INTERACTION DIAGRAM FOR A REINFORCED CONCRETE COLUMN STRENGTHENED WITH STEEL JACKET

Transcription

1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 6, June 2018, pp , Article ID: IJCIET_09_06_153 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed INTERACTION DIAGRAM FOR A REINFORCED CONCRETE COLUMN STRENGTHENED WITH STEEL JACKET Hamza M. Salman Department of Civil Engineering, College of Engineering, University of Baghdad, Baghdad, Iraq Mohannad H. Al-Sherrawi Department of Civil Engineering, College of Engineering, University of Baghdad, Baghdad, Iraq ABSTRACT In this study, the construction of axial load-bending moment interaction diagrams for a reinforced concrete column and a steel jacket column has been presented. For a reinforced concrete column strengthened with a steel jacket consist of vertical steel sections connected by horizontal steel plates (battens), the axial load-bending moment interaction diagrams have been constructed using two analytical methods: strain compatibility method and plastic stress distribution method. In the strain compatibility method, the effects of position of the neutral axis on the stress of concrete, reinforcing bars, and steel section have been taken into consideration. In the plastic stress distribution method, the position of the neutral axis has no effect on the stress of the three components, and the maximum value of the stress has been used for each component. The analytical results obtained by the two methods have been compared with the experimental results for some chosen experimental specimens, and they gave good agreement. Key words: RC column, Interaction diagrams, Steel jacket, Strain compatibility, Plastic stress distribution. Cite this Article: Hamza M. Salman and Mohannad H. Al-Sherrawi, Interaction Diagram for a Reinforced Concrete Column Strengthened with Steel Jacket, International Journal of Civil Engineering and Technology, 9(6), 2018, pp INTRODUCTION The reinforced concrete (RC) column is designed to have a nominal axial resistance. There are many reasons for strengthening of a RC column. Under different conditions like errors in design, overloaded the column above its permissible level, improve the building requirements coensurate with the current code requirements (especially in respect to earthquake loads), editor@iaeme.com

2 Interaction Diagram for a Reinforced Concrete Column Strengthened with Steel Jacket changing the use of the building from residential to public or storage (extra live loads), etc. the non-damaged RC column will not be able to sustain the desired applied load and the strengthening is required. Strengthening with steel jacket consist of four longitudinal steel angles connected with each other by horizontal steel battens with specified spacing will be used in this study. To examine the state of the strengthened RC column under concentric or eccentric applied load if it is safe, unsafe, or at the critical limit, the axial load-bending moment (N-M) interaction diagram needs to be constructed. In this study, two analytical methods will be used to construct the N-M interaction diagram for the strengthened RC column. the first method is the plastic stress distribution method, and the second method is the strain compatibility method and there were two different models used in this method. This study primarily intends to compare between the two analytical methods, which used for constructing the N-M interaction diagrams for strengthened RC columns. 2. LITERATURE REVIEW The steel jacketing technique used in the strengthening of RC columns increased the column ultimate capacity. Due to wide spacing between the steel battens which used in the study, the confinement did not have a noticeable effect on the ultimate capacity and on the ductility of the strengthened RC columns [1]. Tested results showed that the resistance of tested columns will not be affected too much for the same number of steel battens with different spacing unlike the ductility. For columns which have low compressive strength, the strengthening system improved their axial load resistance [2]. The failure type in control specimens was a concrete compression failure. The failure was occurred when the steel cage did not have the ability to confined the concrete. Test results showed that using the strengthening technique increases the axial load resistance and also increase the ductility of the strengthened specimens compared with control specimen [3]. Test results showed that strengthening techniques have been increased the axial load resistance and increased the ductility compared with the control specimen [4]. The test results indicated that the load capacity increases with decreasing strip spacing and increasing the area of the jacket [5]. A selected review of literature on models of confinement for concrete specimens with steel jacketed and a parametric study in which the main confinement parameters predictable by each of models were compared has been presented in [6]. The load carrying capacity of strengthened columns increases when the cross-sectional of angles used increased as well as increasing the coverage area of the strengthening system [7]. Two analytical models to construct the N-M interaction diagram of an RC column strengthened with four vertical steel angles and horizontal steel battens have been presented in [8]. The derivation of expressions was made by assuming equivalent stress block parameters for confined concrete. An analytical model for the hand computation to construct the N-M interaction diagram for an RC column strengthened with steel jacket using the plastic stress distribution method by assuming the strengthened column behaving as a composite column has been adopted in [9] editor@iaeme.com

