EFFICIENCY OF HOLLOW REINFORCED CONCRETE ENCASED STEEL TUBE COMPOSITE BEAMS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 3, March 2018, pp , Article ID: IJCIET_09_03_073 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EFFICIENCY OF HOLLOW REINFORCED CONCRETE ENCASED STEEL TUBE COMPOSITE BEAMS Mohammad M. El Basha Graduate Student, Faculty of Engineering, Ain Shams University, Cairo, Egypt Tarek K. Hassan Professor of Concrete Structures, Faculty of Engineering, Ain Shams University, Cairo, Egypt Mohamed N. Mohamed Assistant Professor of Structural Engineering, Faculty of Engineering, Ain Shams University, Cairo, Egypt Omar A. M. Elnawawy Professor Emeritus of Concrete Structures, Faculty of Engineering, Ain Shams University, Egypt ABSTRACT Composite construction employs structural members that are composed of two materials: structural steel (rolled or built-up) and reinforced concrete, Concrete encased steel tube (CEST) is an example of composite members. Concrete encased steel tubes (CESTs) are efficient members in structural applications including high rise building & bridges, and their use in the building industry is significantly increasing. The concrete encased steel tube (CEST) composite beam members have many advantages compared with the conventional concrete structural members. Steel members have the advantages of high tensile strength and ductility, while concrete members have the advantages of high compressive strength and stiffness. Composite members combine steel and concrete, resulting in a member that has the beneficial qualities of both materials. It is widely recognized that the innovative use of two or more materials in structures generally leads to more efficient economical systems. Hollow (CEST) composite beams can provide an economical form of construction as the hollowed part of the composite beam could be used to reduce the self-weight of the member and enhances the overall stiffness of the member. This paper presents the results of experimental and analytical programs conducted to investigate the flexural behavior of hollow CEST composite beams subjected to pure bending. Experimental results from nine simply supported hollow (CEST) composite beams subjected to pure editor@iaeme.com

2 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams flexural loading, are presented. Three-dimensional non-linear finite element analysis is performed to predict the flexural behavior of this type of composite members. The predictions from the FEA model are in reasonably good agreement with the experimental results. In comparison with the conventional solid reinforced concrete beam specimens, larger elastic deformation, higher strength, higher ductile behavior were observed in the hollow CEST composite beams. Keywords: Concrete encased steel (CES), concrete encased steel tube (CEST), flexural behavior, reinforced concrete hollow beams, composite, encased, steel tube, ATENA. Cite this Article: Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy, Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams, International Journal of Civil Engineering and Technology, 9(3), 2018, pp INTRODUCTION & BACKGROUND Structural members that are composed of two materials: structural steel and reinforced concrete are examples of composite members. Composite beams can take several forms; one of these forms consists of structural steel beams encased in concrete. The inclusion of the contribution of the concrete results in more economical designs, as the required quantity of steel can be reduced. Ducts and pipes associated with the mechanical, electrical, and sewer systems in a building are usually located underneath the floor beams, resulting in a considerable loss in the usable floor height. Passage of these ducts and pipes through web openings in floor beams offers an effective way to utilize the entire floor height, and provides a more economic and compact design. Steel-concrete composite beams have been extensively used in building and bridge structures. Concrete-filled-steel-tube (CFST) structures have the advantages of high strength and ductility due to steel tubes and high loading capacity due to concrete. Steel members have the advantages of high tensile strength and ductility, while concrete members may be advantageous in compressive strength and stiffness. They are comprised of a steel hollow section of circular or rectangular shape filled or centrifuged with plain or reinforced concrete as shown in Fig. 1. Figure 1 Various types of composite elements: (a) concrete encased steel (CES), (b) CFST, (c) combination of CES and CFST, (d) hollow CFST sections and (e) double skin sections. Hollow reinforced CEST sections depends on hollowing part of the area of concrete in any horizontal reinforced concrete flexural member (beams, ribs and slabs) under neutral axis as shown in Fig. 2. The main benefit of this technique is to enhance the effective moment of inertia of the member by using hollow steel sections, and therefore increasing the ultimate carrying capacity of the beam. Thus the structural composite system provides higher strengths even exceeding the sum of the capacities of its individual components editor@iaeme.com

