flexural strength estimation for composite uhpc …jestec.taylors.edu.my/vol 15 issue 3 june...

Journal of Engineering Science and Technology Vol. 15, No. 3 (2020) 1520 - 1541 © School of Engineering, Taylor’s University

1520

FLEXURAL STRENGTH ESTIMATION FOR COMPOSITE UHPC-TUBULAR STEEL BEAM

NASSER H. TU'MA1, MUSTAFA R. AZIZ2,*

1College of Engineering/University of Missan, Missan, Iraq 2Civil engineer/college of Engineering/University of Missan, Missan, Iraq

*Corresponding Author: [email protected]

Abstract

Development research in structural engineering aims to enhance the structural element throughout the service life and structural function of an entire building. Thus, a new structural element with a new concrete target, namely, ultra-high-performance concrete (UHPC), is presented as a composite hollow UHPC beam. This beam has a hollow steel box fully encased in concrete to benefit from its steel properties as a structural function and hollow core as a service utility and economical reduction of dead weight. The production of UHPC is locally performed using different experiments, materials, and curing systems until a target compressive strength of 143 MPa is achieved. This research aims to study the flexural behavior of this new type of element with different parameters, such as type of hollow core material, location, the shape of a steel box, longitudinal reinforcement ratio and a number of shear connectors. The location of the steel box used in this study varies from mid-high toward the tensile zone of the section. Experimental results showed that the new composite hollow beam demonstrated higher flexural capacity and stiffness than solid beam by 40% and 23.5% for both qualities.

Keywords: Composite beam, Hollow beam, Ultra-high-performance concrete.

Flexural Strength Estimation for Composite UHPC–Tubular Steel Beam 1521

Journal of Engineering Science and Technology June 2020, Vol. 15(3)

1. Introduction Composite elements in structural engineering often indicate two connected materials (i.e., steel and concrete) to obtain a single unit that simultaneously exhibits their characteristics. One of the traditional and commonly used composite elements in the latter period comprises a steel beam (I-section) connected to a concrete slab. However, this study presents the development of a new type of composite elements to obtain a new benefit in addition to structural support. In particular, this research uses a hollow steel section fully encased in the concrete section and hollow core to assist in running the service pipes, which is also considered a new economic advantage in decreasing the dead weight and materials of a beam. Shear connectors are often used to resist horizontal shear forces between the members to prevent separation between the concrete and the steel section within the composite elements. In this new type of composite element, the existence of concrete that surrounds the steel section provides a natural bond between the elements through the bonding force. This element reduces the cost of welding shear connectors or number of shear connectors required to obtain a full composite action.

Literature review The literary review of this research indicated that this type of composite encased beams that comprise a concrete surrounding a hollow steel section was not recently used. However, previous studies have used a composite encased beam that consists of concrete surrounding a steel I-section (see Fig. 1).

(a)

(b)

Fig. 1. (a) schematic drawing of the new type composite concrete beam, (b) Typical composite encased members.

Weng et al. [1] studied the splitting shear failure in encased beams. The aforementioned research concluded that the steel flange width should be below 0.67 of

1522 N. H. Tu'ma and M. R. Aziz


the concrete width to prevent splitting shear failure. Shear connectors should be used if the steel flange width is over 0.67.

Nakamura and Nartia [2] experimentally compared two types of the composite encased beam with steel I-section. The first type was a top and bottom flange welded with steel reinforcement, while the second one was an encased beam without any welding. The second was the steel I-section beam only. The comparison result showed that the welded encased beam was over 2.08-fold from the un-welded one, although the latter was still greater than the steel I-section only.

AISC [3] indicated that two types of composite sections were included in this specification. Chapter 1 includes the fully encased composite beams that depend on the natural bonding between the concrete and steel section to ensure the composite action. These beams also include other types of composite sections consisting of steel sections attached to the concrete slab using different types of shear connectors with headed studs and channels.

Kamal [4] studied the effects of the positions of the upper steel flange in fully encased beams on a composite beam’s capacity (see Fig. 2).

Fig. 2. Overall view of the model [4].

