Ahşap iskelet yapıda taşıyıcılık ve koruyuculuk sorunları

Abstract

Ülkemizde I, Derece Tarihi Eserlerin dışında ahşap yapıların tümüyle ahşap olarak onarılma zorunluluğu yoktur. Bunun yanı sıra, ahşap olarak inşa edilen yeni yapılar da yok denecek kadar azdır. Bunun nedenlerinden en önemlisi, ahşabın çabuk bozularak işlev ve estetiğini yitiren, kullanışsız ve pahalı bir yapı malzemesi olduğu şeklindeki genel kanıdır. Bu çalışmada, ahşabın malzeme ve taşıyıcılık özellikleri incelenerek ahşap yapı siste minin bir bütün olarak korunması ele alınmıştır. öncelikle ahşabın doğal bir malzeme olmasından doğan nitelikleri incelenmiş; ağaç halinden yapıda kullanımına dek ahşaba uygulanan işlemler sıralanmıştır. Ağacın rasyonel biçilme yöntemleri, böylece elde edilen standart boyutlar, ahşabın bünyesel kusurları ve buna dayanarak yapılan kalite sınıflandırılması açıklanmış tır. Ayrıca, böcek, mantar, hava koşulları gibi ahşabı yıpratan etkenlere değinilerek "ahşabın çalışması" ele alınmış, bunlara karşı alınabilecek önlemler sıralanmıştır. Daha sonra, ahşabın taşıyıcılık özelliği ele alınarak iskelet konstrüksiyona etkiyen kuvvetler ve iskeleti oluşturan çekme, basınç, eğilme çubuklarının genel özellikleri saptanmıştır. Bununla beraber, iskelet sistemin önemli bir unsur olan düğüm noktaları incelenmiş, böylece oluşan iskelet taşıma sistemleri; tek tabanlı, tek taraflı çift taban, çift aratabanlı ve Amerikan sistemler olmak üzere dört ana başlık altında toplanmıştır. Böylece, oluşan ahşap yapının bir sistem olarak yangına karşı korunumu, ses yalıtımı ve ısı & nem tutuculuk gereksinimlerinin nasıl karşılanabileceği ele alınmıştır.