3 Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy Figure 2 Typical section of hollow reinforced concrete encased steel tube composite beam Encased beams have been used as rigid reinforcement in deck bridges for several decades. Nowadays they are used mainly in railway reconstruction with limited building heights. During the entire period of their exploitation there have not been any changes or modifications in design or structural solution. Eurocodes [1], [2], [3] & [4] allow for plastic design in the ultimate limit state of such members. However, standard verifications of such structure indicate the inefficient use of traditional I-beams only. Therefore, bridge deck specimens with beams of alternative cross-section were designed and experimentally verified in the laboratories by many researchers [5]. Experimental research was conducted by Ammar A. Ali et al. (2012) [6] to investigate the structural behavior of concrete-encased composite beams. Specimens were tested under lateral loading. Test results indicated that the behavior and failure mode of the beams are greatly affected by the steel beam core. The effect of the upper steel section flange position of encased beam on the beam capacity and beam ductility was analyzed by A.Y. Kamal [7]. Three-dimensional non-linear finite element analysis adopted by ANSYS up to failure was performed on twenty one simply supported encased concrete beams. Test results indicate that the behavior of the beam is greatly affected by the steel beam upper flange position. Upper flange width was the most important parameter that influences the beam capacity and ductility. Preliminary criteria for an adequate design was presented. The dual-hazard inelastic behavior of concrete-filled double-skin steel tubes (CFDSTs) is experimentally investigated by P. Fouche, et al. (December, 2017) [8] as a substitute to reinforced concrete columns for bridge piers in multihazard applications. Results demonstrate that CFDSTs exhibit substantial toughness and ductility that can help achieve satisfactory performance when exposed to seismic and blast hazards. Under the cycling loading, for all specimens, yielding of the section preceded buckling of the outside tube. For the blast tests, all the specimens behaved in a ductile manner when subjected to near contact charges but for extreme conditions, sections having large voids in their cross section experienced significant denting. Soundararajan et al. (2008) [9] presented an experimental study of normal mix, fly ash, quarry waste and low strength concrete contribution to the ultimate moment capacity of square steel hollow sections. Results of the experimental investigations showed that normal mix, fly ash, quarry waste and low-strength concrete enhanced the ultimate moment carrying capacity of steel hollow sections. 2. EXPERIMENTAL INVESTIGATION The experimental program in the current consisted of testing nine specimens divided to three solid reinforced concrete beams specimens and six hollow reinforced concrete encased steel tube composite beams specimens. The program was designed to investigate the flexural behavior of this type of beams under pure bending as well as to identify different failure editor@iaeme.com

4 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams modes. The research was extended to determine the effect of the main parameters on the flexural behavior of this type of construction. Details of the experimental program and the tested specimens are summarized in Table 1. All beams were tested with a span of 6 m. The first beam, CB1, had a depth of 600 mm and reinforced with 3No18 bars at the bottom side of the beam to simulate the depth of normal beam designed using conventional steel reinforcing bars (Span/10). The purpose of the current study is to investigate the possibility of reducing the depth of the composite beam using steel tubes without affecting the overall stiffness and strength requirements and yet remains with an acceptable cost figure compared to traditional solution. The target depth of the composite beams was selected as 480 mm which is 20% less than the conventional beam. The second specimen, CB2, had a depth of 480 mm and reinforced with 3No18 bars at the bottom side of the beam. Specimens B1 to B6 had identical depths and reinforcement similar to CB2 but with additional steel tubes with different configurations as detailed in Table 1. Specimen B7 is identical to B1 and B2 but with replacing the steel tube with an equivalent area of conventional steel reinforcing bars Test Set-up A testing frame consisting of two steel beams were fixed to the laboratory s strong floor and were used to support the test specimens. The specimens were supported at both ends on vertical roller support and vertical hinged support. The specimens were fabricated and casted at the Structural Engineering Laboratory of Housing & Building Research Center. A 12-ton over-head crane was used to lift the specimens and place them on the steel supporting beams. Typical test set-up for the nine specimens is shown in Fig. 3 and Fig. 4. All the dimensions of the tested specimens are summarized in Table 1. Typical test set-up Loading Jack Roller support Specimen PI-gauges Hinged support LVDTs Figure 3 Profile view of test set-up Figure 4 Test set-up / typical dimensions legends of all specimens editor@iaeme.com

