SHEAR BEHAVIOUR OF R. C BEAMS HAVING SINGLE SWIMMER … · c + V s is the nominal shear strength,...

International Conference on Civil and Environmental Engineering

Hurghada, Egypt, November 24-26, 2018

SHEAR BEHAVIOUR OF R. C BEAMS HAVING SINGLE SWIMMER

BARS

Yehia A. Hassanean1, Kamal A. Assaf

2,

Khaled Abd El Samee3

and Moamen A. El Hamedy4

1 Professor of Reinforced Concrete Structures, Civil Engineering Department, Assiut University

e-mail : [email protected] 2 Associate professor of Structures Engineering, Civil Engineering Department, Assiut University

e-mail : [email protected]

3 Lecturer of Structures Engineering, Civil Engineering Department Sohag University

e-mail : [email protected]

4 Demonstrator, Civil Engineering Department, Sohag University.

e-mail : [email protected] ;

ABSTRACT: The shear behavior of RC beams at failure differs utterly from their behavior by bending

that is taken into consideration to be a dangerous mode of failure. Shear failure of RC beams is often

explosive and abrupt, whereas not spare advanced warning. Diagonal cracks propagate precipitately as

a result of excess shear forces are significantly wider than the flexural cracks. The single Swimmer bar

system is a new alternative of shear reinforcement. It is a small inclined bars, with its both ends bent

horizontally for a short distance and tied to both top and bottom steel reinforcement. The single swimmer

bar system can be spliced, welded or bolted. The present study simulates the effectiveness of using single

swimmer bars instead of traditional stirrups on improvement of shear performance in reinforced concrete

beams, as well as studying the effect of concrete strength and the type of tying to determine the most

economical strength and the most effective type of tying to carry shear forces. Seven beams are

experimentally tested under static load. The test results are given in shape of photos, figures and tables

including modes of failure, both cracking and ultimate loads, the maximum deflections and maximum

strains. The test results show that using swimming bar as shear reinforcement increase the shear strength

and improve the deformability of the tested beams.

Keywords: High strength concrete, Stirrup, Single Swimmer bar, Shear, Deflection.

1. Introduction

Beams are used to carry loads primarily by internal moments and shears. Beams are always

designed for flexure in the first consideration then designed for shear for safety, since shear

failure is frequently sudden with little or no advanced warning, the design for shear must ensure

that the shear strength for every member in the structure exceeds the flexural strength. The shear

failure mechanism varies depending upon the cross-sectional dimensions, the geometry, the

types of loading, and the properties of the member [1]. A number of investigations were done

mailto:[email protected]

mailto:%[email protected]





on the shear behavior of concrete. Shear reinforcements are necessary to improve the shear

resisting capacity of the beams if the permissible shear stress is lesser than the actual shear

stress. Diagonal cracks are the sign of shear failure. These cracks wider than the flexural cracks

and occurred in the shear zone, which are nearer to the supports. The shear cracks progress

rapidly without warning and the member fails suddenly. The purpose of shear reinforcement is

to prevent shear failure, and to increase beam ductility and subsequently the likelihood of

sudden failure will be reduced. In the past, bent up bars were commonly used in addition to

stirrups to satisfy the minimum code requirement, these bent up bars are the bottom steel

reinforcement bent near the support to join the top steel. In many cases, all the flexure

reinforcement is not needed to resist bending moment. Anyway, using of bent up bars is not

preferred because of its cost and other technical considerations [1]. Nowadays, stirrups are most

commonly used as shear reinforcement, for their simplicity in fabrication and installation.

Stirrups are spaced closely at the high shear region. Congestion near the support of the

reinforced concrete beams due to the presence of the closely spaced stirrups increases the cost

and time required for installation [1-3]. Swimmer bar system is considered a new technique of

shear reinforcement. It distinguishes with more flexibility, simplicity, efficiency, speed of

construction and low cost than bent up bars or stirrups system. One of the most important

advantages that the swimmer bars from plane – crack interceptor system instead of bar – crack

interceptor system when stirrups are used. The single swimmer bar system is more effective

than other types to carry shear loads [1]. Noor (2005) presented several results of experimental

investigation on six R.C beams in which their structural behavior in shear was studied. The

main objectives of that study were studying the effectiveness of adding horizontal bars on shear

strength in rectangular beams, the effectiveness of shear reinforcement, and determining the

optimum amount of both types of shear reinforcement to achieve a shear capacity similar to that

of a normal link system. He was found that, the use of independent horizontal and bent-up bars

as shear reinforcement were stronger than conventional shear reinforcement system [7].

