Kesme dayanımı yetersiz kısa kolonların onarım ve güçlendirilmesinde bir uygulama seçeneği

Abstract

Depremler sırasında yapıların büyük hasar görmesinin en önemli sebeplerinden biri taşıyıcı sistemin içinde yer alan kısa kolonlardır. Kolon serbest yüksekliğinin, kesit derinliğine oranı iki veya ikiden küçük olan kolonlar kısa kolon adı ile adlandırılırlar. Bu tip kolonlardan, özellikle deprem bölgelerinde, gevrek ani kırılmaları nedeni ile kaçınmak gerekir. Ancak, özellikle gelişmekte olan ülkelerde gerek proje, gerek inşaat aşamalarında yetersiz kalan denetim yüzünden ya da sonradan yapılan mimari değişiklikler nedeni ile niteliklerine uygun olmayan kısa kolonlar taşıyıcı sistemin bir elemanı olarak karşımıza çıkmaktadır. Bu çalışmada, kısa kolon olarak projelendirilmemiş ancak çeşitli nedenlerle kısa kolon oluşumu görülen mevcut kısa kolonların, yeni ve etkili bir yöntem ile güçlendirilmesi amaçlanmış ve hasar görmüş olanların onarımı incelenmiştir. Yürütülen çalışmada ülkemiz şartları da göz önüne alınarak sık sık örneklerini gördüğümüz beton kalitesi düşük ve yetersiz enine donatı ile imal edilen beş adet kare kesitli kısa kolona çelik şeritlerle sarma (kuşaklama) adı verilen yöntem uygulanarak onarılmış ya da güçlendirilmiştir. Sonuçlar aynı enkesite sahip ancak TS500/84 ve Deprem Yönetmeliğ/75'in önerilerine uyularak hazırlanan numuneden elde edilen sonuçlarla karşılaştırılmıştır. Paketleme sanayiinde kullanılan çelik şeritlerin beton yüzeyine sarılması ile yapılan kuşaklamanın, kare kesitli kolon yüzeyine tamamen yapışması yada kare kesitli kolon yüzeyi ile aralarında boşlukların kalması durumlarının, kısa kolonun enerji yutma kapasitesine, taşıyabileceği en büyük kesme kuvvetine, sünekliğine ve kırılma mekanizmasına etkileri araştırılmıştır. Deneyler I.T.Ü. İnşaat Fakültesi Yapı-Deprem Mühendisliği Laboratuvarı'nda gerçekleştirilmiştir. Deney numuneleri eğilme momenti, kesme kuvveti ve sabit eksenel basınç altında denenmiştir. Yanal yükleme, depremi benzeştirmek amacı ile tersinir yapılmıştır. Yüklemeler, yerdeğiştirme kontrollü olarak, bilgisayar aracılığı ile denetlenen hidrolik veren yardımı ile yapılmıştır. Yürütülen deneyler sonucunda, uygulanan çelik şeritlerle sarma (kuşaklama) yönteminin, öngörülen güçlendirme ve/veya onarım amacına uygun, kısa sürede uygulanabilen ucuz bir yöntem olduğu görülmüştür.

