Direct and Flexural Shear Strength of Composite Beam with Perforbond Rib

Abstract

When Perfobond Rib shear connectors are used as flexural materials in structures such as bridges, they show flexural shear behavior due to external force, rather than direct shear behavior. The aim of this study is thus to analyze the difference between both behaviors. First, we prepared a specimen to analyze direct shear behavior using Perfobond Rib shear connectors, analyzed the characteristics of behavior with a push-out test and proposed a formula of shear resistance assessment. Proposed formula shows a relatively good fit with less than 10% error. A flexural shear test was then conducted based on the result of the direct shear test. Based on the static flexural test it analyzed the flexural behavior and the flexural shear stress it calculated. Direct shear stress and EN 1994-1-1 to lead and be calculated, it compared the flexural shear stress and it analyzed in about the shear resistance stress which it follows in load direction. Finally, we compared both test results, and the comparison showed that the flexural shear stress is approximately 6% stronger than the direct shear stress.

Keywords

Shear connector Perfobond rib Head stud Direct shear Flexural shear Composite member Shear resistance

1. Introduction

The most important factor to be considered in designing and constructing a composite structure is the combined movement behavior of steel and concrete. Since the deformation characteristics of steel and concrete differ, shear force is generated between the two materials by external force and environmental changes. Shear force is likely to cause relative displacement between the two materials, followed by degradation of stiffness and strength of the structure. Therefore, a shear connector is needed to control the relative displacement and resist the shear force generated in the boundary interface between steel and concrete.

A variety of new shear connectors is currently being developed to overcome the drawbacks of head stud shear connectors. The most active study is that of Perfobond Rib shear connectors. Perfobond Rib shear connectors show excellent shear resistance performance and cause very little relative deformation between steel and concrete before a yield point due to their high stiffness.

For the safe and efficient use of Perforbond Rib shear connectors, it is essential to investigate mechanical behaviors and evaluate the performance of shear resistance. In existing studies, a variety of specimens have been used and the behavior characteristics and shear resistance performance of Perforbond Rib shear connectors have been investigated through Push-Out Tests^2–4. However, when Perforbond Rib shear connectors are used as flexural materials in structures such as bridges, they show flexural shear rather than direct shear behavior, due to external force⁵. A study on flexural behavior is thus needed for this case as the flexural shear behavior significantly differs to that of destruction, compared to direct shear behavior.

Therefore, in this study, a direct shear test and a flexural shear test were performed to more specifically determine the direct and flexural shear behavior of a structure in which Perfobond Rib shear connectors are used. First, we prepared a specimen to analyze the direct shear behavior using Perfobond Rib shear connectors, analyzed the characteristics of behavior using a push-out test and proposed a formula for shear resistance assessment. In addition, we prepared a specimen to analyze flexural shear behavior based on the result of the analyses and investigated behavior of the specimen by conducting a static flexural test for the structure through four points loading. Based on the result of the direct and flexural shear behavior tests, the difference between direct shear stress and flexural shear stress of the specimen using Perfobond Rib shear connectors was identified.

2. Basic Theory

2.1 Calculation of the Degree of Shear Connection(η) of a Composite Structure

The degree of shear connection in a composite structure, which is the level of composition between two materials in a steel-concrete composite structure, can be calculated from equation (1) as follows.

η = \frac{P_{s h e a r}}{{(P_{s h e a r})}_{f s t}} = \frac{P_{s h e a r}}{P_{s t e e l}} = \frac{M_{p s c} - M_{s t e e l}}{M_{f s c} - M_{s t e e l}}

(1)

Where P_shear is the mechanical resistance of a shear resistance device (the horizontal shear strength of a shear connector), P_steel is A_steel × f_y, M_fcs is the theory moment under full junction condition (as for M_n of ASCE), M_psc is the partial shear bonding moment and M_steel is the bending moment of a deck plate.