2. SWIMMER BARS

There are many shapes of the swimmer bar system; single swimmers, rectangular shape,

rectangular shape with cross bracings and swimmer bar planes with vertical and horizontal

stiffeners. Several additions to these shapes can be explored [1]. The single swimmer bar is tied

to both top by the horizontal stiffener bar with flexural steel reinforcement. The swimmer bar

system can be welded, spliced or bolted as shown in [Figure-1. a, b and c]. Swimmer bar systems

are used to improve the shear performance of the reinforced concrete beams instead of

traditional stirrups, reduce the amount of cracks, reduce the width and the length of cracks and

reduce overall beam deflection. The Single swimmer bar with z shape increases shear strength

capacity and flexure capacity. This study focuses on a single swimmer bars which can be

described as a small inclined bar, with its both ends as anchorage length bent horizontally for a



short distance equals to ten times the bar diameter that is used in shear reinforcement, and looks

as a Z shape.

Fig. 1. a. Spliced tying type. b. Bolted tying type. c. Welded tying type.

3. ACI CODE PROVISION FOR SHEAR DESIGN

According to the ACI Code [10], the design of beams for shear is to be based on the following

relation:

𝑉𝑢 ≤ ø𝑉𝑛 (1)

Where: Vu is the total shear force applied at a given section of the beam due to factored loads

and Vn = Vc + Vs is the nominal shear strength, equal to the sum of the contribution of the

concrete and the web steel if present. Thus, for vertical stirrups:

(2)

And for inclined bars:

(3)

Where: Aʋ is the area of one stirrup, α is the angle of the stirrup with the horizontal, ƒyt = strength

of transverse reinforcement and s is the stirrup spacing. The nominal shear strength contribution

of the concrete (including the contributions from aggregate interlock, the dowel action of the

main reinforcing bars, and that of the un-cracked concrete) can be simplified as shown in Eq.4.

𝑉c = 0.17𝜆 √ƒc ′ 𝑏w𝑑 (4)

Where bw and d are the section dimensions, and for normal weight concrete, λ = 1.0. This

simplified formula is permitted by the ACI code expressed in metric units.

4. SUGGESTED METHOD OF DESIGNING SWIMMER BARS

The analysis of needed shear reinforcement swimmer bar system is based on the truss analogy

s

d F A V V

ytv

cu

)cos(sins

d F A V V

ytv

cu



concept. If s1 is the swimmer bar spacing in a single truss analogy, n is the number of bars,

and As is the area of steel of a single swimmer bar [1], then:

s1= 𝑛𝐴𝑠 (5)

(6)

Where Ts is the tension force on the bent bar, s is the spacing of the swimmer bars, α is the angle

between the tension force and the horizontal in the triangular truss and β is the angle between

the simulated concrete strut and the horizontal in the triangular truss. If there are 𝑛 swimmers

bars within the s1 length of the analogous truss chord, and if Av is the area of steel of one

swimmer bar, then:

𝑇𝑠 = 𝑛 𝐴𝑣 ƒ𝑦𝑡 (7)

(8)

In the case of diagonal tension failure, the compression diagonal makes an angle β = 45º with

the horizontal, thus Eq. 6 becomes:

(9)

Which can be simplified as:

(10)

Which is similar to those used by ACI code.

𝑉𝑛 = 𝑉𝑠 + 𝑉𝑐 (11)

5. EXPERIMENTAL PROGRAM

In order to investigate the above mentioned objectives, an experimental program was carried

out to test seven simply supported reinforced concrete beams. Four beams of them were made

from normal strength concrete and the remaining three were made from high strength concrete.

Detailed and description of the specimens, the material properties, mix proportions, test set-up,

test procedure, and measurements were presented in this section.

)cot cot()d-(d sin s

ns

s1

VTT ss

yt

vf)cot cot (sin)d-(d

sn

ns A

ss VT

n

)) cot1( (sin s

)d-(df A V

ytv

s

) cos(sin s

)d-(df

yt

s

vA

V



5.1. Test specimens

The details of the tested beams are shown in [Figures 2 to 4] and listed in [Table- 1]. All beams

were 300 mm height, 150 mm width, and overall length 1400 mm. The beams had three bars

16 mm diameter as main longitudinal reinforcement and two bars diameter 10 mm for

compression steel. The shear span to depth ratio (a/d) was kept constant for all beams and equal

to 1.5. The studied variables were concrete compressive strength, Fcu and type of tying. In

control beam, vertical stirrups 8 mm diameter at 100 mm spacing was used, along the overall

length of the control beam. Six beams were designed for shear with single swimmer bar at 150

mm spacing for third point load. All cubes were tested on the same day on which the respective

beams were tested to ascertain the used concrete compressive strength.