5 Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy All specimens were reinforced with three 18 mm diameter bars as bottom reinforcement and two 12 mm diameter bars as top reinforcement. The specimens were adequately designed for shear using 10 mm diameter stirrups spaced at 125 mm to prevent shear failure. Hollow steel tubes of different dimensions and lengths were pulled through and tied in place above the bottom steel by 25 mm and centered within the beam length and width of specimens B1 to B6. Mechanical shear headed studs (Grade 4.8) are welded to the steel tube at spacing equal to 400 mm at the longitudinal direction from three sides except bottom side to ensure fully bonded connection between the steel tube and concrete and hence to develop a full composite action between the concrete and the steel tube. Specimen B7 has no steel tube but was reinforced instead with additional 16 bars of 8 mm diameter which is equivalent to the area of the steel tube in specimen B1. A sketch of the reinforcing details and the steel tubes arrangement of different specimens is shown in Fig. 5 and Fig. 6. Specimen Table 1 Test Matrix H h t Ls mm mm mm % of L n Area of steel tube mm 2 CB CB B B B B B B B Add Figure 5 CB1, CB2, B1, B2, B3, B4, B5 and B6 reinforcing details showing top & bottom reinforcement and distribution of stirrups across the span of the specimen / view of the hollow steel tube embedded in the beams editor@iaeme.com

6 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams Figure 6 B7 Reinforcing details showing top & bottom reinforcement and distribution of stirrups across the span of the specimen / additional bottom reinforcement Instrumentation & Data Acquisition The Micro-Measurements Strain Smart software was used to calibrate and record the various experimental instruments output. The instrumentation used to monitor the behavior of the beams during testing consisted of a combination of electrical strain gauges, linear variable differential transducers (LVDTs) and PI-gauges. Six LVDTs were used to capture vertical displacement of each beam specimen at different locations, as shown in Fig. 7. Strain gauges were installed along the upper surface of each specimen at mid-span to measure the maximum compressive strain at the top of the beam, as shown in Fig. 7. Strain gauges were installed along the upper and bottom chord of the steel tube as well as in the bottom and top reinforcement to measure the strain, as shown in Figure 8. Two 100 mm displacement type strain gauges (PI-gauges) were installed in the compression and tension sides at mid-span of the beams to measure the compressive strain in the top side of each specimen and the tensile strain in the bottom side of each specimen, as shown in Fig. 7. Figure 7a Different instrumentations (types / locations / naming conventions) used to capture the behavior of the specimens editor@iaeme.com

7 Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy Figure 7b Different instrumentations (types / locations / naming conventions) used to capture the behavior of the specimens 2.3. Material Properties All the nine specimens were fabricated and tested at the Structural Engineering Laboratory of Housing & Building Research Center (HBRC). The following material characteristics are representative for all tested beams. Concrete: For all the specimens, nine standard cubes of 150х150х150 mm were cast during casting of each individual specimen to determine the material properties of the concrete and were tested at 28 days and at the day of testing. The measured average compressive strength at 28 days was 39 MPa editor@iaeme.com