Under the non-composite condition, which indicates that there is no shear connector, η = 0 and P_shear = 0. Therefore, the strength of a composite structure is M_steel, which is the bending strength of a steel part. In addition, the full composition implies the perfectly harmonized behavior of steel and concrete. In this case, η >1 and P_shear > (P_shear)_fsc. However, since in most composite structures, the longitudinal shear transfer device is weaker than the composite beam, they show 0 <η <1. As a result, the flexural force strength, M_psc, of a composite structure can be explained with the degree of P_shear and the degree of shear connection (η).

2.2 Calculation of Flexural Shear Using the Degree of Shear Connection (η)

EN 1994-1-1 (Eurocode 4. ENV:1994)⁶ presents the flexural shear stress of a steel-concrete composite structure using the degree of shear connection (η), as shown in the following equation (2).

τ_{u} = \frac{η N_{c f}}{b (L_{s} + L_{0})}

(2)

Where τ_u is the longitudinal shear stress, which applies to the load in the right angle, η is the degree of shear connection, N_cf is the strength of concrete in the compressed side in the case of complete composition, b is the length of cross section and L_s + L₀ is the distance from the loading position to the end of the specimen.

3. Direct Shear Behavior Test

As shown in section 3.1, specimens have been prepared with 4 variables to calculate the direct shear strength of Perfobond Rib shear connectors. The amount of relative slip, which varies according to changes of load, is measured using the push out test proposed in EN 1994-1-1. With the results measured, the behavior characteristics of Perfobond Rib shear connectors have been analyzed according to four variables.

3.1 Fabrication and Configuration of Direct Shear Behavior of Specimens

With four experimental variables, five specimens were prepared to verify the direct shear strength of a specimen adopting the Perfobond Rib shear connectors. The four experimental variables that determined by reference to previous research include: (i) the existence of holes inside the Perfobond Rib shear connector, (ii) the number of holes inside the Perfobond Rib shear connector, (iii) the existence of reinforced bar in the holes and (iv) the horizontal interval between holes^1–3. Table 1 shows the material properties.

Table 1
Material properties of direct shear behavior test

Concrete Design strength Max. size of coarse aggregate Slump

30 MPa 25 mm 120 mm

Bending steel and shear connector Thickness Yield strength Grade

6 mm 245 MPa SS400

Rebar Diameter Yield strength Grade

16 mm 400 MPa SS400

To test the strength of the concrete specimens, a compression strength test was performed with three of the specimens as proposed in the Korea Structural Concrete Code. The strengths of the three specimens were measured as 32.95 MPa, 26.34 MPa and 33.73 MPa and the average strength was 31.01 MPa. Table 2 shows a configuration of specimens fabricated according to the specifications and test results mentioned above with the ratio of the diameter of a hole against the horizontal interval between holes. Figure 1 shows the front view of a specimen for the direct shear strength test using Perfobond Rib shear connectors while Figure 2 shows the cross-section of the A-A part of the specimen shown in Figure 2 .

Figure 1

Front view of the specmens

Figure 2

Cross-section A-A in Figure 1

Table 2

Names of specimens and parameters

Name of specimen	Parameter
Name of specimen	Number of holes (EA)	Distance of hole (mm)	Number of reinforceing bars
PR-1	None	–	–
PR-2	4	100(2.0D)	–
PR-3	5	75(1.5D)	–
PR-4	3	150(3.0D)	–
PR-5	4	100(2.0D)	2EA

3.2 Configuration Method of Direct Shear Behavior Experimental Device

To identify the difference between the shapes of the shear connectors and performances depending on experimental variables, all shear experimental specimens are tested using the direct shear test (Push-out Test) in accordance with the proposal of EN 1994-1-1. In addition, in order to find a relative displacement in the vertical direction of concrete and steel according to an increase in weight, LVDT is installed and measured. Furthermore, a 20 mm thick neoprene pad is installed under the specimen to minimize destruction of concrete due to the differential stress that occurs in concrete at both sides. Also, oil is applied between the concrete and steel to minimize the resistance caused by friction between the concrete and steel. Static load is applied in the vertical direction.

3.3 Results and Analysis of Direct Shear Behavior Test

The direct shear test on Perfobond Rib shear experimental specimens was performed based on the variables outlined in the previous section. Figure 3 shows the shape of the destructed direct shear experimental specimen and Table 3 summarizes the test results.

