Gövde betonlu betonarme-çelik kompozit kirişlerin negatif moment bölgesindeki davranışı ve taşıma gücünün incelenmesi

thumbnail.default.placeholder
Tarih
1993
Yazarlar
Mengene, Nermin
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
Günümüzde, çelik iskeletli yapılar daha cok kompozit olarak projelendirilmektedir. Kompozit sistemler, sırf çelikten üretil miş taşıyıcı sistemlere göre çok daha az çelik tükettiklerinden önemli bir ekonomi sağlamaktadırlar. Çelik betonarme kompozit yapı elemanlarında, doğrudan çelik kullanılmasına göre sağlanabilen en büyük ekonomi kirişlerde kar sımıza çıkar. Pozitif moment ağırlıklı sistemlerde bu ekonominin mertebesi % 50 civarındadır. Sürekli kirişlerde ise, genelde çe lik profilin üstünde bulunan betonarme tabla, mesnetler yöresinde kompozit çalışmaya fazla bir katkı sağlamamaktadır. Karma kirişlerde, betonarme tablanın altındaki çelik profilin yangından korunmak amacıyla betona gömülmesi ve kolonun her iki yanından sürekli geçen ikiz profiller durumlarında gövde betonlu karma kirişlerle karşılaşılmaktadır. Bu tip kirişler, daha büyük eğilme rijitlikleri sebebiyle sekil değiştirmelerin azalmasını sağladıkları gibi makaslama zorlarına karsı daha dirençlidirler. Deprem kuşağı içinde yer alan Türkiye'de deprem etkilerine en iyi karşılık veren çelik yapıları uygulama azlığı aslında şaşır tıcıdır. Bunun sebebi de, çelik karkas yapıların betonarme kar kas yapılara göre daha yüksek maliyetli olmasıdır. Karma eleman ların, sırf çelikten üretilenlere göre büyük bir maliyet azalma sını sağlamaları dolayısıyla çok katlı yapılarda çelik inşaata değil, kompozit taşıyıcı sistemlere yönelinmesi gerekmektedir. Bu çalışmada, Türkiye açısından Önemi yukarda vurgulanan kom pozit yapı elemanlarında, gövde betonlu karma kirişlerin negatif momentler bölgesindeki davranışları incelenerek bu konudaki bilgi eksiğinin giderilmesi amaçlanmıştır. Bes farklı beton, dört değişik betonarme mesnet donatısı ka litesi, 5m ve 3m gibi birbirinden oldukça farklı iki tip açıklık, Uç değişik boyutta çelik profil, kolon parçalı ve kolon parçasız olma gibi değişkenleri sistematik olarak içeren 18 olağan deney kirişleri I.T.U. Yapı Laboratuvarında denenmişlerdir. Hazırlanan epruvetler, sabit mesnetli olarak 200 kN kapasite li Universal askılı eğilme çerçevesinde, açıklık ortasında tekil yükle yüklenmiştir. Uygulanan yük statik karakterli olup, sıfır dan başlayarak göçme anma kadar kademeli olarak yükseltilmiştir. Her yük kademesinde, açıklık ortasında düşey deplasman ile çelik profil ve boyuna donatıdaki belirli noktaların birim boy değişim leri ölçülmüştür. Elde edilen deneysel değerlerle, teorik olarak çeşitli yön temlerle hesaplanan değerler karşılaştırılmış, tasıma ve kullan ma sınır durumları açısından bu yöntemlerin yaklaşıklık derecele ri saptanmış ve bazı öneriler getirilmiştir. 
