19 

© 2020 Adama Science & Technology University. All rights reserved 

Ethiopian Journal of Science and Sustainable Development  

e-ISSN 2663-3205                                                                           Volume 8 (2), 2021 

Journal Home Page: www.ejssd.astu.edu.et  ASTU  

Research Paper 

Investigation of Reinforced Concrete Columns Confined by Rectangular Spiral 

Reinforcement using Finite Element Analysis 

Thomas Belete1,, Nigatu Chaffo2, Habtamu Melese1, Bobby Lupango1  

1Civil Engineering Department, Institute of Technology, Arba Minch University, Arba Minch, Ethiopia 

 2Ministry of Urban Development and Construction, Addis Ababa, Ethiopia  

Article Info  Abstract 

Article History: 

Received 02 Sept. 2020 

Received in revised form 

02 February 2021 

Accepted 15 February 

2021 

 

 The load carrying and deformation capacity of square reinforced concrete (RC) column 

confined by rectangular spiral lateral reinforcement subjected to axial load were investigated 

numerically using abaqus. Thirty one short square RC columns were modeled where seventeen 

had conventional hoop tie and the remaining fourteen columns had rectangular spiral lateral 

reinforcement configuration. The configuration, amount and spacing of the lateral 

reinforcement were included as a factor of confinement in the investigation. Different amounts 

of lateral reinforcement including 1.4%, 1.6%, 1.8%, 2% and 2.3% were provided in the 

investigated columns. The diameters of lateral reinforcement used for confinement purpose 

were 6 mm, 8 mm and 10 mm. The primary objective of this study was focused on the 

investigation of confinement effect of continuous lateral reinforcement configuration in terms 

of stress carrying and deformation capacity of confined reinforced concrete columns. RC 

columns confined with either configuration (hoop or spiral) of lateral reinforcement and 

provided with amount of reinforcement in the range of 1.6% to 2% where the spacing to core 

depth (s/dc) ratio was maintained in the range of 0.4-0.5 had exhibited improved deformation 

and stress capacity. Accordingly, confined RC columns with rectangular hoop lateral 

reinforcement had 12% and RC columns with continuous spiral lateral reinforcement had 40% 

higher deformation capacity compared to RC columns with the same amount and configuration 

of lateral reinforcement but where the spacing was in such a way s/dc ratio was ≥ 0.5. Clearly, 

the use of rectangular spiral reinforcement as lateral reinforcement could be considered as a 

better alternative for lateral reinforcement of short square RC columns particularly, where the 

deformation capacity is the main importance. 

Keywords:  

Confinement  

Deformation Capacity  

RC column  

Spiral Reinforcement  

1. Introduction 

The ability of structural element to withstand the 

load imposed on without significant deformation 

maintains the stability of the structure. In skeletal frame 

structure, local failure (plastic hinge) in column 

intercepts the flow of load transfer leading to partial or 

global collapse of structure. Hence, the ductile behavior 

of column is required critically for avoiding formation 

of plastic hinge in the column. Due to this reason, 

several National codes (ACI-318-08, 2008; ES EN 

                                                           
Corresponding author, e-mail: fanaisat@gmail.com  

https://doi.org/10.20372/ejssdastu:v8.i2.2021.291 

1998-1: 2013) had incorporated strong column-weak 

beam design philosophy and the flexural strength ratio 

of column to beam (∑ 𝑀𝑐 / ∑ 𝑀𝑏) ≥ 1.2  set to realize 

the above requirement. An effort to enhance the strength 

and ductility of the column member has led to the 

application of confinement. The research on the 

confinement of RC column member around the globe 

could be classified into two categories.  

http://www.ejssd.astu.edu/
https://doi.org/10.20372/ejssdastu:v8.i2.2021.291


Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

20 

The first category includes research studies 

incorporating additional materials to enhance the 

strength and ductility of RC column member. These are 

studies on RC column member confined by fiber 

reinforced polymer (FRP) (Ali et al., 2018), and the 

application of buckling restraint reinforcement (BRR) 

(Lukkunaprasit et al., 2011). However, the additional 

cost of these materials, installation and availability 

hinder the applicability of these confinement techniques 

in ordinary column construction. 