Figure 3

Breaking of specimen in push-out test

Table 3

Result of push-out test

Name of specimen	Cracked load (kN)	Comparison (%)	Maximum load (kN)	Comparison (%)	Relative slip (mm)	Note
PR-1	190	100	213	100	1.22	No Holes, Control
PR-2	345	182				4 Holes
PR-3	320	168	396	186	2.68	5 Holes
PR-4	340	179	382	179	2.00	3 Holes
PR-5	355	187	457	215	3.80	4 Holes with 2 Rebars

According to the analysis of the test results, similar test results were found in all Perfobond Rib shear connectors, except for the PR-1 specimen. All specimens, excluding the PR-1 specimen, show initial cracks on both sides of the concrete between 320 kN and 370 kN. In addition, the specimens were damaged by vertical cracks generated inside the bottom of the concrete because the relative slip amount increases after the maximum load was reached. After the maximum load was reached, the function of the shear connectors had almost dissipated. As a result, relative slip amount significantly increased and the concrete and steel separated. The specimens then became rapidly damaged. However, the PR-5 specimen with lateral reinforced bar inserted shows 5%–19% higher performance on load resistance and has 29.4%–99.8% higher relative slip, compared to other specimens. Figure 4 shows the relationship between the load of shear specimens using Perfobond Rib shear connectors and relative slip.

Figure 4

Load- slip curve between concrete and steel of all specimens

3.4 Analysis of Main Factors According to the Results of Push-Out Test

From the analysis of the test variables, which are the main factors influencing the Perfobond Rib connectors, the structural mechanism of the Perfobond Rib connectors is identified and the formula to assess the direct shear strength of a specimen using Perfobond Rib connectors is proposed.

First, two separate tests for the specimen without holes (PR-1) and those with holes (PR-2, PR-3 and PR-4) were performed to identify the effect of Perfobond Rib shear connectors on the horizontal direction. The number of holes varies depending on the distance between holes. The effect on concrete dowel function was analyzed using the modified variables. The PR-1 specimen resists shear only with the end bearing capacity of the rib because there is no hole inside Perfobond Rib shear connectors. At this time, it is assumed that the end bearing capacity of the PR-1 specimen is 213 kN, which is the maximum resistance of the PR-1 specimen. As shown in Table 3 , the shear resistance performance of the Perfobond Rib connectors with holes significantly differs to that without holes. The Perfobond Rib connectors with holes show 180% higher shear resistance performance than the Perfobond Rib connectors without holes. In addition, it is found that for all specimens shown in Figure 4 , either similar behavior is shown in the relationship between load and relative slip, or there is little relative slip.

In this study, the number of holes differs from 3 to 5, with the distance between holes differing from 1.5D to 3.0D. As shown in PR-2, PR-3 and PR-4., the shear performance of the specimens is not significantly changed depending on the number of holes. However, with regard to the resistance of each hole in the specimen, it is revealed that the resistance of each hole decreases when the number of holes increases from 66.0 kN (3 holes) to 50.5 kN (4 holes) and 36.8 kN (5 holes). According to the studies by Oguejiofor and Hosain (1994) and Medberry and Shahrooz (2002), the dowel effect of concrete decreases when the effects of stress overlap^2–3. Even though the number of holes increases, the effect decreases if the space is not sufficient for the dowel effect of concrete. In addition, according to the paper by Lee (2005), which focuses on the distance between holes, the maximum dowel effect is achieved when the distance between holes is 2.25D¹.

3.5 Proposal of Formula to Assess the Shear Strength

The main factors that have significant effects on direct shear resistance include: (1) the end bearing capacity, (2) dowel effect of concrete and (3) resistance of lateral reinforced bar. With these variables, the formula to assess the shear strength of a structure using Perfobond Rib connectors is proposed. First, with regard to the end bearing capacity, which is the force required to resist at the end of the Perfobond Rib connectors, Oguejiofor and Hosain (1997) suggest that, in order to reflect the bearing strength of concrete, the end bearing capacity of the Perfobond Rib connectors is the compressive force of concrete multiplied by the height (h) and thickness (t) of a rib⁷. Therefore, the bearing capacity of the tip of the connector actually depends on the shape and dimension of a rib.