The variables were different ratios of steel section’s width divided by the concrete section’s width (v = bs / b) values of 0.33, 0.5, 0.67 and 0.86. Each group exhibits variable normalised height (g = hf / hs) values of 0, 0.25, 0.5 and 0.75. The aforementioned study concluded that the ultimate capacity of the fully encased composite beams is particularly high. The increase in the steel section width showed an increase in the ultimate capacity. Moreover, the presence of the upper steel flange in the composite section delayed the initiation of concrete crushing. Accordingly, lowering the position of the steel section towards the tension zone will delay the initiation of concrete cracks. Lastly, the full encasing of a steel I-section in the concrete increases the capacity that is higher than that of the inverted steel T-section.

Neelima and Shingade [5] investigated the flexural and shear behaviour of fully encased composite beams. The aforementioned study showed compared the uses of longitudinal steel rebars and encased rolled steel section as major reinforcements. The resulting load-deflection curve (see Fig. 3) indicated that the use of the encased rolled steel-section (type C) compared with the traditional reinforcing steel rebars (type N) as the main reinforcement was better. The reason is that the curves of the composite section illustrate a high increase in the ultimate strength and minimal deflection resulting from loads.

Neelima and Shingade [6] examined the removal of shear reinforcement from the composite beams to study its flexural and shear behavior with and without shear reinforcement. The research variables were B1 (beam with conventional steel reinforcement), B2 (beams with rolled steel angle sections as reinforcement), and B3 (beams with rolled steel channel sections as reinforcement). The experimental results showed that the crack width markedly increases if the shear reinforcement is unused



compared with beams with shear reinforcement. Composite beams without shear reinforcement failed as a result of concrete crushing in a diagonal tension.

Fig. 3. Load-deflection curves comparison [5].

The development in the concrete materials led to the creation of a new type of cementitious composites. This new type of concrete performs tasks similar to the ordinary one but exhibits superior properties. Thus, this type of concrete is called UHPC. Richard and (Cheyrezy, 1994) first produced this type of concrete (RPC) [7], which currently has an international code of UHPC, such as JSCE [8]. The first structure developed by UHPC in 1997 was the Sherbrooke footbridge in Sherbrooke, Quebec [9]. The favorable mechanical and durability characteristics of UHPC make it an excellent choice for developing highway infrastructure materials. The components of this mixture are summarized in the presence of a high proportion of cementitious materials (cement + silica fume), fine sand, and particularly low w/c ratio. Such a low ratio reduces the workability of the mixture, thereby ensuring that the modern plasticizers can be added to the low w/c ratio. This concrete mixture is characterized by high tensile strength because of the presence of steel fiber in the composition.

The UHPC is characterized by over 150 MPa and 5 MPa compression and pre- and post-cracking tensile strengths, respectively. Shen et al. [10] found that reinforcing UHPC using a steel fiber produces ultra-high-performance steel fiber reinforced concrete, thereby demonstrating many favorable characteristics (e.g., high compressive strength, good workability, and ductility) compared with the traditional UHPC.

Jungwirth and Muttoni [11] investigated the effect of 90° heat treatment on the mechanical properties of UHPC compared with 20° water treatment. The results showed that the use of 90° heat treatment increases the compressive and flexural strengths from (120 to 180) MPa and flexural strength from (25 to 45) MPa, respectively.

2. Materials and methods

2.1. Materials properties The material properties used in the UHPC production are provided as follows. An ordinary Portland cement type II was used. The fine sand used was imported from the DCP Company. Its maximum granule size is 600 μm, which conforms to the BS specification no. 882/1992 [12]. Micro-silica exhibited granules below 0.1 μm, which



conforms to ASTM C1240-04 [13]. The straight steel fiber used was 13 mm in length and 0.173 mm in diameter and imported from China from the Bekaert Corporation Manufacturer. The optimal mixing proportion of UHPC used in this test program was obtained after several trials mix to determine that mixture that contains the maximum ratio of the fine materials (cement + silica fume) and minimum w/c ratios that produced excellent characteristics. The proportion of the mixture used for the structural beams is shown in Fig. 4. The type of molds used for the samples were (200 × 100) mm cylinders for measuring compressive strength and splitting tensile strength (f't and f'c), cubes with 100 mm for compressive strength (fcu) and prisms with (100 × 100 × 500) mm dimensions to measure the modulus of rupture. The results of mechanical properties is shown Table 1

Table 1. Mechanical properties of three samples for each test.