The second category is a widely practiced lateral 

reinforcement confinement. The pioneer works of 

Rechart et al. (1929) demonstrated steel lateral 

reinforcement had significantly improved the strength 

and ductility of axially loaded RC column. Since then 

Sheikh (1983), Mander et al. (1989) and Sheikh and Yeh 

(1990) had conducted investigation on the effect of 

lateral reinforcement confinement on RC column. 

Sheikh (1983) identified core concrete area, nominal 

strength of lateral reinforcement and concrete, spacing 

and amount of reinforcement, and tie configuration as 

the primary factors affecting the confinement in RC 

column. Mander et al. (1989) considered the reduced 

core concrete area in the development of analytical 

model for strength and ductility of confined concrete. 

Sheikh and Yeh (1990) investigated rectangular RC 

column and mentioned that inadequate configuration of 

lateral reinforcement had resulted in unstable strength 

degradation on the post–peak behavior of stress–strain 

curve for tested RC column. Saaticoiglu and Razvi 

(1992) pointed out that the non-uniformity of 

confinement stress at a tie level and along the RC 

column height reduced the improvement on the strength 

and ductility of confined concrete. Sheikh and Toklucu 

(1993) investigated circular RC column, and pointed out 

the contribution of an optimum combination of the 

spacing and amount of reinforcement for enhanced 

strength and ductility of confined concrete.  

The shape of the cross-section of the member under 

consideration had significant effect on the effectiveness 

of confinement provided to the core concrete (Mander 

et al., 1989; Saaticoiglu and Razvi, 1992). Studies on 

circular RC column revealed that both spiral and hoop 

configurations had resulted in significant enhancement 

of ductility and strength of the member (Mander et al., 

1989; Sheikh and Toklucu, 1993). Whereas, on rectangular 

RC column or generally edged cross section the non-

uniform confinement pressure both at tie level and along 

the height of column had reduced the enhancement on 

load carrying and deformation capacity of the column.  

Sheikh and Yeh (1990) and Zeng (2017) 

demonstrated the use of various configurations such as 

diamond and other shapes to eliminate the non–

uniformity at a tie level and also suggested the use of 

additional longitudinal bars in between corners of edged 

cross section RC columns. However, apart from close 

spacing of lateral reinforcement as means to minimize the 

non-uniformity of confinement yet no investigation was 

availed on the effectiveness of continuous confinement in 

reducing the non–uniformity of confinement pressure 

along the length of square RC column. 

Since recent times, several studies have been 

conducted on the use of rectangular spiral reinforcement 

(RSR) in RC beam–column joint. Karayannis and 

Sirkelis (2005) and Saha and Meesaraganda (2018) 

conducted experimental investigation on RC beam-

column joint subjected to cyclic load. These studies 

mentioned that the use of RSR enhanced the shear 

capacity of the joint. It was also observed that the crack 

propagation through the joint was delayed. Athira and 

Remya (2017) numerically investigated beam-column 

joint subjected cyclic load and mentioned that spirally 

tied beam-column joint had exhibited improved energy 

absorption capacity.  

However, studies focused on the load carrying 

behavior of square RC column confined by RSR and 

subjected to axial loading were not or rarely available 

yet. Hence, study had focused on the confinement effect 

of continuous spiral lateral reinforcement on the stress 

and deformation capacity of square short RC column 

subjected to axial load.     

The continuous configuration of rectangular spiral 

reinforcement expected to eliminate the problem related 

to non-uniformity of confinement along the length of 

square/rectangular RC column. Therefore, the main 

objective of this research was focused on the 

investigation of the effect of spiral lateral reinforcement 

on the strength and deformation capacity of square short 

RC column subjected to axial load. In addition, the 

effect of considerable factors affecting RC column 

confinement including the amount and spacing of 

traverse reinforcement was investigated. Finally, the 

enhancement in load carrying capacity and deformation 

capacity due to rectangular spiral configuration was 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

21 

examined in comparison with the conventional (discrete 

hoop) configuration of the lateral reinforcement. 

2. Materials and Methods 

2.1. Materials  

Two materials, concrete and reinforcing bar were 

required to model the RC column in the computer 

program (Abaqus). These two materials were assembled 

together and a model of RC column was created in the 

analysis program.  

The mechanical property of the materials, their 

interaction, load and boundary condition were simulated 

to reflect the actual as constructed RC column. The 

mechanical properties of both materials were taken from 

national code mainly ES EN 1992-1-1: 2013, from 

manuals of analysis program and research papers. 