The principle of the horizontal resistance performance due to dowel effect is similar to that of bolts in double shear. The shear strength of bolts is calculated from the nominal shear strength multiplied by the cross sectional area and can be applied to the evaluation of shear capacity generated by the dowel effect of concrete⁷. Therefore, the shear capacity generated by the dowel effect of concrete can be expressed by multiplying the average shear stress, the cross-sectional area of a hole and the number of holes. However, as noticed in the test results above, it is known that the dowel effect of concrete decreases the shear resistance performance due to the overlapping influence when the distance between holes is reduced even, if the number of holes increases. Although a variety of research and verification is required to evaluate the results quantitatively, it is impossible to perform such a wide range of research and verification in this study. Therefore, this study focuses on ensuring a practical relationship between the number of holes and the distance between holes using numeric and regression analysis. Finally, the effect of shear reinforced bar penetrating a hole of the Perfobond Rib connector can be expressed as a multiplication of the cross sectional area of lateral reinforced bar (A_s), the number of reinforced bars (n) and the yield strength of reinforced bars (f_y), based on the shear friction theory.

With the analysis of the main factors of shear resistance mentioned above, the following equation (3) is prepared to show the factors that influence the shear resistance performance of a structure using Perfobond connectors.

V (k N) = β_{1} h t f_{c k} + β_{2} n_{1} d^{2} \sqrt{f_{c k}} + β_{3} n_{2} A_{s} f_{y}

(3)

where V is the shear resistance performance of the Perfobond Rib connector (kN), β₁, β₂ and β₃ are variables for the regression analysis, h is the height (mm) of the rib, f_ck is the compressive strength (MPa), n₁ is the number of holes inside the Perfobond Rib connector, d is the diameter of the holes inside the Perfobond Rib connector, n₂ is the number of traverse reinforced bars, A_s is the cross-sectional area of the reinforced bars and f_y is the yield strength of the traverse reinforced bars. Equation (4), the strength evaluation formula, used to predict the direct shear strength of Perfobond Rib connectors, is prepared using variable analysis and regression analysis based on equation (3).

V (k N) = 8.49 h t f_{c k} + α n_{1} d^{2} \sqrt{f_{c k}} + 0.38 n_{2} A_{s} f_{y}

(4)

However, the value of α can be changed by β(β = the distance between dowel holes and the diameter of the holes. If β ≤ 1.5, then α ≤ 2.25. If 1.5 ≤ β ≤ 3.0, then α = 0.7732β + 1.2009. If β + 3.0, then α = 3.5.

The difference between the formulas suggested in this study and those used in existing studies is that in this study, a more accurate analysis is sought of the concrete dowel effect with holes and the β value as variables. As shown in existing studies, the concrete dowel effect can be overlapped by the diameter of holes and the distance between holes. As a result, it is necessary to consider such elements.

According to Lee's (2005) study, the dowel effect of concrete is maximized when the distance between holes is 2.25D¹. In addition, his study showed that the dowel effect of each hole was reduced by 12% when the distance is shorter than 2.2D and 6% when the distance is longer than 2.5D. More accurate equations are attempted in this study, considering factors that influence the horizontal components of the concrete dowel, unlike previous lumped variable analysis.

In addition, Oguejiofor and Hosain (1994) analyzed the effect of traverse reinforced bar on direct shear strength using the push-out test, changing the number of traverse reinforced bars and proved that the direct shear strength is enhanced in proportion to the number of traverse reinforced bars. Their result is reflected in equation (5)².

Q = 4.50 h t f_{c}^{'} + 3.31 n d^{2} \sqrt{f_{c}^{'}} + 0.91 A_{t r} f_{y}

(5)

where Q is the shear resistance performance of the Perfobond Rib connector (kN), h is the height (mm) of the rib, t is the thickness of the rib, f'_c is the compressive strength (MPa), n is the number of holes inside the Perfobond Rib connector, d is the diameter of a hole inside the Perfobond Rib connector, A_tr is the cross-sectional area of the reinforced bars and f_y is the yield strength of the traverse reinforced bars.