Modern steel frameworked structures are generally designed as composite system. This is due to the fact that when steel is used alone in carrying structures, it costs much more than other systems made of other materials. On the other hand, in multi storey steel frameworked buildings, almost all steel members are cased in concrete to increase their fire resistance, and the floors are usually made of in situ reinforced concrete slabs. This makes concrete an ever present material in steel structures. Composite members and systems utilize the compressive strength of this concrete or reinforced concrete, which is not added but an existing component. This is why they may provide an important economy. In steel-concrete composite structural members, the biggest economy is obtained in the beams with about 50 % less steel profiles in the positive bending moment zone. In continuous beams, the reinforced concrete slab above the steel section does not contribute to the composite action in the negative bending moment zone. There are two approaches when computing composite beams in negative bending moment zones: a- In the first approach, composite action is avoided, and all of the moment is resisted by the steel profile. To prevent completely the composite action, the top flange of the steel section can be slightly covered by using bitumen when concreting. b- Longitudinal reinforcement is placed in the concrete slab on supports in the negative bending moment zone and the composite action due to the its presence is taken into account. In this second approach, the composite action can again provide an important economy, but the compressive strength of the concrete is overlooked. The second approach is more accepted nowadays, with the exception of bridges. To increase their fire strength, steel profiles of composite beams are sometimes cased in concrete. In other cases, twin steel profiles traversing continuously on both sides of columns are also observed. In such cases, composite beams have web- concrete. This kind of beam shows lower deformation due to their xv bigger flexural stiffness and at the same time, has more resistance against shearing forces. Apart from these advantages, the web concrete conributes to the strength in negative bending zones and therefore, the safety increases even if this contribution is neglected. To take into account the contribution of the compressive strength of web-concrete in negative bending region, the web concrete should not be crushed when loaded. That is why an additional reinforcement like a stirrup is needed in the case of encased profiles. On the other hand, in continuous beams with twin profiles (double I, preferably double channels) such a problem does not appear and the web concrete functions correctly. In this experimental study, the behaviour and the ultimate loads of composite beams with web concrete was investigated in the negative bending moment zone, with the purpose of contributing to the existing literature. Serviceability limit states were studied as well as ultimate limit states. Eighteen composite beams with five various concrete qualities, four different steels as support reinforcements; two different but practically sized spans of 5m and 3m; three different steel profiles with cross sections prepared with or without column elements have been tested in the Structures Laboratories of Istanbul Technical University. Different longitudinal reinforcement steel qualities with smooth and ribbed surfaces have been utilized in slabs on supports, to observe the changes they can bring to the problem. In all test specimens, 0 8 mm diameter stirrups (Steel 37) and 0 12 mm diameter horse shoe formed ordinary reinforcement steel (St» 37) in connectors have been used in slabs. A steel sheet of 1 mm thickness was used only as formwork for concrete, between double channel profiles. This sheet does not act under loading, as the welds connecting it to the profiles eventually breaks. Ordinary wired connections between longitudinal slab reinforcement on supports and the stirrups and shear connectors in the transverse direction, were provided. Beams were tested in the laboratory's bending frame having a capacity of 200 kN. The specimens were simply supported at both ends in the tests and the test frame was arranged to apply a concentrated load P at the midspan. This load P was static in character and was increased from zero to the collapse load stepwise. At each step of the loading, the vertical deflection at the midspan and the supports were measured, as well as the strains on different points of the steel profiles and on the longitudinal slab reinforcements. The vertical deflections were measured by dial gauges with a sensitivity of 1/100 mm and the changes in unit strain were recorded by a data-logger. The main purposes of the experimental research were first to investigate the ultimate limit states of the composite beams with web-concrete in negative bending moment zone, and secondly to xvx study their serviceability limit states. Apart from the economy they can provide, their behaviour under loading was also investigated. The following observations were made by studying the stress distribution diagrams which were deducted from the measured values of strains in the steel profiles and the longitudinal reinforcement. a- For small values of the load, the triangular shaped stress distribution foreseen in elastic design method stress values proportional to the distance from the neutral axis, is entirely valid. b- As the load is increased, the stress in the reinforcements first rapidly approaches yield, and the stress in the steel section, especially in