Kolon-kiriş birleşim yerlerindeki donatı düzeninin değiştirilmesi ile ilgili deneysel sonuçların elde edilmesi

Abstract

Bu tezde sunulan çalışma İstanbul Teknik Üniversitesi İnşaat Mühendisliği Bölümü, Yapı ve Deprem Mühendisliği Laboratuvannda gerçekleştirilmekte olan Devlet Planlama Teşkilatı tarafından desteklenen " Yapıların Depreme Dayanıklılığı Teknoloji Araştırma Projesi " kapsamında yapılmıştır. Projenin amacı, deprem sırasında hasar görmesi muhtemel betonarme kolon- kiriş birleşim bölgesindeki donatı düzeninin değiştirilmesi ile ilgili deneysel sonuçların elde edilmesidir. Bu çalışma kapsamında iki farklı donatı düzenine sahip kolon-kiriş birleşim bölgesi yerdeğiştirme kontrollü tekrarlı statik yükleme altında, birleşim bölgesinde etriye bulunması ve bulunmaması durumu deneye tabi tutulmuştur. Deneylerin sonucunda, hasar mekanizmaları, yük değiştirme ve moment- dönme karakteristikleri ve enerji tüketilme kapasiteleri ile ilgili değerlendirmeler yapılmıştır. Etriyeli ve etriyesiz numuneler arasında yük taşıma kapasitelerine ait olan verilerde büyük farklılıkların olmadığı gözlenmiştir. Ancak enerji yutma kapasitelerinde çok büyük farklılıklar olduğu gözlenmiştir. Deney sonuçlarının her bir numuneye uygulanan kuvvetin normalize edilerek karşılaştırılması durumunda etriyeli numunenin diğer etriyesiz numuneye nazaran daha sünek davrandiği görülmektedir. Bu numunelerde beton kalitesinin yüksek olması, her iki numune arasındaki farkın artmasını engelleyen bir başka husustur.

The work submitted in this thesis is supported partially by State Planning Organization as a research project. The aim of the project is, to study the reinforcement pattern at the inter section of the beam-column which could probably be damaged when an earthquake took place. To get the test results related to this concept, two specimens were tested at the Structures and Earthquake Laboratories of Civil Engineering Department of Istanbul Technical University. Experiences from strong earthquakes evidenced the vulnerability of reinforced concrete buildings to strong ground shakings. During a strong earthquak, beam- column joints are subjected to severe reversed cyclic loading and, if they are not designed and detailed properly, their performance can significantly affect the overall response of the structure. Technical literature on the behavior of reinforced concrete frames became very rich during the past three decades and most of the work carried out has focused on the improvement of design procedures. There are a lot of reinforced concrete buildings that are not designed and constructed in accordance with the modern seismic codes. The most recent major earthquake occured in Erzincan, Turkey on March 13th, 1992 in Dinar on October 1st, 1995, which are two of the most active seismic zones of the world, have shown vulnerability of reinforced concrete buildings and many of them suffered heavily causing loss of lives and economic value due to The insufficiently designed and constructed beam-column joints. The type of failure due to insufficiently designed and/or constructed beam- column joints and beam and column end regions is very common in reinforced concrete structures. Buildings designed and/or constructed so, have great potential to collapse since the confinement is inadequate to provide the necessary ductility. On the other hand, consideration of the real joint behaviour from the stand point of the level of semi-rigidity and complexity of moment-rotation curves and its effect on the overall structural response is a recent subject area of concern which draws attention of many researchers. The present research work has been initiated within the scope of the semi-rigid behaviour characteristics in general and strengthened reinforced-concrete joints, in particular, since the safety and economy of a structure can be achieved by understanding and interpreting the real behaviour. The beam-column subassemblages under consideration are representing the behaviour of two dimensional regularly framed structures against lateral forces as shown in Fig 1. Considering the behaviour of a two dimensional regular frame against lateral forces, with the assumption that the inflection point locations are at midheights of the columns, subassemblages are selected by taking the lower half of the upper column and the upper half of the lower column in a joint. Similarly, considering the location of the zero-moment in the beam, only one half of it has been included in the subassemblage. The specimens have been supported from the upper end of the column in the horizontal direction and from the lower end of the column both in the horizontal and the vertical directions. Also, at both ends, out of plane movements have been prevented. While the weight of the upper stories are simulated by an axial constant load applied to the column from its upper end, the