ES EN 1992-1-1: 2013, classifies concrete into 

several strength classes based on 28 days characteristic 

compressive cylinder strength. Table-2 summarizes the 

mechanical properties of concrete and steel reinforcement 

that were used for modeling specimens in this study. 

Uniaxial compressive stress–strain relationship was 

obtained from the analytical model recommended by ES 

EN 1992-1-1: 2013. The non-linear quasi-brittle 

mechanical behavior of concrete was defined according 

to concrete damage plasticity (CDP) model.  

The elastic nature of both tensile and compressive 

behavior is defined by modulus of elasticity (Ecm) and 

poisons ratio (ν=0.16) (Abaqus get started user’s 

manual, 2014). The inelastic behavior is obtained by 

subtracting the elastic behavior from the total uniaxial 

stress–strain relationship (Abaqus user’s manual, 2014). 

The stiffness degradation beyond the elastic range was 

characterized by tension (dt) and compression damage 

(dcm) variables (Abaqus user’s manual, 2014).  

The CDP model in abaqus defines yield criteria using 

plastic strain in compression and tension (Abaqus user’s 

manual, 2014). Other parameters related to concrete 

flow properties including eccentricity, the ratio of equi-

biaxial compressive yield stress to uniaxial compressive 

yield stress (Kc) and the ratio of stress invariant on 

tensile to stress invariant on compression (σbo/σco) 

(Table 1) were taken from abaqus-6.14 user’s manual 

(Abaqus user’s manual, 2014). Others parameters 

including poison’s ratio, dilatation angle and viscosity 

parameter were determined based on convergence and 

validation test and were summarized in (Table 2). 

Table 1: Flow parameters in concrete damage plasticity 

model 

Dilatation 

angle  

Kc 𝜎𝑏𝑜 /𝜎𝑐𝑜   Eccentricity 

(𝜀) 

Viscosity  

31o 0.67 1.16 0.1 0.0001 

Table 2: Properties of material used 

. Material Yield 

strength 

(MPa) 

Diameter 

(mm) 

Modulus of 

elasticity 

(Gpa) 

Poison’s 

ratio 

1 Concrete 25 - 31.5 0.16 

2 Steel 
Long. Rein. 400 16 200 0.3 

Lat. Rein. 400 6-10 200 0.3 

The yield strength (fy) = 400 𝑀𝑝𝑎 was considered 

for both lateral and longitudinal steel reinforcement. 

The nominal elastic stress–total strain relation was 

determined using the analytical expression from EN 

1992-1-2:2004. The elastic behavior up to the yield 

stress is defined by specifying the modulus of elasticity, 

𝐸𝑠 = 200 𝐺𝑃𝑎 and 𝜈 = 0.3 for poison’s ratio. The 

inelastic behavior was obtained by subtracting the 

elastic behavior from the total uniaxial stress–strain 

relationship (Abaqus user’s manual, 2014). 

2.2. Method  

The investigation was conducted through the use of a 

computer program, Abaqus. In this work pin supported 

short, square RC columns subjected to axial load were 

modeled and simulated in the Abaqus/Standard analysis 

product. And the results of analysis were used to 

investigate the effect of the study variables namely 

amount, spacing and configuration of lateral reinforcement 

on load carrying and deformation capacity of RC column. 

A total of thirty one specimens were modeled (Figure 

1) and submitted into computer program namely Abaqus 

and the result of all thirty one specimens were 

considered in the investigation. All specimens had the 

same size dimension 200 x 200 x 1000 mm. The 

concrete cover was provided according to the provision 

of EN 1992-1-1:2004 for minimum concrete cover. 

Accordingly 25 mm thick concrete cover was provided 

on each sides of the RC column. The middle region 

extending over 250 mm length was identified as test 

region where the remaining top and bottom was 

strengthened by doubling the amount of reinforcement 

to avoid premature failure. 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

22 

 

Figure 1: Typical model of investigated RC columns  

 

Figure 2: Typical cross-section of investigated RC 

columns

Table 3: Summary and detailed data of tested specimens 

S. No. Category Specimen Lat. rein. 

configuration 

Lat. rein. spacing 

(mm) 

s/dc ratio Amount of lat. rein. 