8 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams Reinforcement & steel tubes: Representative Samples of No. 10 bars, No. 12 bars, No. 16 bars, No. 18 bars, steel plates (2.5 mm thickness) and steel plates (3 mm thickness) were tested in tension to determine the mechanical properties as shown in Fig. 8. Figure 8 Tensile tests of different reinforcement bars & steel tubes The measured results were used to determine the average elastic modulus, yield strength and ultimate strength for different bar diameters. The mechanical properties of the steel used in the current study are summarized in Table 2. Bar Diameter Table 2 Mechanical Properties of The Used Steel Bars / Steel Tubes Yield Strength Yield strain Elastic Modulus Ultimate Strength Ultimate strain MPa με MPa MPa mm/mm No No No No mm plates mm plates *The above values were based on the average values from testing three samples for each bar diameter / steel plate. 3. EXPERIMENTAL TEST RESULTS AND DISCUSSION 3.1. Deflection Behavior: The simply supported beams test results showed that introducing hollow steel tube inside the solid beam could be used to increase the flexural strength of the conventional solid RC beams to withstand higher service loads. Test results provided sufficient evidence of the used technique by introducing hollow steel tube in the conventional RC solid beams. The loaddeflection behavior of the ten simply supported beam specimens is shown in Fig. 9. Test results indicated a considerable increase in stiffness and ultimate loads with adding the hollow editor@iaeme.com

9 Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy steel tube inside the solid RC solid beams. Fig. 9. Shows that adding hollow steel tube in specimens B1, B2, B3, B4, B5 & B6 increased the stiffness in the elastic range for these beams till the yielding point by about 33% compared to the stiffness of specimen CB2. The mid-span deflection curves showed traditional non linearity due to the cracking of the concrete and yielding of the steel reinforcement / steel tube. Prior to yielding of the bottom tension steel reinforcement, the stiffnesses of all beams with hollow steel tube were almost the same and were 33% higher than the stiffness of the conventional RC solid beam CB2. Stiffness of the solid beam B7 with additional bottom reinforcement was 27% higher than the stiffness of the conventional RC solid beam CB2. After yielding of the tension steel / steel tube the stiffnesses of all the beam specimens were almost the same, however the stiffnesses of the beams with hollow steel tubes were slightly higher with respect to CB1 & B7. Applied Load Figure 9 Mid-span vertical deflection of all specimens from experimental tests 3.2. Failure Modes Traditional flexural failure due to crushing of the concrete at the mid span section was observed for all the nine specimens. Failure occurred at the maximum moment zone at mid span. The concrete compressive strain, directly behind the location of the applied load, was monitored during the tests using strain gauges. The ultimate concrete compressive strain at failure reached a value of to for all specimens. Typical classical failure due to concrete crushing of the simply supported specimens is shown in Fig Crack Patterns Cracking behavior of the tested specimens was monitored within the maximum moment zone. The first crack was observed at different load levels as shown in Figure 10. Cracks started perpendicular to the center of the bottom of the tested specimens at the mid span section and extended to the top surface. More than 40 cracks were observed throughout the length of the beam as shown in Fig Failure Loads The failure loads of specimens B1 to B7 were 10 % to 33 % higher compared to beam CB1 which had the same geometry and reinforcement but without the steel tube editor@iaeme.com

10 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams 3.5. Effect of Length of Steel Tube The ultimate load carrying capacity for B1 (with length of 0.55 L n ) and B6 (with length of 0.7 L n ) were almost the same, which indicates that the length of the steel tubes of theses beams is greater than the required development length to resist the critical moments at the critical sections of the beams and hence no deponding failure was observed. Experimental results showed that a length of the steel tube of 0.55 L n is sufficient to fulfill the required development length to resist the moment induced at ultimate load carrying capacity. 4. FINITE ELEMENT ANALYSIS 4.1. Methodology The main aim of performing a finite element analysis of the models was to extend the investigations carried out experimentally to have a better understanding of the flexural behavior of reinforced concrete encased hollow steel tube composite beams under different conditions. The analytical program will augment the experimental program by allowing for examination of several parameters that may be cost prohibitive and time consuming to be determined experimentally. Data from the experimental program described above was used to validate, refine, and calibrate the following analytical models. The approach of the analytical phase of this research was to develop and calibrate a three-dimensional nonlinear finite element model (FEM) to study various parameters that have been identified to influence the flexural behavior of hollow reinforced concrete encased steel tube composite beams subjected to pure bending moment. CB1 CB2 B1 B2 B3 B4 B5 B6 B7 Figure 10 Failure modes & typical flexural cracking patterns of different specimens editor@iaeme.com