3 Hamza M. Salman and Mohannad H. Al-Sherrawi A study intended to investigate the performance and behavior of RC columns strengthened with steel jackets under concentric and eccentric axial loads is presented in [10]. Most previous researches focused on strengthened columns loaded with axial loads only without taking the bending moments into consideration, also and for establishing the complete guideline for designing and checking the adequacy of RC columns strengthened with steel angles and battens, more studies on strengthened columns subjected to axial loads and bending moments were required [11]. A non-linear finite element model by using the deactivated and reactivated techniques and stepped loading stages, is used in [12], to simulate and investigate the behavior of adding steel jacket to a preloaded and non-damaged RC column. 3. METHODOLOGY In this study, two analytical methods will be used to construct the N-M interaction diagram for the strengthened RC column. The first method is the plastic stress distribution method. In this method, the plastic stress was used in determining the four points identified Figure 1, which used for constructing the N-M interaction diagram. Point A is the pure axial strength, Point D is determined as the flexural strength of the section, Point C corresponds to maximum flexural strength with an axial strength, and Point B corresponds to a plastic neutral axis location that results in the same flexural capacity as Point D but with twice the axial load of Point C. For any position of the plastic neutral axis (c), the stresses in reinforcing bars and steel angles are reaching its yield value [9]. The second method is the strain compatibility method. In this method, the effects of confinement on concrete compressive strength, stressstrain response of confined concrete and the reducing in the axial resistance of steel angles will be taken into consideration. The analytical equations are developed referring to the four main points. Figure 1 Main points of N-M interaction diagram Two models were adopted (Model I and Model II), the only difference between the two models is the equations that used to include the confinement. In this method, Plain section assumed to remain plain, strain on the concrete varies linearly along the section and the neutral axis located at a distance c from the heaviest loading side (compression side). The strain in the concrete, reinforcing bars and steel angles has been assumed to proportion directly with the distance from the neutral axis. For angles and longitudinal bars in compression and in tension it was supposed that they have to be elastic or yielded [8]. The differences between the analytical equations for the two methods and for the main points have editor@iaeme.com

4 Interaction Diagram for a Reinforced Concrete Column Strengthened with Steel Jacket been illustrated in Table 1. The main dimensions of the strengthened RC column used are given in Figure 2. Figure 2 Details and dimensions of RC column and steel cage 4. VALIDATION OF ANALYTICAL MODEL A set of an experimental investigations presented in [1], [2] and [3] which are (C2T5, C5T5, C8T5, C11T5), (A) and (SCN1), respectively were used to validate the presented models. The Details of these specimens are illustrated in Table 2. For each set, an N-M interaction diagram has been drawn (Figures 3, 4 and 5). The main four points for the two different methods have been obtained and the N-M interaction diagrams have been made. In Figure 3, and by using method I, the comparative with the experimental results shows that specimen C2T5 is not safe while, low conservative N-M values for specimen C5T5 and C8T5. Specimen C11T5 is considered to be safe in this method. This method improves the axial and flexural resistance by 63% and 257% respectively. In method II, Model I gives lower axial resistance (Point A) by 15% than Model II. The difference in the axial resistance comes from the difference in decreasing of the steel jacket axial resistance due to buckling effects. In other points, there is no remarkable difference in both the axial and flexural resistance values. By comparing the two models with the experimental results, it can be noticed that specimen C2T5 is not safe while, the models give low conservative N-M values for specimen C5T5 and C8T5. Specimen C11T5 is safe in the two models editor@iaeme.com

5 Hamza M. Salman and Mohannad H. Al-Sherrawi Table 1 The differences between and I Point A I Model I Model II Point C Model I I Model II [ ] [ ] Point D Model I I Model II & & & Point B I Model I Model II & & & editor@iaeme.com

6 Interaction Diagram for a Reinforced Concrete Column Strengthened with Steel Jacket Specime n Crosssection () Lengt h () C2T C5T C8T C11T Long. bars 4 Φ8 4 Φ8 4 Φ8 4 Φ8 A Φ12 SCN Φ10 Table 2 Details of specimens used Tie Steel section size () Steel strip size () Clear strip Spacing () f c (MPa ) f yr (MPa ) f ya (MPa ) f yh (MPa ) e () 4 L L L L L L Figure 3 Comparison between plastic stress distribution method and strain compatibility method for specimens in [1] Figure 4 Comparison between plastic stress distribution method and strain compatibility method for specimen in [2] editor@iaeme.com

7 Hamza M. Salman and Mohannad H. Al-Sherrawi In Figure 4, and by using method I, the comparative with the experimental results shows that specimen is considered to be safe in this method. This method improves the axial and flexural resistance by 77.5% and 206.5% respectively. In method II, Model I gives lower axial resistance (Point A) by 18.8% than Model II. The difference in the axial resistance comes from the difference in decreasing of the steel jacket axial resistance due to buckling effects. In other points, there is no remarkable difference in both the axial and flexural resistance values. By comparing the two models with the experimental results, it can be noticed that specimen A safe in the two models. It can be noticed in Figure 5 that the improvement of axial and flexural resistances are 85.6% and 601%, respectively. In method II, Model I gives lower axial resistance (Point A) by 8.8% than Model II due to the difference in decreasing of steel jacket axial resistance due to buckling effects, and higher axial values by 20.8% in the next two points (Points B and C) due to the difference in increasing of concrete compressive strength due to confinement. In all points there is no remarkable difference in flexural values. The two models give low conservative N-M values when compared with the experimental specimen result. Figure 5 Comparison between plastic stress distribution method and strain compatibility method for specimen in [3] As shown in Figure 3, Figure 4, and Figure 5, very little differences in the N-M between values obtained in the two methods, because in the plastic stress distribution method the effects of buckling in the steel cage and the effects of position of the neutral axis on the stress of steel section and reinforcing bars have not been taken into consideration, so the axial and flexural resistances of the steel section and reinforcing bars will not decrease, unlike the strain compatibility method. The strain compatibility method increases the unconfined compressive strength of the concrete due to the confinement effect and then the axial and flexural resistances will increase, unlike the plastic stress distribution method in which it uses the actual unconfined compressive strength of concrete. Table 3 Illustrate the improvement in axial and flexural resistances, and unconfined concrete compressive strength for RC column using the two methods editor@iaeme.com