compression flange, will be different from the triangular shaped distribution. This condition appears before reaching the ultimate limit states calculated from the ultimate load. As loading progresses, the stress distribution in the steel section tends to the characteristic double rectangular shape or very similar shape of plastic design method distribution. From above results, the computation of the composite beams with web-concrete between double channels in negative bending region, it confirmed that the use of plastic design method does not show any contradiction in the stress distribution. Because plastic design approach brings some economy, 40 %, its experimental practicability is justified. On the other hand, although there is longitudinal reinforcement in the tensile zone, that the neutral axis in the stress distribution diagram is drawn nearer to the compression flange, the important contribution of the web-concrete to computation is confirmed. Experimental results show that, in the negative bending moment zone the ultimate loads of composite beams with web- concrete between twin profiles can be computed according to the theoretical relationship suggested by Arda and Yardimci but not yet supported by experimental investigation. This experimental study brings clarity to the material safety coefficients =0 which means neglecting the effect of web-concrete and using plastic design method service loads as explained above were compared to the values by taking cu»=0.8 and using again its plastic design method service loads, the values with =0 are found to be smaller. This comparison shows that the contribution of the web-concrete is about 23 %. In the test specimens, the shearing force in the negative bending moment zone never exceeds 30 % of the elastic shear force carrying capacity of the steel profile, and so the ultimate moment values and the ultimate loads have not been affected by the interaction of the shear forces (Turkish Standards 4561). For the tested specimens working as two cantilever beams staying at both sides of a column, the serviceability limits are taken equal to be to that of a cantilever beam. The limits for such a case are L/150 = 0.0067 according to Turkish Standards 648, and 0.006 L according to Eurocode No.4, here L being the cantilever span. The moment of inertia of the cross-section which is the most important factor affecting the deflections, can be calculated using three different assumptions: a- As that of moment of inertia of gross (uncracked) cross- section, b- As that of the moment of inertia of a cracked cross-section, not considering the concrete in the tensile zone, c- As the average moment of inertia of the cracked and uncracked cross-sections as foreseen by Eurocode. To transform the area of concrete to an equivalent steel area, in these three different assumptions mentioned, concrete widths have been divided by 2n (n being the ratio of the modulus of elasticity of steel and concrete) to take into account long-term creep-effects for concrete. When the load-deflection diagrams are examined, they appear to be of practical interest only in a limited area bounded by the serviceability limit f i 0.006 L line, and the plastic design xviii method service load (P«t/1.7) line. In this utilizable area, the experimental load-deflection diagram shows a linear trend. That is why the real experimental deflection values can be reached by multiplying one of the theoretical values by a characteristic coefficient. Comparing, the ratios of the experimental deflection values to those of theoretical one with the three different moment of inertia assumption cited above under the service loads obtained from the ultimate loads, it can be seen that the best approach is obtained with the moment of inertia of the cracked cross-section which gives a characteristic value equal to 0.994. This value is very close to unity as well as being on the safe side. As opposed to this the theoretical deflections calculated using moment of inertia of the gross section, as well as those with the average moment of inertia, are not in the safety side and, quite a bit away from the experimental deflection values. This investigation shows clearly that the deflections calculated with the moment of inertia of the cracked cross-section are more realistic. Eurocode 4' s suggestion to calculate the deflections with average moment of inertia of cracked and uncracked cross- section, does not give such closer and safer results. As the load-deflection diagram shows a linear shape, the moment of inertia of gross (uncracked) cross-section can be used for quick results in precalculation and then multiply the obtained deflection value fi with the characteristic factor belonging to Wfi ratio. Comparing the serviceability limit state load Puis to the service load Pudi corresponding to the experimental ultimate limit state and also to the service load P«ti corresponding to the theoretical ultimate limit state, it can be determined that the serviceability limit state is more significant than the ultimate limit state for the span equal to 5 m. But the contrary is valid for beam specimens with a span of 3 m. If the plastic design method taking into account the contribution of the web-concrete is used, an excessive load is obtained. That is why serviceability state limits are more important.
Açıklama
Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1993
Thesis (Ph.D.) -- İstanbul Technical University, Institute of Science and Technology, 1993
Anahtar kelimeler
Betonarme kiriş, Kompozit kirişler, Momentum, Çelik-metal, Reinforced concrete beam, Composite beams, Momentum, Steel-metal
Alıntı