Fig. 2. Longitudinal Sec in control beam

Fig. 3. Longitudinal Sec in beam with single siwmmer bars

Fig. 4. Cross Sec in beam with single siwmmer bars



Table: 1. Test specimens details

Beam

No

Fcu

N/mm2 Shear web reinforcement Type

Stirrups Single swimmer bar

B1 30 8@100mm ---------- ---------

B2 30 ---------- 1Ø12 @150mm Spliced

B3 30 ---------- 1Ø12 @150mm Welded

B4 30 ---------- 1Ø12 @150mm Bolted

B5 40 ---------- 1Ø12 @150mm Spliced

B6 60 ---------- 1Ø12 @150mm Spliced

B7 80 ---------- 1Ø12 @150mm Spliced

5.2. Materials

Concrete mixes were designed to produce normal and high strength concrete, and have cubic

compressive strength of 25, 40, 60 and 80 N/mm2 after 28 days. Ordinary Portland cement with

a grade of 42.5 N was used for all specimens, the used cement was tested experimentally. 20

mm maximum nominal size crushed gravel which has specific gravity 2.53 was used as coarse

aggregate in normal strength concrete, crushed basalt with a maximum nominal size of 20 mm

and specific gravity 2.78 for high strength concrete and local natural sand has specific gravity

2.5 and fineness Modulus 2.4 was used as a fine aggregate. To enhance the high strength

concrete by means of pore – refinement, Silica Fume has a specific gravity of 2.1 and a

maximum dose to be 20% of cement as a replacement of cement was used. A high-range water-

reducing admixture was used to improve the workability of the mix. Deformed bars of about

400 N/mm2 proof strength and having a diameter 10, 12, and16 mm were used as main bars and

shear reinforcement bars. The stirrups used were made of 8 mm diameter smooth bars of 330

N/mm2 yield strength. All used materials are agreed with ECP [203].

5.3. Mix proportions

Four mixes were produced according to concrete strength. The quantities of materials used for

these mixes are illustrated in [Table-2]. The Concrete compressive strengths of tested cubes

after 28 days were 28.6 MPa, 45.3 MPa, 67.2 MPa and 83.4 MPa, respectively. The slump test

was carried out on fresh concrete and the values are listed in [Table- 2].

Table 2: Mixing proportions for the designed mix mixes.

Target

strength

N/mm2

Cement

Kg/m3

Sand

Kg/m3

Coarse

Aggregate

Kg/m3

Water

Liter/m3

Silica

fume

kg/m3

Super

plasticizer

kg/m3

Fcu

N/mm2

Slump

value

(cm)

25 350 570.0 1140 210 0 0 28.6 8

40 500 535.0 1070 200 0 0 45.3 8

60 475 576.5 1153 164 71.25 8.2 67.2 7.5

80 475 596.5 1193 125.5 95.0 14.25 83.4 7.5



5.4. Test procedure

Before testing, the surface of the specimens was painted with white emulsion to make the cracks

more visible and easy to be observed. At the age 28 days, the reinforced concrete beams were

prepared for testing. To ensure that shear cracks will occur near the support, third point load

was applied to the tested beams with shear span to depth ratio equal to 1.5. Vertical deflections

at third-span were measured by LVDTs. At each load stage, the deflection readings are

recorded, the cracks are marked on the surface of and cracks width was monitored at each load

increment. All shear and flexure reinforcement strains and the ultimate load were also

measured.

Fig. 5. Test setup

6. Test results and discussion

6.1. Crack pattern and mode of failure

Beam B1, which reinforced with traditional stirrups was dedicated as a control beam to beams

with single swimmer bars. Diagonal shear cracks observed at a load of 100 KN. These cracks

were extended and widened as the load increased. The cracks emigrated for point of loading

and increased to the support. Flexure cracks appeared at a load of 160 KN along the unloaded

span. [Figure-6] shows the crack pattern and mode of failure of control beam and other tested

beams. The behavior of other normal strength beams with single swimmer bar system was

similar to the mode of failure of control beam, but there were multi diagonal cracks through the

shear span. In high strength concrete beams flexure cracks and diagonal shear cracks were

appeared and the mode of failure was converted to shear –flexure failure. [Figure-7.a and b]

shows the values of crack width in normal and high strength concrete beams, respectively.

Using of the swimmer bar system decreases the value of crack width and length compared to

using of stirrups system. Bolted tying beam improves the property of crack width more than

other types of tying but, it is considered much cost. The higher concrete strength beams reduce

the values of crack width to lower limits.