Sa. No. Compressive strength (f'c)

Splitting Tensile Strength

Modulus of rupture

1 138.2 11.9 19.8 2 133.5 11.8 19.6 3 120.51 10.6 15

Av. 130.7 11.4 18

Fig. 4. Mix proportions, max. Compressive strength (fcu).

2.2. Steel materials

• Hollow steel box Several steel hollow boxes were used in different shapes and dimensions. Square hollow box with three different dimensions (60×60, 80×80, and 100×100) mm². A steel rectangular hollow section with dimensions of 50*100 mm². Steel circular hollow section with 60mm diameter. All sections have the same thickness (2.8 mm). All the steel sections were tested according to ASTM A370-10 [14]. Table 2 shows the mechanical properties of the steel boxes.

• Steel bars In this research, a four size of steel bars were used (ϕ6, ϕ10, ϕ12 and ϕ16). All of the steel reinforcement tested using machine SANS 1000 kN, according to ASTM C78-02 [15]. Table 3 shows the mechanical properties of steel bars.

Table 2. Mechanical properties of the steel sections.



Dimensions of steel hollow box

Average Yield Tensile Strength (MPa)

Average Ultimate Tensile Strength (MPa)

60*60 320 375 80*80 317 380

100*100 316.97 395.39 60 327 446

Table 3. Properties of steel reinforcement. Bar diameter

(mm) Bar area

(mm²) Yield strength fy

(MPa) Tensile strength fu

(MPa) 6 28.26 533 631

10 78.5 515 624 12 113.04 493 583 16 201.06 500 610

• Shear connectors To obtain composite interaction between hollow steel box and the concrete surrounding it. Shear connectors (head studs) were used to a prevented separation between them by resisting the opposite shear forces between them during the loading. Figure 5 shows the shear connectors and welding process of the shear connectors.

Fig. 5. Shear connectors and its welding process.

2.3. Methods and variables used The experimental investigation in this research aims to determine the flexural behavior for a new type of structural elements composed of hollow steel section encased in a UHPC section. This research was divided into five groups to study the effect of the five main variables. G1 studies the effect of the type of material used in fabricating the hollow core in the section. G2 examines the effect of a steel hollow box’s location within a beam’s cross-section. G3 investigates the effect of longitudinal reinforcement ratios for these new elements (composite hollow beams). G4 and G5 determine the effects of the shear connector arrangement in composite hollow beams and changing steel box shape, respectively. The tenth beams were cast with cross dimensions, while the overall depth, width, and length are 220, 150, and 1500 mm, respectively. The transverse reinforcements were maintained constant in all beams (ϕ10 @ 60 mm). The longitudinal reinforcement in the control beams was (2ϕ 12). The geometry of the solid beam (control) is shown in Fig. 6 (all dimensions. in (mm).



Fig. 6. Flexural test and geometry of tested beams.

Group No.(1): Comparison between the solid and hollow beam and composite hollow beam

Group No.(2): Changing the locations of steel box

Group No.(3): Changing longitudinal reinforcement

Group No.(4): Changing the arrangement of shear connectors

Group No.(5): Changing the shape of the steel box

Fig. 7. Geometry and details of tested beams in groups (all dimensions. in mm).



2.4. Molds A total of 10 plywood molds were prepared. The beams in the present work demonstrated hollow cores. Thus, an opening was made from both sides of the mold to encase a hollow steel box or compressed a cork inside the mold and install it 10 cm to the left of the steel box or cork outside the mold. This procedure will ensure that no movement occurs during casting (see Fig. 8).

Fig. 8. Prepare molds before casting.

