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International Journal of Civil Engineering and Technology (IJCIET)

Volume 9, Issue 3, March 2018, pp. 720–735, Article ID: IJCIET_09_03_073

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=3

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

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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

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. 720–735.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=3

1. 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.

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



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.

2.1. 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.

Figure 3 Profile view of test set-up

Figure 4 Test set-up / typical dimensions legends of all specimens

Typical test

set-up Loading Jack

Specimen

Hinged

support

Roller

support

PI-gauges

LVDTs



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.

Table 1 Test Matrix

Specimen

H h t Ls Area of

steel tube

mm mm mm % of

Ln mm

2

CB1 600 ----- ---- ----- -----

CB2 480 ----- ---- ----- -----

B1 480 100 2.5 55 800

B2 480 73 3.0 55 798

B3 480 185 2.5 55 1225

B4 480 107 3.0 55 1002

B5 480 140 2.5 55 1000

B6 480 100 2.5 70 800

B7 480 ----- ----- ----- Add.

803.8

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



Figure 6 B7 Reinforcing details showing top & bottom reinforcement and distribution of stirrups

across the span of the specimen / additional bottom reinforcement.

2.2. 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



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.



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.

Table 2 Mechanical Properties of The Used Steel Bars / Steel Tubes

Bar Diameter

Yield

Strength Yield strain

Elastic

Modulus

Ultimate

Strength

Ultimate

strain

MPa με MPa MPa mm/mm

No. 8 316 1580

200000≈

438 0.2375

No. 10 570 2850 670 0.1633

No. 12 501 2505 690 0.1803

No. 18 482 2410 693 0.1626

2.5 mm plates 313 1565 392 0.385

3.0 mm plates 301 1505 365 0.363

*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 load-

deflection 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



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.

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 0.003 to 0.0035 for all specimens. Typical classical failure due to

concrete crushing of the simply supported specimens is shown in Fig. 10.

3.3. 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. 10.

3.4. 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.

Applied Load



3.5. Effect of Length of Steel Tube

The ultimate load carrying capacity for B1 (with length of 0.55 Ln) and B6 (with length of 0.7

Ln) 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 Ln 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.

Figure 10 Failure modes & typical flexural cracking patterns of different specimens

CB2 CB1 B1

B2 B3 B4

B5 B6 B7



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.

4.3. Modeling description of reinforced concrete encased hollow steel tube

composite beams

The analysis in the current study was conducted using ATENA version 4.3.1., “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



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.

Figure 12 Mid-span vertical deflection of all specimens from FEA models

Applied Load



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.

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

0.003.

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.

Applied Load Applied Load

Applied Load Applied Load



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.

Table 3 Failure Loads

Specimen

Failure Load

(Experimental) R1 R2 Failure Load (FEA)

Pu

(Predicted) FEA/EXP.

kN ----- ----- kN kN -----

CB1 148 1.00 1.26 140 132 0.946

CB2 117 0.79 1.00 108 102 0.923

B1 170 1.15 1.45 167 151 0.982

B2 164 1.11 1.40 160 152 0.976

B3 197 1.33 1.68 186 164 0.944

B4 174 1.18 1.49 172 160 0.989

B5 184 1.24 1.57 179 159 0.973

B6 167 1.13 1.42 166 151 0.994

B7 163 1.10 1.39 158 162 0.969

*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 $113.64 for one meter cube of concrete, $1136.36 for one ton of steel bars and

$2272.73 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



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 40-70

% 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.



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 1994–1–1: Eurocode 4: Design of Composite Steel and Concrete structures. Part

1–1: General Rules and Rules for Buildings.

[2] STN EN 1994–2: Eurocode 4: Design of Composite Steel and Concrete Structures. Part 2:

General Rules and Rules for Bridges.

[3] STN EN 1992–1–1: Eurocode 2: Design of Concrete Structures. Part 1–1: General Rules

and Rules for Buildings.

[4] STN EN 1993–1–1: Eurocode 3: Design of Steel Structures. Part 1–1: General Rules and

Rules for Buildings.

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