Fig.6. Crack patterns at failure of tested beams

Fig. 7. a. Effect of type of tying on values b. Effect of strength of concrete on values

of crack width of normal strength beams. of crack width of normal strength beams.

6.2. Load – deflection

To assure the objectives of this paper, beams with single swimmer bars must be compared to

beams with traditional stirrups. [Figure- 8. a and b] show the curves of load deflection for

0

100

200

300

400

0 0.5 1 1.5

TOTA

L LO

AD

(K

N)

CRACK WIDTH(MM)

B1 B2 B3 B4

0

100

200

300

400

500

0 1

TOTA

L LO

AD

(K

N)

CRACK WIDTH(MM)

B5 B6 B7



normal and high strength concrete beams, respectively. It is noticed that spliced tying beam

reduces the maximum deflection value to 15% compared to control beam. It is observed that

welded tying beam improves the maximum deflection value to 40.13% with respect to control

beam. The maximum deflection value of the bolted tying beam decreased to 30.5% compared

with maximum deflection of control beam. It is observed that the use of swimmer bars in each

shear span gives values of less deflection at a higher rate in the case of normal concrete than

that of high strength concrete and that is agreed with [2]. From last curves, we notice the

behavior of spliced tying beam is similar to the behavior of beam with traditional stirrups.

Fig. 8. load deflection curves for tested beams.

6.3. Cracking and Ultimate loads and stresses

[Table:3] Shows the values of ultimate loads for all tested specimens, and it is observed the

efficiency of using swimmer bars over than stirrups in RC beams. Spliced tying beam increases

the ultimate load about 11.88% in comparison with control beam. it is obviously observed

improvement in ultimate load capacity by 23.36% in the case of using welded tying beam. Using

higher strength beams leads to a massive improvement in ultimate load capacity for example,

Beam with compressive strength 80 N/mm2 increases the ultimate load up to 80.33% with

respect to control beam.

6.4. Ultimate shear load and Ultimate Shear stress

The ultimate shear stress is very important in the shear behavior especially for high strength

concrete [2]. The ultimate shear load and stress of the tested beams are calculated and given in

[Table- 3]. It was noticed that, the ultimate shear load increased by about 23.36 % from beam

with stirrups to welded typing beam. Shear stress, increased by about 61.33% of the normal

strength spliced tying beam to beam with strength 80N/mm2, and so on for other beams.

0

50

100

150

200

250

300

350

0 2 4 6

TOTA

L LO

AD

(K

N)

DEFLECTION (MM)

B1 B2 B3 B4

0

100

200

300

400

500

0 5 10 15 20 25

TOTA

L LO

AD

(K

N)

DEFLECTION (MM)

B5 B6 B7



Table:3. Values of cracking, ultimate loads and ultimate shear stress for tested beams.

Where: Pu = Ultimate load of the tested beam

Pu control = Ultimate load of control beam

6.5 strain in shear web reinforcement

Shear strains were installed for each swimmer bar as shown in [figure-5], the maximum values

of shear strain were listed [Table-4]. Also, it was found that, the values of shear strain by using

single swimmer bars were higher than those by using traditional stirrups and single swimmer

bars reached to yielding value of steel but stirrups didn’t yield. And shear strain values in high

strength concrete were more than those in normal strength concrete. That, in turn illustrates the

efficiency of using the single swimmer bar system over the stirrups system in shear strength,

and also the shear performance is better in higher strength beams than normal strength beams.

6.6 strain in main reinforcement

Steel strains were installed to the main reinforcement at the location of loading. Values of

flexure strain were recorded and drawn in [figure-9]. We can notice that using of beams with

swimmer bars with Z shape decreases flexure strain values that in turn leads to improvements

in flexure strength. The increase depends on the type of tying of swimmer bars with the

longitudinal reinforcement. Flexure strength capacity increased to by 14.67% in the case of

using spliced type beam with respect to control beam. Bolted tying type beam improves the

flexure strength capacity to by 20.77% compared to control beam. The best improvement in

flexure strength was about 24.57% in the case of using welded tying type.

6.7 Ductility and toughness

Ductility of reinforced concrete beams can be measured based on structural characteristics such

as: third-span deflection, curvature. The displacement ductility (μD) considered here was

measured as the ratio between maximum deflection (Δmax) (corresponding to 90% of the

maximum recorded load) and the deflection corresponding to cracking load (Δcr). Moreover,

Beam

No

Cracking

load (KN)

Ultimate

load (KN)

Ultimate shear

load (KN)

Ultimate

Shear stress

(N/mm2)

B1 83 244 0 163.48 3.63

B2 97 273 10.62 182.91 4.06

B3 91 271.5 11.27 181.905 4.04

B4 95 301 23.36 201.67 4.48

B5 80.1 350 43.44 234.5 5.21

B6 147 429 75.82 287.43 6.39

B7 152 440 80.33 294.8 6.55

100P

PP

control u

control uu



toughness or energy absorption capacity up to failure is represented by the area underneath the

load-deflection curve and the values shown in [Table-5]. Single swimmer bars improve stiffness

and modulus of elasticity by decreasing crack width and deflection values for tested. This could

be attributed to the increase in strength of beams.