effects of the lateral forces are simulated by a reversed cyclic trasversal load applied to the beam-end. Considering the subassemblage support and loading conditions, it is concluded that the moment and the shear diagrams of the subassemblage are simulating the earthquake induced internal forces. N H L/2 Fig 1 Beam-Column joint subassemblage The purpose of the conducted tests was to investigate the behaviour and the connection characteristics of the regularly reinforced, lightly reinforced beam-column joints under reversed cyclic loading. The specimens are of external type beam-column joints of a two dimentional reguler frame. The columns have cross sectional dimensions of 400mmx250mm and a length of 3000mm, and the beam hase cross sectional dimensions of 400mmx250mm and alength of 1500mm. Approximate weight of a specimen is 11KN. All the specimens have end-plates, anchored to the specimens at the free ends of the beam and the columns to lift and to support them and to apply the loads. As the subject scheme was aiming seismic strengthening, it has been decided to conduct, rather then monotonic, cyclic type of tests, which are necessary for structures or structural details for which the expected load condition is characterized by a load history of a cyclic type as much with a known amplitude and distribution as with an unknown ones. On the base of speed of application of the control parameter, the cyclic type of tests can be classified in three groups; dynamic, pseudo dynamic and quasi-static tests. It was decided to carry out quasi-static type of tests, which are characterized by the application of forces by means of displacement controlled static jacks. Regarding to another classification which considers the aim of the experimental study, the tests were, rather than qualification or modelling, comparison tests. The two specimens tested were all made up of C 20 class concrete.High strength steel deformed bars have been used as longitudional reinforcements for both the beam and column. The two internal reinforcement configurations used during the preparation of the specimens are given in Fig 2, Fig 3. 15Qcn 40cn 190cn -300cn- Fig 2 Column-Beam joint having stir-ups J, j, 15den 19C 40cr\ -300crr- Fig 3 Column-Beam joint no having stir-ups The aimed concrete compressive strength for the concrete was 20 MPa.The concreting job for the two specimens have been performed in two parties. During each party, four cylindrical concrete specimens have been prepared from the same batch and they have been prepared from the same batch and they have been tested just before the experiments on the specimens started. The specimens have end-plates, anchored to them, giving possibility to lift and to support them and to apply the loads. In order to trasfer the applied loads and/or reaction from the plates to the specimens safely, the connection detail given below was provided. Four 500 mm long L 60.60.6 mm angles are fixed to the 30 mm thick end-plate by continuous fillet welds applied all along the perimeter of the angles. In addition, against slippage of the angles, short pieces of reinforcing bars were welded trasversely as shear studs. Also, additional ties are provided near the plates and all longitudional reinforcing bars were connected to the end-plates by means of spot welding. The test set-up is fixed to the strong floor using four threaded steel anchor rods and screws.In addition, using a tendon-tensioning machine, a post-tensioning force of 140 KN has been applied to each rod and while the rod is in tension. The screws have been fixed in order to have a permanent tension in them,in order to develop high friction forces between the test set-up and the strong floor against any relative movements. The same procedure has been followed while fixing the top piston to the reaction wall. In order to see the behavior of the specimens, not only in the plastic region, but also in the relatively elastic region; the displacement function is so chosen that it starts with small amplitudes of displacements. From the tip of the beams,the specimens have been loaded by a displacement controlled forcing function, which are repeating themselves 3 times at every step as shown in Fig 4. Displacement 20T mm Fig 4 Displacement pattern In this study two beam-colum connection details having different reinforcement arrangements are tested under displacement controlled static loading. During the experimental work the specimen is set up in one day afterwords in three days the loading and the acquisition of data are achieved. The specimen is subjected to displacement controlled lateral loading with the help of a hydraulic jack. The loading was performed with a constant loading rate which is controlled by a valve manually. But a little difference at loading rate occured between two experiments since the valve is controlled manually. Since