2.5. Mixing and curing of specimens Mixing is the most challenging part in the UHPC mix casting because of its low w/c ratio. So, the method of mixing is an essential issue in this mix because the UHPC mix needs high-speed mixing. The past researchers usually using a horizontal mixer with a vertical shaft and rapid mixing. However, in the present work, two manual mixers with (120-500r/min) which give a rapid rate of mixing to obtain the UHPC. Heat treating is performed after the concrete has set, as shown in Fig. 9, by simply heating at ambient pressure for seven days. Heat-treating at 90°C substantially accelerates the pozzolanic reaction, while modifying the microstructure of the hydrates which have formed. However, these hydrates remain amorphous”. This curing process was adopted by past researchers such as Mahdi [16] and Jungwirth [11].

Fig. 9. Heat treatment for specimens.



3. Experimental Result Beams were transferred to the testing machine after the curing time was completed. The load was applied in the center of the beam specimens with two bearing plate under two-point load by increasing the pressure of hydraulic jack (see Fig. 10). The testing set up machine consists of two essential parts, two points loads and supports. Were two supports used with clear distance of 1400 mm and 2 point loads with a clear distance of 466 mm, the position of dial gages was at the lower center of the beam specimens. The dial gauges were INSIDE type with a maximum measuring of 30 mm and precision of 0.01 mm. The loads are applied in successive increments of 5 kN until reaching the failure load.

Fig. 10. Test setup of the experimental study.

3.1. Ultimate load, ultimate deflection, yield load, and yield deflection comparison

The results of the load-deflection values for the different stages and failure modes for all tested beams are shown in Fig. 11 and Table 4.

Fig. 11. yield, the ultimate results of the tested beams.



Figure 11 shows the values of the loads and deflections in the yield and ultimate points. The loads and deflection values did not substantially change when a hollow core was fabricated in a non-composite hollow beam (B2). However, these values increased when the hollow core was fabricated using a steel hollow section in a composite hollow beam (B3).

The deflections of a composite hollow beam (B3) at the yield and ultimate points were higher than those of the non-composite beams B1 and B2 because of high resistance to deflections. Thus, the arrival of the composite hollow beam at the yield and ultimate points is delayed because of the high yield and ultimate loads. Figure 11 also shows that the yield and ultimate loads increased when the steel box towards the tensile zone was lowered in (B4). Therefore, the yield and ultimate deflections also increased because the section’s arrival at the yield and ultimate phase are delayed.

Figure 11 also shows a significant increase when the percentage of longitudinal reinforcement ratio increased. Moreover, (B3), which contains a steel box and shear connectors on four plated boxes, demonstrated immense load capacity. Therefore, the deflections are also high at the yield and ultimate points. The composite hollow beam (B8) that possessed no shear connectors linking the box plates with surrounding concrete presented smaller values for the yield, ultimate load, and deflections than (B3) and (B7).

The composite hollow beams with numerous shear connectors reached the yield stage at a higher load than those with few shear connectors. Additionally, the composite hollow beam (B8) encased with a rectangular steel box with geometric properties A = 50 × 100 mm² and I = 416,6666 mm4 presented higher capacity than (B3) with steel box’s geometries A = 60 × 60 mm² and I = 108,0000 mm4 compared with (B9) with steel box’s geometries A = (𝜋𝜋 × 6024) mm² and I = 63,6172 mm4.

Table 4. Load &deflections comparisons of tested beams. G1 Beams

Variable: hollow core material

ultimate load

ultimate deflection

yield load

yield deflection

B1 Solid Concrete 110 9.34 80 2 B2 Cork 105 9.6 80 2.04 B3 Hollow steel box 230 15.81 180 6.77 G2 Beams

Variable: Changing the locations of the steel box

ultimate load

ultimate deflection

yield load

yield deflection

B3 Middle Hollow steel box 230 15.81 180 6.77 B4 Bellow Hollow steel box 240 12 160 5.45 G3 Beams

Variable: Changing longitudinal reinforcement

ultimate load

ultimate deflection

yield load

yield deflection

B3 2𝞍𝞍 12 230 15.81 180 6.77 B5 2𝞍𝞍 10 200 14.76 160 6.2 B6 2𝞍𝞍 16 290 17.65 260 9.7

G4 Beams

Variable: Changing the arrangement of shear

connectors

ultimate load

ultimate deflection

yield load

yield deflection

B3 Weld on Webs & flanges 230 15.81 180 6.77 B7 Weld on flanges only 220 15.71 160 6.2 B8 Without weld any studs 200 14.7 140 5.65