11 Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy 4.2. Effect of Relevant Parameters Analytical finite element model has been conducted to simulate hollow reinforced concrete encased steel tube composite simple beams loaded by a vertical concentrated static load at mid span to examine and evaluate the effects of several important parameters. The main variables in the analytical investigation are as follows: 1- Concrete compressive strength Fcu, 2- Dimension / thickness / span (Ls/Ln ratios) of hollow steel tube section and 3- Location of the hollow steel section relative to N.A. A comparison of experimental and analytical results is conducted. Based on the research results, a rational model was developed and calibrated with experimental results Modeling description of reinforced concrete encased hollow steel tube composite beams The analysis in the current study was conducted using ATENA version , Advanced Tool for Engineering Nonlinear Analysis. The material models adopted by the program are capable of simulating the characteristic failure modes of reinforced concrete structures with a sufficient accuracy. The program has been extensively validated by several researchers and excellent correlation with the experimental behavior has been observed and documented in the literature [10-12]. Modeling of the compressive behavior of concrete follows the generally accepted principles of plasticity, though these principles were modified for the unique and computationally demanding aspects of concrete response. The concrete material model includes non-linear behavior in compression including hardening and softening, fracture of concrete in tension based on the nonlinear fracture mechanics, biaxial strength failure criterion, reduction of compressive strength after cracking, tension stiffening effect, and reduction of the shear stiffness after cracking (variable shear retention). Details of the concrete model can be found elsewhere [11]. The basic constitutive model in ATENA is based on the smeared crack concept and the damage approach. The material axes of cracked concrete, the orthotropy axes, can be defined by two models: rotated or fixed cracks [11]. In the rotated crack model, the crack direction always coincides with the principal strain direction. In the fixed crack model the crack direction and the material axes are defined by the principal stress direction at the onset of cracking. In further analysis, this direction is fixed and cannot change. An important difference in the above approaches is in the shear model on the crack plane. In the fixed crack model, a strain field rotation generates shear stress on the crack plane. Consequently the shear model becomes important. In the case of the rotated cracks, a shear on the crack plane never appears and the shear model needs not to be employed. The stress response is based on the damage concept and is defined by means of the equivalent uniaxial stress strain law. This law describes the development of distinct material variables and their damage and covers the complete material behavior under monotonically increasing load including pre- and post-peak softening in compression and tension. In case of a uniaxial stress state, it reflects the experimentally observed uniaxial behavior. In a biaxial state, the equivalent strain is calculated using the current secant inelastic elastic modulus. In the uncracked concrete, the material is considered isotropic and one elastic secant modulus is defined corresponding to the lowest compressive stress. In cracked concrete, which is orthotropic, two moduli are defined, the first one for compressive and the second one for tensile material axes respectively. It is known from material research that post-peak softening is a structure-dependent and a simple strain-based model is not objective, but dependent on the finite element mesh due to strain localization in softening. Therefore, a fracture mechanics approach based on the crack band model and fracture energy is implemented in ATENA [13]. An elastic perfectly-plastic behavior was assumed for the reinforcing bars. The modulus of elasticity was set to 200 GPa editor@iaeme.com