Table 4 shows a comparative analysis of the cases calculated using equations (4) and (5). ⓐ is the resistance capacity of the front of the rib, ⓑ is the resistance capacity due to the dowel of concrete and ⓒ is the resistance capacity changed due to the presence of reinforced bars. Reviewing Table 4 , Equation (4) is relatively accurate with the error rate of 0.1%–5.2%. It is revealed that the second term due to the dowel effect is the main error factor. For the second clause, the error is considered to have occurred due to the regression analysis because it determines the β₂ value. The measurement, of the effect of the tip bearing and the traverse reinforced bars are relatively accurate.

Table 4

Comparison between equation (4) and equation (5)

Name of specimen	Maximum load (kN)	Equation (4)					Equation (5)					Note
		Resistance of parameter				Rate of error	Resistance of parameter				Rate of error
		ⓐ	ⓑ	ⓒ	Total	Rate of error	ⓐ	ⓑ	ⓒ	Total	Rate of error
PR-1	213	212.8	0	0	212.7	0.1	112.7	0	0	112.7	47.1	No Holes, Control
PR-2	396	212.8	162.8	0	375.5	5.2	112.7	196.1	0	308.8	22.0	4 Holes
PR-3	396	212.8	166.7	0	379.3	4.2	112.7	245.1	0	357.8	9.6	5 Holes
PR-4	382	212.8	155.5	0	368.3	3.6	112.7	147.1	0	259.8	32.0	3 Holes
PR-5	457	212.8	162.8	60.4	435.9	4.6	112.7	196.1	144.6	453.4	0.8	4 Holes, 2 Rebars

The resistance element in front of the Perfobond Rib and the bearing effect of the traverse reinforcing bars in the results from Equation (5), considered in the formula, were 47% less and 2.4 times greater, respectively, than those of Equation (4). In addition, a 5.8% difference was observed between the results when only ⓐ and ⓒ are considered, excluding the concrete dowel effect. However, in the case of Equation (5), a 9.6%–32.0% difference was found in the concrete dowel effect. Such difference occurs because the diameter of holes and the distance between holes differ, which significantly affects the dowel effect. As the effect of dowel can be overlapped when the distance between holes becomes narrow, it is assumed that Equation (5), which does not consider the distance between holes as a variable, shows higher deviation than that of the test result².

4. Test of Flexural Shear Behavior

The purpose of this study is to identify the relationship between flexural and direct shear stress. To achieve this, a steel-concrete composite deck, which is the specimen used to investigate the flexural shear behavior, was prepared with the Perfobond Rib connectors tested in Section 2.2. The four point loading flexural test was performed on the specimen and the flexural shear behavior was analyzed. In addition, a specimen constructed with Perfobond Rib shear connectors only and a specimen made of Head Stud shear connectors and Perfobond Rib shear connectors together (as is recommended in the existing standard for road bridge design), were prepared to analyze the flexural behavior and load resistance capability.

4.1 Fabrication and Composition of Flexural Behavior

Two specimens with and without head stud shear connectors were prepared for the flexural shear behavior test. Table 5 shows the material properties.

Table 5
Material properties of flexural shear behavior test

Concrete Design strength Max. size of coarse aggregate Slump

30 MPa 25 mm 120 mm

Steel plate Thickness Yield strength Grade

6 mm 245 MPa SM400

Rebar Diameter Yield strength Grade

19 mm 400 MPa SS400

The dimensions of the flexural shear behavior specimen are 1000 mm (the width) × 200 mm (the height) × 3500 mm (the length). The sizes of the Perfobond Rib and Head Stud shear connectors are t6 × 133 mm and $φ 16 \times 80$ mm, respectively. The number of holes in the Perfobond Rib shear connector is $φ 65 \times 13$ holes × 2 sides (symmetrical), while the arrangement of Head Stud shear connectors is 3 holes × 6 lines × 2 sides (symmetrical). The distance between the traverse reinforced bars is 13@125 = 1625 mm. The strength of concrete specimens was determined based on the average compressive strength of three concrete specimens of 32.63 MPa, according to the Korea Structural Concrete Design Code. Figure 5 shows the cross-sections of flexural behavior specimens. Table 6 describes the general specification of specimens made according to the following composition.