G5 Beams

Variable: Changing the shape of the steel box

ultimate load

ultimate deflection

yield load

yield deflection

B3 Square 60×60 mm² 230 15.81 180 6.77 B9 Rectangular 50×100 mm² 239 13 180 6.8 B10 Circular 𝞍𝞍 60 mm 200 15.38 140 6.4

3.2. Cracks and failure patterns with load-deflections The cracks and failure patterns of the tested beams shown in Figs. 12-21. Evidently, flexural cracks formed at the beam center. These cracks formed at the bottom of the beams progressed toward the top and widened during ultimate loads. Generally, all the beams exhibit a flexural failure mode.

Fig. 12. Load-deflection behavior and crack patterns of the solid beam (B1).

Fig. 13. Load-deflection behavior and crack patterns of the hollow beam (B2).



Fig. 14. Load-deflection behavior and crack patterns of the composite beam(B3).






Fig. 21. Load-deflection behavior and crack patterns of the composite beam (B10).

All beams of this category were designed to fail in flexure with tensile mode, which is characterized by the formation of cracks in the tensile stress zone, then yielding of steel bars and shifting the neutral axis toward the compression zone. The general experimental behavior of the composite hollow beams recorded during the test can be summarized as follows. When the applied load was increased, the first crack occurred in the tension zone, thereby indicating that the concrete loss is tensile strength. The applied loads increased until a sudden non-linear increase occurred in the dial gauge reading, thereby indicating that the steel section has been yielded. The applied loads further increased until the concrete has been crushed in its compression fiber of the section. The upper flange of a steel box has been buckled with a further increase of the loads. As expected, the main cracks for all test beams commenced in the middle zone.

3.3. Stiffness Stiffness is the required load for causing one unit deflection. The stiffness value can be calculated by dividing the ultimate load by the maximum deflection in the tested beam. Generally, the beam with high ultimate load and minimal deflection



will show a high stiffness value. The stiffness values at ultimate loads are presented in Fig. 22 and Table 5.

In G1, the composite hollow beam (B3) demonstrated higher stiffness than the non-composite one (B2) by 33% and higher than the solid beam (B1) by 23.5%. This finding indicates that when the hollow steel section is encased in concrete, the section stiffness will increase with the increase of the ultimate load of the beam.

In G2, when the hollow steel section is lowered towards the tension zone in the composite hollow beam (B4), the stiffness increase is higher by 37.4% than (B3). This finding indicates that the stiffness of the hollow composite beams increases depending on the location of the hollow steel section in the tensile zone of the section.

In G3, the composite hollow beam (B6) with high longitudinal reinforcement demonstrates stiffness higher than (B3) and (B5) by 12.9% and 21.2%, respectively. This result indicates that the stiffness of the hollow composite beams increases as the longitudinal reinforcement ratio increases.

In G4, the composite hollow beam (B3) with shear connectors linking the encased steel box with concrete by welded studs on four plates (flanges and webs) of the steel box presented stiffness higher by 3.9% than (B10), thereby demonstrating that studs welded only two plates (only flanges) of the steel box and is higher by 6.9% than (B11) which is without shear connectors connecting steel box with concrete. This finding indicates that the stiffness of the composite hollow beams increases as the number of shear connectors increases.

In G5, the composite hollow beam (B9) with encased steel section with geometric properties of [A box = 50 × 100 mm², A box = 2.88% × Ac, box shape = rectangular, I box = 416.6 cm4 and I box = 3.23% × Ic] showed stiffness higher by 26.3% than (B3) with geometric properties of [A box= 60×60 mm², A box = 2.17% × Ac, box shape = square, I box = 108 cm4 and I box = 0.81% × Ic]; and higher by 41.38% than B11 with geometric properties of [A box= (𝜋𝜋 ∗ 6024) mm², A box = 1.66% × Ac, box shape = circular, I box = 63.61 cm4 and I box = 0.48% × Ic]. Consequently, the composite hollow beams exhibiting steel box moment of inertia ratio of 3.23% presented stiffness higher than those that have steel box inertia ratios of 0.81% and 0.48%. This finding indicates that the stiffness of the composite hollow beams increases as the steel box moment of inertia ratio increases.