In our country, as a consequence of rapid progress achieved in most sectors including construction, timber constructions have been unable to compete with steel and especially concrete systems. In addition, several prejudices such as; the short service life of timber buildings, their weakness against fire, and lack of qualified timber workmanship minimize the survival chance of timber buildings.However, if necessary precautions are taken to protect timber from natural conditions which shorten its life, and to maximize its structural endurance during fire, then timber becomes as good a building material and structural element as the others, in some cases even a better one.The aim of this work is to determine the problems that arise when using timber framework structures and timber as a building material, and to suggest solutions to these problems.First of all, the properties of wood are examined. Since wood is a natural material consisting of fibres, it shows different mechanical properties in three different directions which makes it a unique material.Several criteria such as climatic conditions, growth deformations, species of trees ... affect the endurance and strength of timber. As wood is a poor heat conductor, heat doesn't affect its properties very much, whereas humidity is indirectly proportional with the strength of timber. As humidity increases the dimensions of the wood also increase. This is called swelling. Naturally, as a result of a decrease in humidity shrinkage occurs.Because of its cellular structure, timber is a light material and it is easy to carry;therefor, it can be used in structures which should be constructed in a short period of time. However, the most important disadvantage is that it is affected by physical and biological factors.Trees are cut and processed in order to obtain timber. 25-30 % of the tree is wasted during this process. Consequently, several types whose dimensions range from 26/50-mm to 200/200 mm,and lengths ranging between 1-5 m are acquired.Physical deformations which happen during the growth of the tree determine the strength and quality of timber. Eccentric growth, formation of knots, cracks caused by extreme temperature changes or thunders, disoriented growth are all deformations which reduce the mechanical abilities of timber. These factors are used to determine the classification of timber.Class I timber, whether oak or pine, is timber with highest mechanical capability. Class II is defined as normal, whereas Class III is low quality. While classifying, apart from physical deformations the effect of rots and insects, if any, are also taken into consideration.In order to fight with rots and insects,the first step to be taken is identifying them. Climate, another destructive agent should also be considered while taking steps to avoid their effects. A special feature of wood which is its ability to swell and shrink causes it to change dimensions in different percentages in three directions. This is a main disadvantage for its use in buildings since it endangers the structural capability of the building. A variety of precautions are taken to minimize the disadvantages of timber. The natural durability is the primary factor which decreases the effects of the destructive agents. Moreover, there are traditional, semi-industrialized and industrialized methods:Traditional methods can be summarized as passive precautions such as; designing details which enable timber to move freely without damaging the structure, and active precautions such as the preference of use of plywoods.Semi-industrialized methods are the use of liquid, gas,smoke or mayonaise paste insecticides, brushing or spraying chemicals to the surface to avoid rots, woodstains or paints to stop the effects of the rain,sun and pollution.Industrialized methods are dipping chemicals into chemicals, chemical diffusion method and impregnation. Impregnation is an advanced method where certain protective chemicals such as copper-chromium-arsenic (CCA) are forced into the cells of wood with application of pressure by two similar applications named Vacuum & Pressure and Double Vacuum systems.There are external and internal forces affecting a structure. Earthquake and wind are dynamic , weight and structural loads are static loads which affect a structure externally. Stresses and moments caused in a cross-section of the structure is named as internal forces.Simple structural elements forming the timber framework are traction, pressure and bending elements.Traction elements are rods which contain only tractive forces in their sections along their length. They are used as wind or stability connections. The single or multi piece cross section of the rod does make no difference whatsoever in the bearing capacity.Traction elements can be joined or extended using adhesives, nails, bolts, screws... one important point is to make sure that the extended part is symmetrical according to the axis of rod and axis of the extension.Pressure rods are elements which only transfer pressure loads. Pillars are considered of this group. The more a pillar is reinforced by horizontal connections, the more its bearing capacity becomes. Unlike the traction elements, the pressure rods are affected by buckling effect.In multi piece pressure rods,stress calculations should be done for both xx and yy axes. Multi piece pressure rods are uneconomical in terms of timber wastage. The best form for a pressure element is a circle, square or a rectangular cross section close to a square follows.While joining pressure elements, the angle formed between the direction of the force and the fibre direction of the second piece is important. Calculations are done according to three situations;the value of the angle being 0, 90, and in between the two.If the cross-sectional dimensions of a rod is determined by bending moment, the element is then a bending element. They can be used as joists, purlins and cladding. There are simple joists, cantilevered joists and and Gerber joists.Bending elements can also be single or multi piece. Usually the height is much greater than the width of the cross section. When single piece section is insufficient, then multi piece solutions formed by adhesives, nails...are used.In order to join elements transferring forces at the same point joining accesories are used. Their functions are extending the length, forming knot points in structures and forming composite cross sections.Main joining elements are nails, bolts, keys, screws and adhesives. Nails are circle cross sectioned elements made of steel which can endure 6-8000 kg/cm2 of traction stress.Its diameter, thickness of the wood, pressure endurance of wood and spaces between the nails limit the bearing capacity of the joint. Cylindrical metal elements which can be tightened and placed perpendicular to the sliding surface between the joined pieces are named as bolts and dowels. It is dangerous to use them in structures where dislocation value of the pieces are large.They should be placed in holes previously drilled. Minimum diameter of bolt should be 12mm, and dowel should at least have a diameter of 8 mm. Minimum 2 bolts or 4 dowels should be placed symmetrical to the joining line.Keyed joints are formed by rectangular or cylindrical wooden or.specially designed steel elements which are kept under pressure or shear stresses. They are either placed in holes previously prepared or buried inside the wood.They should be produced of Class I hardwoods such as oak. As a rule, keyed joints must be reinforced by bolts which can later be tightened.The screw consists of two different cross sections; cylindrical body and a screw thread. They are made of steel and their minimum diameter is 4 mm. They are placed in holes previously drilled. Distance between two screws must be less than 40 times the diameter paralel to the fibres and less than 20 times the diameter perpendicular to the fibres.If two pieces are joined by adhesives the joint should be rigid and it should bear traction. Albumin based adhesives obtained from animals or plants are not durable to water and humidity. Synthetic adhesives which are classified as thermoplastic and heat resistant resins are more durable. Because of simple workmanship thermoplastic resins are widely used. However adhesives formed by formaldehites and PF/RF/UF are better quality products.The most rigid joining element is the adhesives. Nailed joints are looser than that. The most loose one is the bolts. Therefor Adhesives and bolts should not be used together.Another method to extend and join timber elements is to use fittings. Various forms are given to the edges of the elements to be joined and the pieces are then fitted together.With this method, pillar and joists, joist and joist, joist and diagonal support joints can be established.With the organization of structural elements and joints a timber framework structure is formed. Types of timber framework structures are:-Single based system which consists of joists placed on pillars in two directions on top of which the floor beams are placed. This is also examined in two classes: Single systems where "Riegel" system formed by the sinking of floor beams into the joists in every direction ,single and double story buildings consisting of joists placed paralel to each other in a single direction are available.Multi systems which, include the "Zange" system, where pillars are tightened in two directions forming a stabile knot point, and divided pillar system where the continuous joist passes through the pillar which is divided into two.-Systems which are double based in one direction:In this system, in one direction, floor beams are placed on joists which rest on the pillars. In this direction, the pillars of the second story are placed on a second joist which rests on the floor beams. Whereas in the other direction floor.beams which are on the edges, rest on pillars directly. -Double based systems: In this system, edges in both directions are designed as double based; floor beams are placed between two joists which rest on and under the pillars. System is reinforced by short beams placed perpendicular to the floor beams heading the edge.-American system: Formed by the use of standard dimensions such as 5 x 10 cm. The speciality of this system arises from density of the pillars and beams. The joints are usually obtained with nails and by diagonal cladding stability is ensured.After the timber framework structure is formed, necessary precautions should be taken to protect the system against undesirable incidents.Since wood is a burnable material, measures which lengthen the time it can support loads during fire should be taken. The first method to achieve this is to increase the dimensions of the unprotected timber pillars and joists to obtain cross sections which can continue supporting load for 30 or 60 minutes. The second method is to apply chemicals to the surface of timber, by brushing or spraying, which with temperature rise become foams, reducing the burning speed of timber. Chemicals can also be applied by impregnation method. Another method is to cover either the whole system or just the structural elements by asbestos-cement or plasterboards.To avoid the passing of the undesirable noises from one room to another, either the weight of the partition element should be increased or a sound insulator should be used. The most widely used insulator, mineral wool, is placed between the joists in floors and between pillars in walls. Heat and humidity effects in timber buildings should be considered together like any other system. Since wood is a poor heat conductor, it is a good insulator. However, the spaces between the pillars and joists still have to be protected. There, PS Boards or mineral wool should be placed. When heat isolator is placed inside or in the middle condensation is probable. To avoid this vapour barrier sheets are placed before the insulator.With the explained precautions taken, it is likely that timber framework structures will become a more serviceable and enduring system.