(ρS) in (%) 

Bar dia. of lLat. 

rein.(mm) 

1 Group 1 

R50Am14 Hoop tie 50 0.33 

1.4 

6 

R90Am14 " 90 
0.6 

8 

R140Am14 " 140 
0.93 

10 

S50Am14 Spiral 
50 

0.33 
6 

S90Am14 " 90 
0.6 

8 

3 Group 2 

R45Am16 Hoop tie 45 0.3 

1.6 

6 

R60Am16 " 60 
0.4 

7 

R80Am16 " 80 
0.53 

8 

R120Am16 " 120 
0.8 

10 

S45Am16 Spiral 
45 

0.3 
6 

S60Am16 " 60 
0.4 

7 

S120Am16 " 120 
0.8 

120 

3 Group 3 

R40Am18 Hoop tie 40 0.27 

1.8 

6 

R60Am18 " 60 0.4 7.4 

R80Am18 " 80 0.53 8 

R110Am18 " 110 0.73 10 

S40Am18 Spiral 40 0.23 
6 

S60Am18 " 60 0.42 7.4 

S110Am18 " 110 0.73 

33 
10 

4 Group 4 

R35Am20 Hoop tie 
35 

0.23 

2.0 

6 

R60Am20 " 60 
0.4 

8 

R100Am20 " 100 
0.67 

10 

S35Am20 Spiral 
35 

0.23 
6 

S60Am20 " 60 
0.4 

8 

S100Am20 " 100 
0.67 

10 

5 Group 5 

R30Am23 Hoop tie 
30 

0.2 

2.3 

6 

R55Am23 " 55 
0.37 

8 

R85Am23 " 85 
0.57 

10 

S30Am23 Spiral 
30 

0.2 
6 

S55Am23 " 55 
0.37 

8 

S85Am23 " 85 
0.57 

10 

Core depth (dc = 150mm) 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

23 

The typical cross-section for all investigated RC 

column specimens was given in Figure 2 and for all 

columns 16 mm diameter longitudinal bars were 

assumed in assembling the column model. At a time all 

variables reasonably were kept constant except the 

variable to be studied. The investigated column specimens 

vary either in amount, spacing or configuration of the 

lateral reinforcement. In assembling most columns, 

commonly known diameters of lateral reinforcement 

(Table 3) were used but, on few columns where particular 

spacing of lateral reinforcement was needed for 

comparison, unfamiliar diameters of lateral reinforcement 

were assumed for analysis purpose. 

Sheikh and Toklucu (1993) mentioned that for amount 

of lateral reinforcement ≤ 1% and ≥ 2%, changing the 

spacing of lateral reinforcement had no significant effect 

on stress– strain relationship of axially loaded RC column. 

In this research, in order to have enough data for 

comparative investigation, the columns with five different 

amounts of lateral reinforcement including (1.4%, 1.6%, 

1.8%, 2% & 2.3%) were investigated. 

The spacing of lateral reinforcement was determined 

using equation 1 and it is a function of amount of lateral 

reinforcement (ρs), core depth of the column cross-section 

(dc) and cross-sectional area of the lateral reinforcement 

(as). At a time one amount of lateral reinforcement (ρs) was 

selected and kept constant, and since the cross-section of 

all columns was constant, the spacing of lateral 

reinforcement was varied through altering the diameter 

(db) or cross-sectional area (as) of the lateral reinforcement. 

𝑆𝑝𝑎𝑐𝑖𝑛𝑔 (𝑠) =
4𝑎𝑠(𝑑𝑐 − 𝑑𝑏 )

𝜌𝑠 𝑑𝑐
2

… … … … . (1) 

The name of model specimens (Table-3) follows from 

spacing, configuration and amount of reinforcement. For 

instance, in designation of specimen R40Am18: R stands 

for rectangular hoop configuration, 40 stands for spacing in 

mm and Am18 indicate the amount of lateral reinforcement 

in percent (ρS = 1.8%). Similarly, in designation of specimen 

S60Am18: S stands for continuous spiral configuration, 60 

stands for spacing in mm and Am18 indicate the amount of 

reinforcement in percent (ρS = 1.8%). 

3. Result and Discussion 

Specimens with different amount of reinforcement 

and spacing had been modeled and analyzed using the 

computer analysis program- Abaqus 6.14. The analysis 

result for different comparable RC column models were 

contrasted with each other, mainly based on the 

maximum stress developed on the test region of RC 

column, the strain at maximum stress, and overall non-

linear stress–strain curve behavior. In general, the 

investigated models could be identified as model with 

continuous spiral and hoop tie lateral reinforcement 

based on the lateral reinforcement configuration. 