8 Interaction Diagram for a Reinforced Concrete Column Strengthened with Steel Jacket Table 3 Improvement in axial and flexural resistances, and unconfined concrete compressive strength for RC column I Specimen Model I Model II %M %N % %M %N % %M %N % [1] [2] [3] CONCLUSIONS Adding of a steel jacket will produce a new column in which gives the ability to handle the additional applied loads that the unstrengthened column cannot handle. For a RC strengthened with a steel jacket consist of vertical steel sections connected by horizontal steel plates (battens), the N-M interaction diagrams have been constructed using two analytical methods: strain compatibility method and plastic stress distribution method. The analytical results obtained by above methods have been compared with the experimental results for some chosen experimental specimens, and they gave good agreement. The plastic stress distribution method gives higher N-M values than the strain compatibility method in the zone of compression failure, due to neglecting the effect of the position of the neutral axis on the concrete, the reinforcing bars, and the steel jacket stresses, and also the effect of buckling of the steel jacket. REFERENCES [1] Elsamny, M. K., Adel, A. H., Amr, M. N., and Mohamed, K. A. Experimental Study of Eccentrically Loaded Columns Strengthened Using a Steel Jacketing Technique, International Journal of Civil, Environmental, Structural, Construction and Architectural Engineering, 7(12), 2013, pp [2] Areemit, N., Faeksin, N., Niyom, P., and Phonsak, P. Strengthening of Deficient RC Columns by Steel Angles and Battens Under Axial Load, [3] Tarabia, A. M., and Albakry, H. F. Strengthening of RC Columns by Steel Angles and Strips, Alexandria Engineering Journal, 53(3), 2014, pp [4] Khalifa, E. S. and Al-Tersawy, S. H. Experimental and Analytical Behavior of Strengthened Reinforced Concrete Columns with Steel Angles and Strips, International Journal of Advanced Structural Engineering, 6(6), [5] Hoque, M. M., Islam, N., Hossen, M. R., Sarker, S. K. and Hossain, M. A. Experimental Investigation on the Axial Load Capacity of Reinforced Concrete Columns with Steel Jackets, International Conference on Recent Innovation in Civil Engineering for Sustainable Development (IICSD-2015), [6] Cavaleri, L., Di Trapani, F. and Ferrotto, M. F. Steel Jacketing of RC Columns: Reliability of Capacity Laws for Concrete, Europe and the Mediterranean towards a Sustainable Built Environment International Conference, SBE 16 Malta, 2016, pp [7] Ezz-Eldeen, H. A. Steel Jacketing Technique Used in Strengthening Reinforced Concrete Rectangular Columns under Eccentricity for Practical Design Applications, International Journal of Engineering Trends and Technology (IJETT), 35(5), 2016, pp [8] Al-Sherrawi, M. H. and Salman, H. M. Analytical Model for Construction of Interaction Diagram for RC Columns Strengthened by Steel Jacket, International Journal of Science and Research, 6(10), 2017, pp editor@iaeme.com

9 Hamza M. Salman and Mohannad H. Al-Sherrawi [9] Al-Sherrawi, M. H. and Salman, H. M. Construction of N-M Interaction Diagram for Reinforced Concrete Columns Strengthened with Steel Jackets Using Plastic Stress Distribution Method, Civil Engineering Journal, 3(10), 2017, pp [10] Debasish, S. Behaviour and Strength of RC Columns Retrofitted with Steel Angles and Strips under Eccentric Axial Loads, Department of civil engineering Bangladesh University of engineering and technology Dhaka, Bangladesh (2017). [11] Sen, D., and Begum, M., 2017, A Comparative Study of Steel Angles and Strips Strengthened RC Columns, Asian Journal of Civil Engineering (Bhrc), 18(3), 2017, pp [12] Salman, H. M. and Al-Sherrawi, M. H. Finite Element Modeling of a Reinforced Concrete Column Strengthened with Steel Jacket, Civil Engineering Journal, 4(5), 2018, pp [13] Vigneshvar R, Hareesh M and Sekar S K, Analysis of Tensile Capacity of Reinforced Concrete Columns and its Ductility Performance towards Seismic Behavior. International Journal of Civil Engineering and Technology, 8(8), 2017, pp editor@iaeme.com