Table: 4. Shear strain values

Fig: 9 Strain in main reinforcement values at failure

Table:5. Ductility and toughness values of tested beams

Beam No (Δcr) (Δmax) Ductility (μD) Toughness (kN.mm)

B1 1.035 3.675 3.55 817.09

B2 1.47 4.48 3.05 1174.83

B3 0.891 3.61 4.05 790.19

B4 0.936 3.517 3.76 792.00

B5 0.988 4.516 4.57 2608.26

B6 1.16 8.3 7.16 4435.10

B7 1.375 14.61 10.62 8900.18

7. Conclusions

Based on the experimental program results, the following conclusions can be drawn:

Using of single swimmer bars system with z shape increases the shear resistance and

reduces the deflection values with respect to using the traditional stirrups system in

normal and high strength concrete beams.

Shear performance of single swimmer bar system increases in higher strength concrete

beams over normal strength beams, diagonal shear failure occurred in normal strength

beam, however, using of higher strength concrete beams converged the failure to shear-

flexure failure.

In high strength beams, the cracks propagate in a slower rate than in normal strength

beams, the failure in HSC is more explosive and sudden than NSC and the surface of

failure in HSC is sleek but the surface of failure in NSC is rough that in turn, ensures

the fact that high strength concrete is a more brittle more material than normal strength

concrete.

Beam

No

Strain in web reinforcement

at failure 10−7

B1 980

B2 2508.6

B3 2163.58

B4 2940.47

B5 2801.825

B6 4667.55

B7 5550.67

0

1000

2000

3000

B1 B2 B3 B4Flex

ture

str

ain

(µ

)



Max deflection value in welded tying beams reduced by 40.13% compared with control

beam, bolted tying by 30.5 % compared and spliced tying by 15% for normal strength

beams.

Using of single swimmer bars system with z shape increases the flexure strength by

14.67% to 24.57 %, according to type of tying as well increase shear strength.

9. References

1. Al-nasra, M. M., Duweib, I. A. and Najmi, A. S. The Use of Pyramid Swimmer Bars as

Punching Shear Reinforcement in Reinforced Concrete Flat Slabs. J. Civ. Eng. Res. 3, 75–80

(2013).

2. Mohamed, H. A. Effect of using swimmer bars on the behavior of normal and high strength

reinforced concrete beams. Ain Shams Eng. J. 8, 29–37 (2017).

3. Al-nasra, M. M., Asha, N. M. and Najmi, A. S. Optimizing the use of swimmer bars as shear

reinforcement in the reinforced concrete beams. Int. J. Civ. Structural Eng. 3, 75–80 (2013).

4. Saravanakumar, P. and Govindaraj, A. Influence of Vertical and Inclined Shear

Reinforcement on Shear Cracking Behavior in Reinforced Concrete Beams. Int. J. Civ. Eng.

Technol. 7, 602–610 (2016).

5. Al-nasra, M. M. Investigating Alternatives in Shear Reinforcements in the Reinforced

Concrete Beams ACI Code Provision for Shear Design. 27–31 (2015).

6. S, R. and Joy, A. The Use of Swimmer Bars as Shear Reinforcement in RC Deep Beams.

IOSR J. Mech. Civ. Eng. Ver. III 12, 2278–1684 (2015).

7. Noor Hamid (2005). The Use of Horizontal and Inclined Bars as Shear Reinforcement,

Master Thesis, University of Technology, Malaysia.

8. Abu-Lebdeh, T., S.A. Hamoush, W. Heard, and B. Zornig, 2010. Effect of matrix strength

on pullout behavior of steel fiber reinforced very high strength concrete composites”,

Construction and Building Materials Journal.

9. Lesley H. Sneed and Julio A. 2008, Effect of Depth on the Shear Strength of Concrete Beams

without Shear Reinforcement, USA Portland and cement Association.

10. ACI 318-11. Building code requirements for structural concrete commentary; 2011.

SHEAR BEHAVIOUR OF R. C BEAMS HAVING SINGLE SWIMMER … · c + V s is the nominal shear strength,...

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