As it will be discussed in the proceeding section, 

varying the spacing of lateral reinforcement for the amount 

of reinforcement (ρS) ranging from 1.6% − 2% had a 

significant effect on stress-strain behavior of RC column 

particularly on post peak stress-strain curve behavior. 

3.1. Effect of lateral reinforcement spacing 

The stress-strain diagram (Figure 3 through 12) show 

the effect of lateral reinforcement spacing with respect 

to the amount of lateral reinforcement. Reducing the 

spacing almost by half such that from 90 mm to 50 mm 

had no significant effect on stress carrying and 

deformation capacity of both (R90Am14 and R50Am14) 

tested columns, indeed the stress–strain curve of each 

column overlap over each other (Figure 3) and both 

columns attain peak stress at the same strain (Table 4).  

Similarly, in spiral tied specimen with amount of 

lateral reinforcement 1.4%, column S50Am14 and 

S90Am14 where the lateral reinforcement spacing was 

50mm and 90mm respectively, both had nearly the same 

peak stress (Figure 4) and attained the peak stress at the 

same strain. Thus, reducing the spacing between lateral 

reinforcement for specimens with amount of 

reinforcement 1.4% had insignificant improvement on 

the stress and deformation capacity of tested confined 

concrete RC column specimens. The effect of lateral 

reinforcement spacing was more significant on columns 

with amount of reinforcement in the range of 1.6-2%. 

 

 

 

 

 

 

 

 

 

Figure 3: Stress-strain curve for hoop tied specimen (ρS 

= 1.4%) 

0.0

10.0

20.0

30.0

40.0

0 0.005 0.01 0.015 0.02 0.025
Strain

R50Am14

R90Am14

R140Am14

S
tr

e
ss

 (
M

p
a

) 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

24 

0

10

20

30

40

0 0.005 0.01 0.015 0.02 0.025

S
tr

e
ss

 (
M

p
a

)

Strain

R45Am16 R60Am16

R120Am16 R80Am16

0.0

10.0

20.0

30.0

40.0

0 0.005 0.01 0.015 0.02 0.025

Strain    

S60Am18

S40Am18

S110Am18

S
tr

e
ss

 (
M

p
a

) 

 

 

 

 

 

 

 

Figure 4: Stress-strain curve for spirally tied specimen 

(ρS = 1.4% 

 

 

 

 

 

 

 

 

Figure 5: Stress-strain curve for hoop tied specimen (ρS 

= 1.6%) 

 

 

 

 

 

 

 

 

Figure 6: Stress-strain curve for spirally tied specimen 

(ρS = 1.6%) 

 

 

 

 

 

 

 

 

Figure 7: Stress-strain curve for hoop tied specimen (ρS 

= 1.8%) 

 

 

 

 

 

 

 

 

Figure 8: Stress-strain curve for spirally tied specimen 

(ρS = 1.8%) 

 

 

 

 

 

 

 

 

Figure 9: Stress-strain curve for hoop tied specimen (ρS 

= 2%) 

 

 

 

 

 

 

 

 

Figure 10: Stress-strain curve for spirally tied specimen 

(ρS = 2% 

 

 

 

 

 

 

 

Figure 11: Stress-strain curve for hoop tied specimen 

(ρS = 2.3%)

S
tr

e
ss

 (
M

p
a

) 

0

10

20

30

40

0 0.01 0.02 0.03Strain

S50Am14

S90Am14

S
tr

e
ss

 (
M

p
a

) 

0

10

20

30

40

0 0.005 0.01 0.015 0.02 0.025

Strain 

S60Am16

S45Am16

S120Am16

0.0

10.0

20.0

30.0

40.0

0 0.01 0.02 0.03
Strain

R40Am18 R60Am18

R110Am18 R80Am18S
tr

e
ss

 (
M

p
a

) 

0.0

10.0

20.0

30.0

40.0

0 0.005 0.01 0.015 0.02 0.025

Strain

R60Am20

R100Am20

R35Am20
S

tr
e
ss

 (
M

p
a

) 

0.0

10.0

20.0

30.0

40.0

0 0.01 0.02 0.03Strain

S100Am20

S60Am20

S35Am20

S
tr

e
ss

 (
M

p
a

) 