Fig. 22. Stiffness values in five groups.

Table 5. Stiffness comparisons of tested beams.



G1 Beams

Variable: hollow core material stiffness Increasing or decreasing

percentage % B1 Solid Concrete 11.78 (control) B2 Cork 10.94 -7.6 B3 Hollow steel box 14.55 2.2

G2

Beams Variable: Changing the locations of the steel box stiffness Increasing or decreasing

percentage % B3 Middle Hollow steel box 14.55 (control) B4 Bellow Hollow steel box 20 37.4

G3

Beams Variable: Changing

longitudinal reinforcement stiffness Increasing or decreasing percentage %

B3 2 𝞍𝞍 12 14.55 (control) B5 2 𝞍𝞍 10 13.55 -7.3 B6 2 𝞍𝞍 16 16.43 21.2

G4

Beams Variable: Changing shear connectors arrangement stiffness Increasing or decreasing

percentage % B3 Weld on Webs & flanges 14.55 (control) B7 Weld on flanges only 14 -3.9 B8 Without weld any studs 13.61 -6

G5

Beams Variable: Changing the shape

of the steel box stiffness Increasing or decreasing percentage %

B3 Square 60×60 mm² 14.55 (control) B9 Rectangular 50×100 mm² 18.38 26 B10 Circular 𝞍𝞍 60 mm 14 -3

3.4. Load vs. deflection comparison The load-deflection behavior for all tested beams is shown in Fig. 23.

The specimens in G1 demonstrated that the non-composite hollow beam (B2) shows higher deflections compared with the solid beam (B1) because of the decrease in moment of inertia capacity of the section, primarily owing to the existence of hollow core in the section. However, the behavior of the solid beam (B1) was similar to the hollow beam (B2), but the curve gradually deviated with increasing loads. This finding indicated a slight decrease in (B2) strength because of the presence of a hollow core. When the hollow steel box was encased in the section in the composite hollow beam (B3), the deflections decreased that are below those of (B1) and (B2) in the specific loads. The large increase in the maximum deflection in (B3) is due to the high difference in the ultimate loads between the beams.

In G2, (B4) containing a steel box in the tension zone exhibited higher load capacity and less maximum deflection than (B3). Thus, lowering the steel box towards the tension zone will result in high ultimate load and maximum deflections, but the deflections along the curve were decreased at specific loads compared with (B3) containing a steel box in the middle zone.

In G3, the composite hollow beams show high capacity and maximum deflection when the longitudinal reinforcement increased, but the deflection values along the curve will be decreased.

In G4, the composite hollow beam (B3) exhibiting a higher number of shear connectors (studs) shows higher ultimate load and maximum deflection compared with (B7) and (B8), but the deflection values along the curve decreased.



In G5, the composite hollow beam (B9) demonstrating a rectangular steel box shows higher ultimate load and deflection resistance compared with those with square and circular steel boxes (B3 and 10, respectively). The effect of the increasing moment of inertia of the section and its influence against bending stresses are evident because the rectangular section demonstrated a moment of inertia higher than the other sections when all other factors were considered the same.

Fig. 23. Comparison of load-deflection behaviour for the tested beams in five groups.



3.5. Flexural load capacity The flexural load capacity for all the tested beams is shown in Fig. 11.

The variation of the hollow core material in G1 indicates that the non-composite hollow beam (B2) decreased by 4.76% less than that of the solid beam (B1). The load capacity of the composite hollow beam (B3) increases by 109% higher than that of the solid beam (B1), thereby indicating that the encased steel hollow box increased the load capacity of the beam specimen.

In G2, the variation of the steel hollow sections location shows that the flexural capacity of the composite hollow beam (B4) with a steel box below the section demonstrated a load capacity of 4.3% higher than that of (B3) containing a steel box in the middle of the section. This finding indicates that lowering the steel hollow section increases the load capacity in the composite hollow beams.

