Hasır çelik takviyeli çelik-beton kompozit döşeme plaklarının pozitif moment bölgesindeki davranışı ve taşıma gücü

Abstract

Son yıllarda, çelik iskeletli yapıların yerinde dökülen betonarme döşeme plaklarında, kalıp olarak katlanmış çelik saçların kullanılması yaygınlaşmıştır. Kalıcı kalıp olarak kullanılan çelik saçın aynı zamanda donatı görevini üstlenmesi ve dolayısıyla sağlanabilecek ekonomi fikri katlanmış çelik saç-beton karma döşeme plaklarının kullanımını ortaya çıkartmıştır. Bir karma (kompozit) döşeme plağında, kalıcı kalıp olarak kullanılan katlanmış çelik saç, yeni dökülen plak betonu ve donatısını, kendi ağırlığını ve inşaat sürecindeki hareketli yükleri taşımakta dır. Beton mukavemetini kazanıp karma çalışma gerçekleştiğinde de, işletme yüklerini taşıyan plağın donatısı görevini görebi1mesi, yani karma çalışmanın gerçekleşebilmesi için, kayma bağlantısı olarak bilinen araçlarla sağlanan bir mekanik kilitlenmeye ihtiyaç vardır. ülkemizde, kompozit plaklarda kullanılan katlanmış çelik saçlar üretilmesine ve zaman zaman ölü kalıp niteliğinde betonla birlikte kullanılmasına rağmen, çelik saç- beton kompozit plak uygulamasıyla karşılaşılmamaktadır. Yapılan deneysel araştırmada, Türkiye'de rahatlıkla uygulanabilecek bir kayma bağlantısı aracılığıyla birbirine bağlanan katlanmış çelik saç ve beton plakların, pozitif momentler bölgesindeki taşıma ve kullanma sınır durumları incelenmiştir. Çalışmada, İTÜ İnşaat Fakültesi Yapı Laboratuvarında toplam 15 epruvet denenmiştir. Epruvetlerin hazırlanma sında t=0.75 mm,t=1.00 mm. t =1. 20 mm olmak üzere üç fark lı çelik saç kalınlığı ve kayma bağlantısı olarak da kat lanmış çelik hasır (B.Ç.IV) kullanılmıştır. Deney düzeninde, iki uçlarından sabit mesnetli olan döşeme kirişlerine, açıklıkların üçte birlerinde uygulanan statik P tekil yükü, sıfırdan başlayarak, bel1i artımlarla göçmeye kadar kademeli olarak yüklenmiştir. Bu yüklemelerin her kademesinde açıklık ortasındaki ve açıklık üçte birindeki düşey deplasmanlarla, çelik saç ve beton da be lirli noktalardaki birim boy değişimleri ölçülmüştür. Elde edilen deneysel değerlerle, teorik olarak çeşitli yöntemlerle hesaplanan değerler karşılaştırılmış, taşıma ve kullanma sınır durumları açısından bu yöntemlerin yaklaşıklık dereceleri saptanmış ve öneriler getirilmiş tir. Ayrıca, deney epruvetlerinin kayma bağlantısı hesabı için deney sonuçlarıyla uyumlu bir öneri de sunulmuştur.