0.0

10.0

20.0

30.0

40.0

0 0.005 0.01 0.015 0.02 0.025Strain

R55Am23

R85Am23

R30Am23

S
tr

e
ss

 (
M

p
a

) 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

25 

Table 4: Summary of values on the stress-strain curve for specimens (ρS = 1.4%) 

S. No. Specimen name Max. stress 

(MPa) 

Strain at max 

stress 

Strain at 85% of max 

stress 

Strain ductility 

ratio 

1 R50Am14 34.76 0.0024 0.0165 6.9 

2 R90Am14 34.46 0.0024 -- -- 

3 R140Am14 33.66 0.0023 0.0062 2.7 

4 S50Am14 34.35 0.00234 0.0115 4.9 

5 S90Am14 34.44 0.00234 -- -- 

-- = stress capacity not reduced to 85% of the maximum stress

 

 

 

 

 

 

 

 

Figure 12: Stress-strain curve for spirally tied specimen 

(ρS = 2.3%) 

As summarized in Table 5, column specimens 

(R45Am16, R60Am16 and R120Am16) had nearly the 

same peak/maximum stress, however indispensable 

enhancement was observed on the strain at which the 

peak stress was attained. Specimen R60Am16 attain 

peak stress at 12.5% higher strain compared to strain at 

peak stress in specimen R120Am16 respectively. 

Similarly, in spirally tied specimen the peak stress for 

specimen S60Am16 was 2.3% higher compared to the 

peak stress for specimen S120Am16. Regarding the 

deformation capacity, the strain at peak stress for 

specimen S60Am16 was 54.5% higher (Table 5) than 

the strain at peak stress for specimen S120Am16. 

In specimen under Group-3, with 1.8% lateral 

reinforcement, reducing the spacing from 110 mm in 

specimen R110Am18 to 60mm in specimen R60Am18 

resulted in 2% higher peak stress and offset the strain at 

peak stress by 12.5% (Table 6) compared to that in 

specimen R110Am18. Similarly, in spirally tied 

specimen, the peak stress and strain at peak stress in 

specimen S60Am18 was 2.6% and 12.5% higher (Table 

6) compared to that in specimen S110Am18, respectively.

Table 5: Summary of values on the stress-strain curve for specimens (ρS = 1.6%) 

S. No. Specimen name Max. stress 

(MPa) 

Strain at max. stress Strain at 85% of 

max. Stress 

Strain ductility ratio 

1 R45Am16 34.92 0.00244 0.0092 3.77 

2 R60Am16 35.08 0.0026 0.0115 4.423 

3 R80Am16 34.77 0.00245 0.0143 5.84 

4 R120Am16 34.63 0.00231 0.0189 8.2 

5 S45Am16 34.94 0.00254 0.0082 3.23 

6 S60Am16 35.2 0.00377 0.0087 2.31 

7 S120Am16 34.4 0.00244 0.0132 5.41 

Table 6: Summary of values on the stress-strain curve for specimens (ρS = 1.8%) 

S. No. Specimen name Max. stress 

(MPa) 

Strain at max stress Strain at 85% of max 

stress 

Strain ductility 

ratio 

1 R40Am18 35.25 0.0025 0.0106 4.24 

2 R60Am18 35.4 0.0027 0.01 3.7 

3 R80Am18 35.06 0.0024 0.0097 4.04 

4 R110Am18 34.73 0.0024 0.0093 3.87 

5 S40Am18 34.35 0.00234 0.0084 3.6 

6 S60Am18 35.38 0.0027 0.0083 3.07 

7 S110Am18 34.5 0.0024 0.0094 3.92 

0.0

10.0

20.0

30.0

40.0

0.00 0.01 0.01 0.02 0.02 0.03

Strain

S30Am23

S85Am23

S55Am23

S
tr

e
ss

 (
M

p
a

) 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

26 

The same trend was observed on columns with 2.0% 

lateral reinforcement, the peak stress in specimen 

R60Am20 was 2.2% higher compared to peak stress in 

R100Am20 and the strain at peak stress was enhanced 

by 10% (Table 7) than that in specimen R100Am20. 

Similarly, in spirally tied specimen S60Am20 with 

reduced lateral reinforcement spacing had 2.6% higher 

peak stress capacity than that of comparable specimen 

S100Am20 (Table 7) and offset the strain at peak stress 

by 54% compared to that in specimen S100Am20. 