12 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams for reinforcing steel bars. The concrete was modeled using a combination of brick and tetrahedral elements using 20 and 10 nodes, respectively. More information about the capability of the software can be found elsewhere [11 & 12]. Typical hollow reinforced CEST composite beam was modeled using large number of bricks and tetra elements with max mesh size of 0.05 m. The mesh configuration used in FE analytical model is shown in Fig. 11. The large number of elements was necessary to maintain a sufficiently fine mesh around all of the loading and boundary conditions as well as in the middle region of the beam where flexural failure is expected to occur. Individual bars were modeled by truss elements embedded in concrete elements with axial stiffness only. Hollow steel tubes were modeled by 3D solid tetra elements. Three rigid steel plates were modelled, one at the middle of the top face of each beam and two at the right and left ends of the lower face of each beam, to simulate the real boundary conditions. The loading was applied as a concentrated load at one node on the top loading plate. The load was increased incrementally, at load of 2 kn per step, until failure. Newton-Raphson iteration method was used to find equilibrium within each load step (increment). The right plate was restrained from translation and rotation in x, y and z directions to simulate the hinge support, while the left plate was restrained from translation in z direction only to simulate the roller support. Figure 11 Mesh configuration for FE analytical model 5. VERIFICATION OF FE MODELS {COMPARISON BETWEEN ANALYTICAL & EXPERIMENTAL RESULTS} To verify the FE model, a comparison of the experimental results and those from the FE analyses was carried out. It can be seen that, the FE model simulated the ultimate behavior in a satisfactory way. It was observed that the finite element model (FEM) underestimated the ultimate load carrying capacity of beams by approximately 0.60 to 8.04 % as shown in Fig. 12. Applied Load Figure 12 Mid-span vertical deflection of all specimens from FEA models editor@iaeme.com

13 Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy 5.1. Vertical Deflections Comparisons between Load-deflection curves for all the tested specimens, measured from the experimental test and monitored from the FEMs at mid-span of the tested specimens are presented in Fig. 13. Applied Load Applied Load Applied Load Applied Load Figure 13 Comparison between experimental and analytical load- deflection curves for sample of the specimens 6. CRACKED-SECTION ANALYSIS The tested beam specimens were also analyzed using strain compatibility and internal force equilibrium procedures to predict the flexural response up to failure. The concrete was assumed to be subjected to uniform uniaxial strains over the entire width of beam. Strains were assumed to vary linearly over the depth of the section. The predicted ultimate moment was determined at each section according to the following procedures: a) Assume a strain at the extreme compression surface of the concrete at failure to be b) Assume a neutral axis depth. c) Determine the internal forces in compression and tension zones based on the tensile strains at every layer of the reinforcement as shown in Fig. 14. d) Check the equilibrium of the section. e) Revise the assumption of the neutral axis until equilibrium is satisfied. f) Calculate the internal moment of the section by taking the moment at the neutral axis editor@iaeme.com

14 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams Figure 14 Stress strain distribution in a beam section at ultimate limit state A comparison between measured and predicted values is presented in parallel with the results of the finite element analysis in Table 3. Specimen Failure Load (Experimental) Table 3 Failure Loads R1 R2 Failure Load (FEA) Pu (Predicted) FEA/EXP. kn kn kn CB CB B B B B B B B *R1 = Failure load of specimen / Failure load of CB1 ** R2 = Failure load of specimen / Failure load of CB2 7. COST ANALYSIS: One of the prime objectives of this study was to provide a cost-effective analysis. The approximate cost calculation for each beam specimen is shown in Fig. 15. The cost analysis was based on $ for one meter cube of concrete, $ for one ton of steel bars and $ for one ton of structural steel tube. The total construction cost accounts for the cost of materials, equipment needed during construction and labour. The percentage increase in the flexural capacity and the construction cost for each specimen are shown in Fig. 15. Figure 15 Cost analysis of all specimens editor@iaeme.com