In G3, the variation of the longitudinal reinforcement ration shows that (B6), which presented a high longitudinal reinforcement, demonstrated a flexural capacity higher by 26% and 45% than composite beams (B3) and (B5). This result indicates that the flexural capacity of the composite hollow beams increases as the longitudinal reinforcement ratio increased.

In G4, the composite hollow beam (B8) showed a higher flexural load capacity than (B3) and (B7), thereby showing the effect of shear connectors arrangement on flexural capacity.

In G5, (B9) shows flexural capacity higher than (B3) and (B10), thereby indicating the steel box shape effect on flexural capacity.

3.6. Ductility characteristics Ductility is one of the important features that should be considered when designing structures that are exposed to substantial inelastic deformations resulting from different loading conditions. This feature can be defined as the structural member’s ability to undergo inelastic deformations beyond yield deformation without substantial loss in its load carrying capacity. The ductility in the flexural member can be obtained through its load-deflection curve. Ductility is the ratio of the deflection value when the member completely fails to that at the yield point. The output is known as ductility factor (μ = ∆u / ∆y). The ductility factor is calculated by dividing the maximum deflection (∆u) by the yield deflection (∆y). Figure 24 shows the ductility factors for all the tested beams.

In G1, Fig. 12 also shows that ductility is increased by 0.85% when the hollow core was fabricated in the non-composite hollow beam (B2). This condition is due to the brittle property of the concrete, thereby presenting a brittle behavior. Thus, the presence of a hollow core in the concrete in (B2) led to a minimal decrease in the proportion of brittleness of the section. In addition, the decrease in B2 stiffness led to an increase in ductility. Meanwhile, the hollow beam (B2) demonstrated a moment of inertia I =13202 cm4 that is less than that of the solid beam (B1) I = 13310 cm4. Thus, the presence of the hollow core leads to a decrease of the moment of inertia of the section by 0.81%, thereby resulting in the early arrival of the specimen at the yield stage. Ductility decreased when the hollow core was fabricated using a steel box in (B3). This decrease is due to the increase in the ultimate load of (B3). The addition of the steel box led to an increase in the amount



of steel reinforced to resist deflections, thereby making the section reach the late stage of loading to yield. Thus, the deflection value at yield (∆y) will be considerably high. The ductility factor for (B3) was less than that of the solid beam (B1) because the value of ∆y is inversely proportional to the ductility factor (μ = ∆u / ∆y). This finding is attributed to the minimal difference between yield and maximum deflection in the composite beam compared with that in the solid one. Specifically, the ductility in the non-composite hollow beams is higher than those of the solid and composite ones.

In G2, the ductility in the composite hollow beam with a steel box below the section (B4) decreased by 5.9% compared with the composite hollow beam containing steel box in the middle of the section (B3). This condition is due to the greater stiffness of (B4) compared with (B3), and the presence of the hollow steel box below the section leads to the weakening of the concrete strength against tensile stresses, thereby leading to the emergence of cracks in the early stages of loading and resulting in a decrease in the section ductility. Therefore, the best location for the steel box when considering section ductility is in the middle of the concrete section.

In G3, (B6) contained high longitudinal reinforcement and exhibited less ductility factor compared with (B3) and (B5). This condition is attributed to the great ultimate load of the section that contains substantial longitudinal reinforcement. Hence, the section reaches yield failure late, thereby resulting in higher elastic than a plastic phase, while increased yield deflection value (∆y) leads to decreased ductility factor (μ = ∆u/ ∆y).

In G4, (B3) demonstrated decreased section ductility by 8% and 11% compared with (B7) and (B8), respectively. This finding indicates that the increase in the number of shear connectors led to increased section stiffness and decreased ductility.

In G5, (B9) presented a rectangular steel box and showed minimal ductility factors by 25.6% and 22.5% compared with (B10) and (B3), respectively. This condition can be explained by the load-deflection curve in Fig. 24, where in B9 evidently shows less ultimate deflection compared with (B10) and (B3).

Fig. 24. Ductility factor for the tested beams in five groups.