Short column failures are among the major causes of damages in buildings. Especially in many developing countries inadequate control both in the design and the construction levels are the reason for short columns. The failures are due to low concrete strength, poor detailing, and alterations of architectural layouts without consulting to structural engineer. In this study, an efficient and reliable method to repair and strengthen reinforced concrete short columns in existing and damaged structures is presented. A research project is in progress at the Faculty of Civil Engineering of Istanbul Technical University for the last four years to develop the methods of retrofitting techniques for reinforced concrete buildings of prime importance such as schools and hospitals which were built according to former seismic codes. Most reinforced concrete buildings failed and were heavily or lightly damaged during the earthquake of March 13, 1992 in Erzincan, Turkey. Post-earthquake damage assessment of some of these buildings revealed that the damages were due to poorly reinforced structural elements and low concrete strength. Field reports after damaging earthquakes often indicate that columns are vulnerable elements, particularly if they fail in shear. Shear dominated behavior is most common in short columns. Short columns are defined according to their column height to depth (h/d) ratios equal or less than 2. Large degree of protection against shear failure in columns must be provided, simply because column shear failure are brittle. Quite a high number of failures were due to short columns. It is observed that most of the short columns are created by the placement of non-structural partition walls, addition of some partitions for window openings at the basement floor. Therefore, a cost-effective technique of strengthening and repairing short columns is needed. THE TECHNIQUE The technique is actually to provide enough confinement of concrete against sudden brittle failure. Two approaches were used. The first is the application of two U-shaped steel laminates to the specimens using epoxy adhesives. Although this approach is found to be satisfactory, it requires skilled and careful workmanship and is expensive. Therefore, the second approach of xn the application post tensioned metal strips to columns utilizing a strapping tool together with metal clips is adopted. Metal strips availability has been examined by packing industry. Various types of specimens were tested under bending, shear, and axial compression to simulate the effect of earthquake loading. The experiments were carried out using displacement controlled static alternating hydraulic jacks. The results show that the failure of short columns during earthquakes can be prevented by strengthening with the post tensioned metal strips around columns. The results also show that the technique increases both the strength and ductility of the short column. Available literature shows that a large number of tests have been performed to study the behavior of concrete beams in shear. There have been some work in the area of the shear design of reinforced concrete members (Aoyama, 1993). There have been also some tests on columns (Jirsa and Umehara, 1982). Shear capacity of columns under combined bending, shear, and axial loading has been investigated with increasing loading (Gerçek and Boduroglu, 1994). Repair and strengthening of columns are as important as the shear capacity of columns. There are different temporary and permanent restoration techniques developed such as using steel bar, wirerope, adhesive tape, steel plate which are used to increase the lateral confinement of columns under axial load only (Okada ef a/., 1987). Strengthening of a column can also be done by covering columns by carbon fibers under lateral load (Katsuma et al., 1987). Another recent method of repair and strengthening reinforced concrete beams and columns is to wrap the member by metal strips (Pilakoutas and Dritsos, 1992; Frangou and Pilakoutas, 1994). This technique is applied experimentally to beams and beam-columns. Taking into account all these techniques, the last approach is adopted for repair and strengthening of short columns. In this study, extremely short column is defined as a column with a clear height to depth ratio equal or less than two. EXPERIMENTAL SET-UP AND PROGRAM In the experimental program, six test specimens, representing full scale models of short columns in a medium-rise reinforced concrete building. This investigation was a study of behavior of repaired and strengthened short columns subjected to cyclic lateral deformations. The column is loaded axially together with shear and end moments to simulate the similar loading conditions during an earthquake. For test set-up, a special reinforced concrete reaction wall is designed to provide lateral support for the specimen and the hydraulic jack. The hydraulic jack is modified into a displacement controlled loading mechanism with special software and mechanical equipment. The axial load in the column is provided by a hydraulic jack. The schematic test set-up is shown in Fig.1. xiu reinforced concrete block (140x140x140cm) reinforced concrete block (140x140x140cm) Figure 1. The reinforcement arrangement, dimensions of the specimens and internal force diagrams are shown in Fig.2. A «J 25 A - A JQ H 40 f Figure 2. xiv In preparing the test specimens different hoop space is used to observe the failure type. Considering the practice in uncontrolled construction of placing the hoops with large spacing, certain specimens are prepared without any hoops along the height of the short column. A specimen was subjected to a constant axial load and lateral load reversals maintaining the top and bottom stubs parallel. The loading history consisting of several cycles of increasing lateral displacements is shown in Fig. 3. A/Ai A/Ai cycles 100kN cycles Lateral Deformation 3 6 Axial Load Figure 3. The lateral displacement was measured at column mid height. As a result lateral load vs. displacement response, stiffness and strength degradation and ductility of the specimens were obtained during the application of loading history. RESULTS The column shear vs. displacement curves of a typical short column without hoops is shown in Fig.4. The sudden brittle failure occurs due to a slight increase in the controlled displacement. The failure shear force was compared with the shear force formulas of (JSIM - Japanese Seismic Indexing Method, 1977), (ACI-318, 1989) and (TS500, 1984). This sudden brittle failure was not forecasted by neither of these formulas. The similar column was tested with very small increments in controlled displacement to catch the brittle failure of concrete in shear. The test specimens were presented in Appendix B during and after test. The damaged specimen was repaired using steel strips and was retested to failure. It can be seen that the energy dissipation capacity of the repaired specimen is much larger than the previous case. The results of the test without hoops and of strengthened column specimen are shown in Fig.4. and Chapter 4. xv Lateral Force -10. ? *?.£&-:. ;Q-.v£?»H ' -i-w.-fy.-yf - 40 -60 -80 -100 ? '.>j '..I* ii : -' ' ^ * " * _ ». £ * *. *.* K * &<}*. * -,?»?'» ;%*-"- *-2 - ""î~~~ 6 8 10 Lateral Displacement A [mm].STRENGTHENED ?WITHOUT HOOPS Figure 4. CONCLUSION The test results show that wrapping of columns by metal strips can improve the earthquake-resistant capacity of existing and/or damaged reinforced concrete short columns for square cross-sections. The result is also applicable to columns of rectangular and circular cross sections. The metal strips should be protected as usual against corrosion and fire. The technique is easy to apply and very cheap. ACKNOWLEDGMENTS This study has been supported State Planning Organization under the Technological Research Project of "Investigation on Seismic Capacity of Reinforced Concrete Buildings of Prime Importance During and After an Earthquake ". TEKFEN Construction Inc. is gratefully thanked for providing materials of the specimens and concrete reaction walls.