15 Mohammad M. El Basha, Tarek K. Hassan, Mohamed N. Mohamed and Omar A. M. Elnawawy The figure indicates that beam specimen B3 provided the maximum increase in strength. Using an efficiency scale, E, defined by Equation (1), the efficiency of specimens B1 to B7 was evaluated as shown in Fig. 16. Equation (1) Figure 16 Efficiency of all specimens The results show that using encased hollow steel tube is an efficient technique in terms of strength improvement and construction cost compared to the conventional RC solid beams depending on the steel tubes configurations. 8. CONCLUSIONS Based on the results of the current investigations, the following conclusions shall be drawn: 1. For hollow composite beams, the usage of concrete encased steel tube system increases the ductility of the beams as compared with traditional solid beams. 2. The ultimate flexural strength, ductility and energy absorption capacity can be enhanced by providing the hollow steel tube embedded in the beam as a heavy reinforcement. 3. A comparative study between conventional reinforced concrete members and hollow CEST composite members has been made. This shows that high ductility as well as high moment carrying capacity could be expected from hollow CEST composite members. The fracture of the hollow CEST members depends to a great extent on the fracture of the concrete encasement and thus special care must be given to the casting of the concrete. 4. The bending strength and the flexural stiffness of the hollow CEST section increases with increasing the height of the steel tube for the composite beams with the same area of the embedded steel tubes. 5. The hollow steel tube (for specimen B1 to B6) provided a larger elastic deformation capacity for hollow CEST composite beams compared to specimen CB1. Moreover, the maximum strength of hollow CEST composite beam specimens was about % higher than that of the control conventional Solid RC beam specimen CB2. 6. Behavior of hollow reinforced concrete encased steel tube composite beams in bending can be predicted using non-linear finite element analysis. The analysis is capable of predicting vertical in-plane deformation, crack pattern, mode of failure and ultimate load carrying capacity with a sufficient accuracy. Deflections and strains predicted by the FE models compare favorably with the experimental tests. Finite element analysis was shown to be a promising method to obtain data for the development of design aid for hollow CEST composite beams editor@iaeme.com

16 Efficiency of Hollow Reinforced Concrete Encased Steel Tube Composite Beams 7. Despite the high initial cost of structural steel tubes, the efficiency of the CEFT beams proven to be much higher than that of conventional RC beams. Such an improved performance is more apparent with increasing the stiffness of the steel tube itself. REFERENCES [1] STN EN : Eurocode 4: Design of Composite Steel and Concrete structures. Part 1 1: General Rules and Rules for Buildings. [2] STN EN : Eurocode 4: Design of Composite Steel and Concrete Structures. Part 2: General Rules and Rules for Bridges. [3] STN EN : Eurocode 2: Design of Concrete Structures. Part 1 1: General Rules and Rules for Buildings. [4] STN EN : Eurocode 3: Design of Steel Structures. Part 1 1: General Rules and Rules for Buildings. [5] V. Kvočáka, V. Kožlejováa and D. Dubeckýa: Analysis of encased steel beams with hollow cross-sections, Institute of Structural Engineering, Civil Engineering Faculty of Technical University in Košice, Vysokoškolská 4, Košice, Slovakia, Steel Structures and Bridges 2012, Procedia Engineering 40 (2012) , 2012 Published by Elsevier Ltd. Selection and review under responsibility of University of Žilina, FCE, Slovakia. [6] Ammar A. Ali, Saad N. Sadik and Wael S. Abdul-Sahib: Strength and Ductility of Concrete Encased Composite Beams, Eng. & Tech. Journal, Vol.30, No.15, [7] Ahmed Youssef Kamal; (April, 2015), Encased Beam with Variable Upper Steel Flange Position, International Journal of Application or Innovation in Engineering & Management (IJAIEM), Vol. 4, Issue 4, pp [8] P. Fouche, A.M.ASCE; M. Bruneau, F.ASCE; and V. Chiarito, D. Denavit, (December, 2017), Dual-Hazard Blast and Seismic Behavior of Concrete-Filled Double-Skin Steel Tubes Bridge Pier, Journal of Structural Engineering (ASCE), Vol. 143, No. 12, pp. ( )-( ). [9] A. Soundararajan and K. Shanmugasundaram: Flexural behavior of concrete filled steel hollow sections beams, Journal of Civil Engineering and Management, 2008, 14(2), [10] Hassan, T., (2012), Influence of Shear Reinforcement Detailing on the Behavior of Concrete Column Heads, IABSE Conference, Sharm El-Sheikh, Egypt, May [11] Cervenka, V., Constitutive Model for Cracked Reinforced Concrete, ACI Journal, V.82, No.6, 1985 pp [12] ATENA Theory Manual, Cervenka Consulting, Czech Republic, 2007, 231 p. [13] Bazant Z P. and Oh B H, Crack band theory for fracture of concrete. Materials and structures, 16: editor@iaeme.com