Generally, using this type of steel hollow sections in strengthening or encasing beams will lead to diminished ductility and increased stiffness and rigidity. The decrease in ductility for these steel sections is due to the values of yield tensile stress (fy), which are usually less than the usual steel reinforcement rebar. However,



the ultimate tensile stress (fu) is higher. Thus, the conduct behavior of these steel sections is similar to a linear relationship.

3.7. Energy absorption capacity Energy absorption capacity can be obtained by calculating the area under the load-deflection curve. Figure 25 shows the energy absorption for all tested beams.

The energy absorption in G1 was increased in the non-composite hollow beam (B2) by 0.69% higher than the solid beam (B1). However, the energy absorption increased in the composite hollow beam (B3) by 213% than the solid beam (B1). Accordingly, encasing the hollow steel section in the concrete section enhances the energy absorption capacity better than using cork for fabricating the hollow core process.

As shown in G2, the energy absorption capacity is decreased by 30% when the hollow steel box is lowered in the tensile zone in (B4). This condition is due to the presence of the hollow core in the tensile area, which weakens the susceptibility of the concrete to resist the increasing tensile stresses below the section.

Figure 25 also shows that the increase in the longitudinal reinforcement in this new type of beams will enhance the energy absorption in G3.

In G4, using the sufficient number of shear connectors (studs) in (B3) will result in high energy absorption in these sections. Figure 25 illustrates that the composite hollow beam (B3) exhibited toughness higher by 22.4% and 26.5% compared with (B10) and (B9), respectively.

In G5, the composite hollow beam (B3) embedded with a square steel box presented toughness higher than those of (B9) and (B10) by 26.5% and 22.4%, respectively.

Fig. 25. Energy absorption capacity in five group.

4. Further Research The analysis presented in this study aims to develop a new structural element (i.e., composite hollow beams) with a structural and service benefit by studying its flexural behavior. However, the shear capacity behavior of this structural element was neglected. Therefore, many tests are required in this field to study the behavior of several parameters for these sections. This study performed a related test using two point-loadings. Nevertheless, testing the behavior using different load conditions, such as distributed and cyclic loads, is suggested in future studies.



5. Conclusions The current experimental research investigates the flexural behavior of a new type of composite beams, which is composed of a hollow steel section that is fully encased in the concrete with several variables, such as steel section location, longitudinal reinforcement ratio, steel box shape and arrangement of the shear connectors, which are regarded as highly important. The main conclusions obtained in this study are as follows:

• The fabricating of hollow core in the middle of the section by cork did not significantly affect the capacity of the section where it decreased by only 1% when a hollow core was made by (60×60) dimensions of cork material.

• The fully encased steel hollow section in the concrete beam is an effective technique for enhancing its flexural capacity. The load capacity of the composite hollow beam (B3) increases by 109% compared with the solid beam (B1) and 119% than the non-composite hollow beam (B2).

• Lowering the steel box in the composite hollow beams leads to an enhancement in the ultimate load. The load capacity of B4 that contains a steel box in the tension zone increases by 4.3% compared with B3 that consists of a steel box in the middle zone of the section.

• Generally, fabricating the hollow core in the section in composite or non-composite hollow beams leads to a highly significant effect in the development of the structural characteristics of the structural member for the continuity of services and economic benefit, specifically when the UHPC mixture was used, thereby exhibiting high cost.

• The stiffness values of the composite hollow beams are higher than those of the solid and non-composite hollow beams. Such values increase as the longitudinal steel reinforcement increases. Stiffness increases with increasing steel box ratio or moment of inertia of the steel box.

• The energy absorption capacity improved when the hollow core was fabricated in the beam. The non-composite hollow beams demonstrated higher energy absorption compared with the solid ones by increasing the energy absorption when the steel box was encased in the concrete section.

Nomenclatures

b Width of concrete beam, mm bs Width of steel section, mm fc Concrete compressive strength, MPa fy Yield strength of longitudinal reinforcement, MPa hf Depth of concrete flange, mm hs Depth of steel section, mm

Greek Symbols φ Diameter of reinforcement, mm

Abbreviations

AISC American Institute of Steel Construction B Beam JSCE Japan Society of Civil Engineers RPC Reactive powder concrete



UHFRC Ultra-high fiber reinforced concrete UHPC Ultra-high performance concrete

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