the loading was statically; this difference at the rates was not important. In the experimental work several displacement measurements are performed, with the help of displacement transducers on the reference frame, which is considered to be rigid during the experiment. The axial force applied mechanically to the column is controlled by a load cell and has a constant value of 200KN. Also this constant value of axial force was under control during the experiments but small deviation from the proper value occurred. This is not taken into account during the evaluations of test data. The positions of the displacement transducers are controlled during the test so that the data collected from them is dependable. The displacement transducer which is positioned at the point of maximum deflection is changed by an other transducer with 1/4 less precision but the displacement measured by this new transducer is four times higher than the first one. At specimen#l, having stir-ups in the connection, first cracks are formed at the beam as expected in a weak beam-strong column design. Cracking began to form when the loaded part of the beam has a lateral displacement of 1mm as the load was 15KN. The second crack was formed at the second cycle when the lateral displacement was 2mm and the load was 32KN. In the following steps of the experiment, the first crack formed at the connection of beam to column did not extend, but other cracks formed at this connection zone. In the second specimen cracking first formed at the beam and 12cm the column above face, when the lateral displacement was 1mm and the load was 17.3KN in the first cycle. In the following steps of the experiment, flexural cracks formed near to the loading point and at the same time the crack formed in the beam-column connection zone began to extend. The crack in the connection zone was 8mm at the first cycle and at a loading value of 4KN. The hydraulic actuator applying force to the edge of the beam was controlled by computers. Also the lateral displacement data of the beam was collected by the help of computer. The curves are plotted from the data collected by the computers during the experiment. In order to obtain the rotation of the section just at the beam-column connection, displacement transducers are positioned vertically at the same direction with the vertical reinforcement and the horizontal displacement was measured. The measured final horizontal displacement in the first experiment was 343 mm, and 346mm in the second experiment. Moment at this section is evaluated for plotting moment-rotation curves. As seen from these curves energy absorbing capacity of the specimen having stir-ups at the beam-column connection zone is higher than the specimen having no stir-ups at the beam-column connection zone. If the collected test data is analysed, it can be said that the first crack in the specimen having stir-ups at the beam-column connection zone, is formed at on earlier step of loading, than the specimen with no stir-ups. The load value was 15KN m the second specimen, while the corresponding value in the first specimen was 17.3KN. The formation of cracks at the beam-column connection zone in the first specimen occured at a load value of 32KN, but the cracks began to form at an earlier loading step, 20KN in the second specimen. Also the cracks in the second test specimen were extended in the following steps of loading and no other cracks formed later. But in the first specimen the first crack did not extend in the following steps, instead of this other small cracks formed during the experiment. The results of the tests were compared considering the damage mechanisms, load- displacement and moment-rotation characteristics and energy consumption capacities of the specimens. It is seen from the experimental work that there is no significant difference at the load carrying capacity of two specimens. But great difference is observed at the energy absorbing capasities. If the normalized applied loadings are compared with each other, it is observed that the load carrying capacity of the specimen having stir- ups at the beam-column connection zone is higher than the other specimen. It can be stated that the behaviour of the first specimen is more ductile than the one having no stir-ups at the beam-column connection zone. From the comparison of moment-rotation curves of the two specimens the importance of arranging stir-ups at the beam-column connection zone is clear as shown in Fig 5. Since the concrete quality is high for both specimens, the difference in behaviour is small for the two specimens. It is obvious that the arrangement of stir- ups in the first specimen causes difficulty in workmanship. This difficulty at the arrangement of stir-ups usually results no stir-ups at the beam-column joint which is a frequent cause of damage in reinforced concrete frames.