Figure 5

Cross-section of proposed each deck profile

Table 6

Names of specimens and parameters

Name of specimens	Specifications (mm)	Parameter (head stud)	Note
HS-PR-Test	3,5001,000200	°	Head Stud + Perfobond Rib
PR-Test	3,5001,000200	x	Perfobond Rib

4.2 Configuration Method of Flexural Shear Behavior Experimental Device

As shown in Figure 6 , a four point load test was conducted on the flexural shear behavior specimens. The shear span to depth ratio (a/d) was 4.0. The distance from the hinge part to the end of the specimen was 125 mm. An LVDT was installed in the centre of the specimen to measure deflection. In addition, a steel-strain gauge was installed on the bottom of the steel to measure the strain of the bottom steel due to the increase in weight.

Figure 6

Set-up for flexural test

4.3 Results of Flexural Shear Behavior Test

The test results of the flexural shear behavior specimens are shown in Table 7 . P_max is the maximum load and d_max is deflection at the maximum load. P_a and δ_a are the yield strength and the yield deflection, respectively. Figure 7 shows the typical destruction patterns from the flexural shear test and Figure 8 shows the relationship between the load and deflection of the flexural shear behavior specimens. The initial cracks of the HS-PR-Test specimens were found at L/4 point in the end of the specimen under 411 kN. When the load reaches 656 kN, cracks start generating at the concrete compression area. The maximum test load was 678 kN and the deflection at the maximum load was 37.3 mm.

Figure 7

Breaking of specimen in flexural test

Figure 8

Load-slip of flexural shear specimens

Table 7

Results of flexure shear test

Name of specimens	P_max (kN)	P_a (kN)	d_max (mm)	d_a (mm)	Note
HS-PR-Test	678	626	37.3	27.6	Head Stud + Perfobond Rib
PR-Test	565	503	37.1	24.2	Perfobond Rib

The separation of the bending steel and concrete in the PR-Test specimen made of Perfobond Rib shear connectors only started at the end of the specimen when the load reached 245 kN and showed significant separation and destruction when the load reached 539 kN. The maximum load was 565 kN. The deflection was 37.1 mm at the maximum load. As shown in Figure 10, all specimens show similar behaviors up to approximately 150 kN. On the other hand, the PR-Test specimen without Head Stud shear connectors showed significantly decreased maximum load and increased deflection due to the splitting failure between the bending steel and concrete. For the specimen without the Head Stud shear connectors, the bonding strength between steel and concrete at the earlier stage was reduced and, as a result, the specimen was destroyed due to the rapid separation of steel and concrete. Moreover, it is shown that the maximum flexural resistance capability was decreased by approximately 10%.

4.4 Degree of Shear Connection of Steel-Concrete Composite Specimen

In this study, the degree of shear connection of PR-Test and HS-PR-Test specimens was analyzed based on the results of flexural shear behavior. The degree of shear connection was calculated from Equation (1) in accordance with the method suggested in EN 1994-1-1. The larger value of η implies superior composition performance. If η ≥ 1, it is assumed that the composition status is perfect. To calculate the yield strength of the composite slab, the value of shear strength in the longitudinal direction of shear span should first be calculated. With this result, h, the degree of composite, can be drawn with Equation (1). The partial shear bonding moment, M_psc, can then be calculated. In this study, the degree of steel composition for deck plate variables was indirectly evaluated using the maximum moment, M_max, instead of M_psc for the evaluation of the steel-concrete composite deck slab⁸. Table 8 shows η of each specimen calculated from Equation (1).