During recent years, in floor construction of build ings, there has been an increased use of profiled steel sheeting in formwork stage. An efficient and economical light-weight floor system is created by integrating the structural proporties of concrete and formed steel decking. This immediately suggests that, there is a possibility for composite action between the steel deck and concrete. Of course, these two materials may also be used in a non-composite fashion. In either case, the steel deck serves both as a form for the wet concrete and working platform during the construction stage, with the steel deck remaining permanently in place. Before the advent of composite steel decks, the deck was often designed to carry all dead and live loads. When the concrete steel deck floor is designed as a non-composite system, the steel decking serves only as a self sustaining structural form, supporting the construction and wet concrete loads. After the concrete hardens, the steel deck serves no further purpose, and the reinforced concrete slab must support the superimosed live loads. However, when the steel deck can provide composite interlocking with the concrete, it is capable of performing the dual role of functioning, as a form during the construction stage, and as positive reinforcement for the slab under service conditions. Thus, the only additional steel neccessary in the slab is that required for shrinkage and in the case of. continuous construction, to resist negative bending. In order to permit the steel deck to serve as tensile reinforcement and act compositely with the concrete, a means for positive mechanical interlock is needed. This mechanical interlocking is realized either by the shear transferring devices, the geometry of the steel sheet profile or with the combination of both. Their function is to provide the necessary composite action between the steel deck and concrete, preventing the horizontal slip and the vertical separation between the two materials. In some cases however, the shape of the deck itself (closed deck) provides the function of XV vertical separation resistance. In such closed profiled steel sheetings, the horizontal slip is prevented by the holes punched in sides of the deck so that the concrete penetrates throught the holes. But in most cases, the composite action between the steel sheeting and concrete is totally provided by the mechanical shear transferring devices. These are: - Indentations or/and embossments rolled into the deck, - Reinforcing steel lattice welded on steel sheeting - Stud connectors welded on supports. In Turkish market, closed profiled steel decks are not available and the deck surfaces are commonly smooth, without any embossments and indentations. For these reasons and also since the quality of connection provided by a welded steel lattice was not exactly known, the steel deck-concrete composite slab designing is generally avoided.- However, in assuring a perfect adherence between the steel folded sheeting and the concrete, a reinforcing lattice welded on the steel deck is quite practical and shows features easily usable in this country. This welded lattice, on the other hand, is also functioning as a load distributing reinforcement in transverse direction. In this experimental study; a primary series of 6 tests and a main series of 9 tests are carried out in the positive bending moment zone to investigate the elastic and plastic behaviour of the composite slabs in which the adherence between the concrete and the steel sheeting is obtained by welding a reinforcing lattice on the profiled steel sheetings. The profiled steel sheetings used in test specimens are galvanized and show three different thicknesses (t=0.75 mm, t=1.00 mm, t=1.20 mm), same width (b= 860 mm) and depth (d = 27.5 mm). Tension tests were performed on the steel sheetings in accordance with the Turkish Code TSE 138. The yield stresses are 2850 daN/cm2 for steel sheetings with t= 0.75 mm and t= 1.20 mm thicknesses and 2 2400 daN/çm for steel sheeting with t=1.00 mm thickness. In the tests specimens, with different sheet thicknesses, the concrete was casted in such a way that the total depth of slab always is equal to d = 10 cm. The experiments were performed at the Structural Laboratories of Istanbul Technical University. The test specimens were simply supported at both ends and the test frame was designed to apply two concentrated loads at the one thirds of their span lengths. The load P applied to the test specimens was static in character and was increased in steps xv i starting from zero up to the level where the failure mechanism was observed. At each increment of the loading, the deflections at the mid-span (L/2) and at one thirds of the span (L/3) along with the strains at various locations of the steel sheeting and the concrete, were both measured. Comparameters with accuracy of l/100mm were used in measuring deflections and the unit strain changes were measured employing strain-gauges. The strain changes were recorded by a data-logger using jj/m as a unit. These values are the s unit strain values at the specified locations. The main purpose of the present experimental study is to investigate ultimate limit states of the composite slabs in positive bending moment zone. Therefore the primary test series are conducted to determine a suitable shear connection which could prevent an adherence rupture between the folded steel sheeting and the concrete. In evaluating the results of the primary series of tests a method of calculation has been developed for the selection of a suitable steel reinforcing lattice welded to the folded steel sheeting to provide shear connection. The reinforcing lattice designed using this method, shear-bond failure has never been observed in the test specimens before the flexural failure. The main test series which followed the primary ones consist of three sub-series. Each contains three identical test specimens possessing the same sheet thicknesses. In addition to the ultimate limit state loads of the test specimens, their serviceability limit states were also investigated. The load carrying capacity of the tested slabs was first analysed employing elastic and plastic design methods to explore their suitability in interpreting experimental results. In the elastic desing method, the values of stresses were determined assuming triangular stress distribution in the cross section and the calculations were carried out as those of a rectangular concrete section with an effective slab depth of d s The location of the neutral axis of the equivelant cross section can be determined from. nA 8 x= XVI 1 Where n is the ratio of the modulus of elasticity of steel to the modulus of elasticity of concrete (n=E /E, >. s b The theoretical bending stresses in the concrete and in the steel can be found as in the following: 2M °W bx F^H M a St A s K7^ In the relations above, A is the cross sectional area of the steel deck, d is the effective slab depth, b 3 is the width of the slab and M is the bending moment. The experimental values of bending stresses ta, a od s a were compared with those determined theoretically, a, D t a. It is observed that for the concrete, the a, /a s t od ot ratios for all of the tested slabs are always less than unity, only in a few cases, equal to 1. This means that the. calculations made with the elastic method will give results that are on the safe side for the concrete. Contrarily, in the profiled steel sheeting, the experimental stress values measured in test specimens exceed theoretical stress values calculated with the elastic method when they reach at 40 percent of the yield stress. Since the allowable stress in the elastic method is equal to (0.60 a ), it can be clearly seen ys that the elastic method is not in aggreement with the experimental results. On the other hand, when the variation of the stress values along the depth of the steel sheeting for each load increment is examined, an approach to rectangular stress-distribution corresponding to yielding of steel can be easily seen. Thus, it is cuncluded that the plastic design method was more suitable and the theoretical ultimate loads of the slabs were calculated according to various standards. Comparisons were made with the experimental ultimate loads to determine the most agreeble desing method. In these standards (EC4 and ASCE), both for the steel and the concrete, a rectangular stress-distribution is accepted and the location of the neutral axis can be found as, XVI 11 y= ot A a f d a s ys ^ J O a, o\ b I d /2 b br s { and then, the ultimate moment of the slab in positive bending moment zone is M = ot A a u as ys d - Where, A is the cross-sectional area of the steel a deck, a is the yield strength of the steel, a is ys br the compressive strength of the concrete, d is the effective slab depth which is defined as the (distance from the extreme concrete compression fiber to the centroidal axis of the steel deck's full cross section), d is the depth of the concrete, above the top o corrugation of the steel deck and, b is the width of the slab. The theoretical ultimate values are fully conform with the experimental ultimate values if factors of a, = 0.80 and a. = 1.00 are utilized. P,/P values which b a ud u t are the ratios between the experimental and theoretical ultimate loads, are obtained very close to 1 and are never less than 1, when a and ot as stated above are employed. b On the other hand, when comparisons are made concerning the serviceability limit states, in transforming the concrete cross-section to an equivalant steel cross-section in order to calculate the moment of inertia, the width of the concrete part must be divided with 2n, in which n= E /E is the ratio of the modulus of s b elasticity of steel to concrete. In calculating the theoretical deflections for a comparison with the experimental deflections, the moment of inertia were obtained separately for the cracked and uncracked cross-sections and, following the procedures suggested by two standards