı farklılıkların sonuçlara önemli etkiler yapacağı görülmüştür.

Fire, or combustion, is the process of burning. In essence, fire is a chemical reaction in which a substance combines with oxygen; heat is released and the form of the substance is destroyed. Three things are essential before a fire can occur; fuel, oxygen and an initial source of heat. Since the process of burning releases heat at temperatures in the region of 800 C to 1200 C, the fire will then continue, unless extinguished, until either the fuel or the oxygen runs out. A fire can start in almost any building and it is not possible to eliminate the danger entirely. It is possible, however, to produce a building in which the chances of a fire to develop into a serious incident are reduced. The severity of a fire is.# measure of the rate at which heat is produced. In a building, the severity of fire will largely determine its effect on the surrounding structure, contents and atmosphere and fire load is one of the main factors which determines the degree of fire severity. The fire load is the expected maximum amount of combustible material in a single fire area, and, in the normal building, the fire load consists of the combustible structural elements and the combustible contents subject to a single fire loss. The severity of a fire is determined by the material burned and its rate of burning. Arrangement of the material has a marked effect on the rapidity of combustion. The fire resistance grading of structural elements used in buildings is determined by tests carried out in accordance with standards. Fire resistance is measured by the number of hours that an element (e.g. wall, floor, columnor beam) will continue to function when subjected to heating in a prescribed manner designed to simulate actual fire conditions. In general, fire tests are used for a number of purposes in fire safety. Some of the current uses are as follows : 1. to define non - combustible materials, 2. to define the degree of hazard for content materials, 3. to evaluate the surface burning characteristics of material. 4. to define the period of fire resistance for structural elements of a building, 5. to quantify the amount of smoke produced by a material. There is a common pattern among the different fire tests. They usually have some exposure conditions to high temperature, certain observations and measurements are made during the test, and finally, certain criteria or conditions of acceptance are prescribed which define the performance of material. The objective of each test is usually welldefined. The exposure conditions and the manner in which the specimen is placed in the test chamber of apparatus should be related to its end - use conditions. For instance, a wall should be tested as a wall and a door should be tested in a door frame mounted in a wall. A framework can be introduced for considering fire resistance tests which can be extended to include most other fire tests. The first task is to develop the specific fire safety goal that a particular fire test is expected to evaluate. Next, based on an understanding of the controlling variables, it is necessary to choose a fire exposure which should simulate the conditions of an axpected fire. Then the test specimen which might be a building element or a building assembly is exposed to the fire environment representing the testing process. Finally, the response of the specimen determines the basis for evaluation, a process that depends heavily on the criteria stemming from the safety goal. The purpose of fire resistance testing is to abtain information on the ability of structural elements, such as walls, floors, doors, roofs, beams and columns, to confine a fire to compartment where it started. In general, to confine a fire, elements must possess such resistance to heat exposure that they will not allow excessive heat transmission to other compartments. This implies that the elements must have a certain thermal resistance and must not collapse during the fire or develop openings that will permit hot gases or flames to flow to other compartments. During a fire resistance test this is examined under conditions that are made as similar possible to those met with in fully developed fires. Fire resistance testing is performed in many countries; it is still the most common method of determining the fire resistance of a structural element, although the use of theoretical prediction VII methods is increasing. The testing is carried out according to specifications of the particular country concerned, but in principle the testing method is very nearly the same in all countries. The fire resistance test is carried out by exposing certain surfaces of a test specimen to heating in a furnace so as to simulate its exposure to heating in a fire. The test specimen is in general representative of the construction for which classification is desired as to materials, dimensions of components and workmanship. When necessary the test specimen is conditioned before testing in order to bring its strength, moisture content and material properties to a state that is normal in practice. As far as possible the specimen is tested under supporting or restraining conditions that are similar to those in service. Load bearing elements are subjected before heating to loading that produces in the critical regions stresses of the some magnitude as would be produced normally in a full size element when subjected to the design load., The loading is maintained constant during tne test. The heat input to the test furnace is controlled in such a way as to have the average temperature of the furnace follow as closely as possible a standard temperature - time curve. Furnace temperatures are in general measured by means of thermocouples, which are placed at several locations within the furnace to give an approximation to its average temperature. During the test, temperatures are recorded at significant points of the test specimen. For walls and floors, for instance, these points lie on the unexposed surface of the specimen, when the surface temperature exceeds a certain limit the risk of combustible materials at the unexposed side of the wall becomes great. For proctected steel beams and columns of which the steel is the main load bearing component, only the steel temperature is of importance. The higher the steel temperature the lower the strength of the beam. A specimen is considered still fire resistant during a tast as long as it satisfies certain criteria with respect to: a) Stability: The test specimen should not collapse in such a way that it no longer continues to perform the function for which it was constructed. There is at present no generally accepted criterion of collapse. For beams and floors the attainment of a certain rate of deflection or certain total deflection during test is often regarded as collapse. VIII b) Integrity: For elements of structure such as walls and floors that have the function of separating two parts of a building, the formation in the test specimen of openings through which flames or hot gases can pass should not occur. c) Insulation: For elements of structure such as walls and floors that have the function of seperating two parts of a building it iş required that the average temperature of the unexposed face of the specimen shall not increase above the initial temperature by more than a certain amount which is stated in the standards. Heat transfer to a test specimen in a furnace takes place simultaneously by convection and radiation, and the rate of heat transfer achieved' is variable with time of exposure and with the characteristics of the furnace and the nature of the flames. Convection coefficients may vary by a ratio of five to one due to variations in gas velocity and turbulence in the furnace. Therefore, considerable differences in convection heat transfer to the specimen may accour depending on the location of the specimen in the furnace and on the flow characterististics of the furnace gases. Radiant heat transfer to a test specimen during a furnace tes-i will depend on the radiation properties of the flame and of the furnace enclosure. If the flame is transparent, radiation will be largely from the walls of the furnace which will be at a lower temperature than the flame. In the interest of convenience and safety, many fire - testing laboratories operate their furnaces at slightly negative pressures. This condition of operation prevents smoke and hot gases from entering the laboratory and reduces the danger of making observations through the viewing ports in the furnace walls. The most important result of negative furnace pressure during a fire test is the tendency for cool air from the laboratory to leak into the furnace through cracks or other openings in the furnace or specimen. The effect of such leakage on fire resistance ratings may be most signigicant for specimens of construction that permit the leakage to enter the furnace through construction. Minimum dimensions for fire test specimens are specified in the standards of a number of countries in which acceptance fire tests are carried on. The size of area exposed to the fire, relative to its thickness, IX is important in fire tests when the criteria relative to structural performance and passage of flame are applicable if a specimen is restrained or is required to carry load during the exposure, the effects of restraint and load are more pronounced in large specimen than in small specimens, assuming that the thickness of both are the same. During a standard fire test a load bearing element or assembly shall support a superimposed load calculated to develop stresses equal to those which would be produced in the construction carrying the maximum load for which it is designed. This usually means that the test load in a standard fire test consist of one dead load plus one live lod. Live loads are applied by means of hydraulic cylinders or by the use of weights. The limit on temperature rise of the unexposed surface is meant to be a safeguard against the ignition of flammable materials on the side of the floor, roof or wall a way from the fire. Experimental studies have indicated that measuring the.uxexposed surface temperature by thermocouples placed under asbestos pads caused flaws. Because of large differences in the sorption characteristics of various building materials and the inadequacy of defining and measuring the moisture content of test specimens, the fire endurance values for various constructions probably 'include markedly different contributions from moisture. One of most important effect of moisure content of the specimen is that causes spalling which reduces the fire resistance ability of the construction during testing period.