Table 8
Degree of shear connection

Name of specimens P _max η Parameter (head stud) Comparison of η

HS-PR-Test 678 2.76 ° 1

PR-Test 565 2.22 X 0.80

Name of specimens	P _max	η	Parameter (head stud)	Comparison of η
HS-PR-Test	678	2.76	°	1
PR-Test	565	2.22	X	0.80

5. Analysis of Direct Shear Stress and Flexural Shear Stress

In this study, the direct shear resistance performance of Perfobond Rib shear connectors is analyzed and a direct shear stress formula is proposed for the direct load given in Section 3.5 on the basis of such test results. Furthermore, an analysis is performed of the flexural and direct shear stress and the shear stress in the direction of the load, by comparing the direct shear stress formula and the flexural shear stress formula suggested in Section 2.2 (EN 1994-1-1).

Many domestic and foreign studies, including Rui(2008), focus on methods to calculate shear force in the perpendicular direction, called flexural shear force⁹. In this study, the relationship between flexural shear force and direct shear force is analyzed by calculating the flexural shear force using Equation (2), adopting the degree of shear composition (η) suggested in EN 1994-1-1 and comparing the results with the direct shear stress from Equation (4).

According to the calculation of the HS-PR-Test specimens, it is shown that the flexural shear stress is 2.90 kN/mm² and the direct shear stress is 2.81 kN/mm². The HS-PR-Test specimen is made of a Head Stud and Perfobond Rib shear connectors. For the PR-Test specimen, the flexural shear stress is calculated as 2.33 kN/mm² and the direct shear stress is 2.19 kN/mm². As for existing study results, the flexural shear stress is stronger than the direct shear stress because of the stress redistribution through load steps from the vertical direction to the horizontal direction. Equation (4) suggested in this study does not show clear results for all specimens using Perfobond Rib connectors in each case. However, it is known that general flexural shear stress is similar to, or the same as, direct shear stress through this study.

According to a recent study by Shin (2005), obvious differences are observed between composite behaviors of shear connectors obtained in the standard Push-Out Test and shear composite behaviors occurring in an actual composite beam. Especially, it is revealed that the difference is even greater when a beam is partially composited¹⁰. This is demonstrated by the fact that a concrete slab of a composite beam in a static moment section is under a higher compressive stress condition than in the standard Push-Out Test. The test result shows that, in some cases, a composite beam designed as a partial shear composition shows actually perfect shear composition behaviors when comparing the designing value with the actual value obtained in the test; this concurs with the above argument.

6. Conclusions

In this study, specimens were prepared for a direct shear behavior test and a flexural shear behavior test using Perfobond Rib connectors and a comparative test was performed to analyze the results. The following conclusions were made.

According to the direct shear test of the Perfobond Rib shear connectors, the main factors that influence direct shear resistance are the tip resistance of the concrete of the rib, the dowel effect of concrete and the bearing capacity of the traverse reinforced bars. In addition, the concrete dowels can be reduced by overlapping of stress fields as the distance between holes.

For practical application of Perfobond Rib shear connectors, a regression analysis was performed using the results of direct shear strength test. Equation (4), as a formula to assess the direct shear strength of Perfobond Rib shear connectors, was also proposed. According to the results, Equation (4) shows a relatively good fit with less than 10% error. It is found that the most significant cause of the error is the second term, which is related to the dowel effect. This can be considered as an error of the regression analysis. In addition, the effect on the end bearing and the traverse reinforced bars shows a relatively good fit.

Based on the result of the flexural shear test, the degree of shear composition (η) is calculated and the flexural shear stress is drawn. It is revealed that the flexural shear stress is approximately 6% stronger than the direct shear stress, according to the comparison of the flexural stress drawn in this process and the direct shear stress identified in the push-out test.

Footnotes

Acknowledgment

This study was supported by Research Fund of National Research Foundation of Korea(NRF)-2015R1C1A1A02036540. The authors gratefully acknowledge this support.

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Concrete	Design strength	Max. size of coarse aggregate	Slump
Concrete	30 MPa	25 mm	120 mm
Bending steel and shear connector	Thickness	Yield strength	Grade
Bending steel and shear connector	6 mm	245 MPa	SS400
Rebar	Diameter	Yield strength	Grade
Rebar	16 mm	400 MPa	SS400