In case of specimens with amount of reinforcement 

2.3%, the peak stress in specimen R55Am23 was 0.8% 

higher than the peak stress in specimen R85Am23, and 

the strain at peak stress was enhanced by 8.3% (Table-

8) compared to the strain at peak stress in specimen 

R85Am23. Similarly, in spirally tied specimen both 

(S55Am23 and S85Am23) had equal peak stress and the 

strain at peak stress for specimen S55Am23 was 7% 

(Table 8) higher than that in specimen S85Am23. 

Clearly the enhancement of stress and deformation 

capacity through reducing lateral reinforcement was 

lower in specimens with 2.3% lateral reinforcement 

compared to the enhancement on specimens included in 

Group 2 - 4 with 1.6% - 2% reinforcement. On the other 

hand, reducing the lateral reinforcement spacing beyond 

certain limit had no significant effect on the stress 

carrying and deformation capacity of confined concrete. 

As shown in Table 5, specimens R60Am16 with wider 

spacing had relatively higher stress capacity and the 

strain at peak stress was 6.5% higher compared to 

specimen R45Am16 which had close spacing of lateral 

reinforcement.  

The same result was observed on columns with 1.8% 

lateral reinforcement. The stress-strain diagram (Figure 

7 & 8) for specimen R60Am18 and S60Am18 with 

wider spacing than R40Am18 and S40Am18 

respectively had exhibited similar stress strain curve, 

indeed specimen R60Am18 and S60Am18 attain peak 

stress at comparatively larger strain (Table 6). Thus, 

reducing the lateral reinforcement spacing beyond 

certain limit such that s/dc ratio less than (≤0.35) 

couldn’t enhanced the stress and deformation capacity 

of tested columns with both configurations.  

The results observed on columns with 2.0% and 

2.3% lateral reinforcement bear a witness to the same 

conclusion. As shown in Table 7, specimens R60Am20 

and S60Am20 with wider spacing of lateral 

reinforcement had the same peak stress capacity 

compared to the corresponding specimens R35Am20 

and S35Am20, respectively. Indeed, the strain at peak 

stress in specimen S60Am20 and R60Am20 was 4% 

and 8% higher compared to the peak stress in S35Am20 

and R35Am20, respectively. 

Actually, specimen S30Am23 had 10% and 70% 

higher peak stress and strain at peak stress, respectively 

(Table-8) compared to that of specimen S55Am23. 

However, the rapid strength degradation on the falling 

branch of the stress-strain curve reduced the 

significance of reducing the lateral reinforcement in 

columns with the amount of reinforcement 2.3%. 

Hence, the selection of lateral transverse reinforcement 

spacing should consider the important outcome due to 

redistribution of forces through deformation. 

It was noted that, specimens R80Am16 and 

R80Am18 with lower spacing (s/dc = 0.53) had 

exhibited no significant improvement on load carrying 

and deformation capacity relative to comparable 

specimens R120Am16 (s/dc = 0.8) and R110Am18 (s/dc 

= 0.73), respectively. Thus, more reduction of the lateral 

reinforcement could not cause significant improvement 

on stress and deformation capacity of the investigated 

column specimens. Therefore, an optimum combination 

of spacing and amount of lateral reinforcement had to 

be provided to improve the load carrying and 

deformation capacity of axially loaded RC column.    

Referring to the cases observed on specimen with 

lateral reinforcement of 1.4%, 1.6%, 1.8%, 2% and 

2.3%, reducing the spacing of lateral reinforcement had 

significant effect on the stress and deformation capacity 

such that, for the amount of reinforcement in the range 

of 1.6% - 2 percent. Reducing the spacing where the 

amount of reinforcement was in the mentioned range, 

improved the peak stress capacity, on average by 2.3% 

and enhanced the strain at the peak stress, on average, 

by 12% and 40% in hoop tied and rectangular spiral tied 

square RC column specimens respectively. The spacing 

to depth (s/dc) ratio of these specimens was existed in 

the range of 0.4-0.5.  

Clearly, the rectangular spiral configuration of 

lateral reinforcement had contributed to the observed 

higher deformation capacity of investigated confined 

RC column specimens. Spiral configuration of the lateral

 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

27 

Table 7: Summary of values on the stress-strain curve for specimens with (ρS = 2%) 

S.  No. Specimen name Max. stress 

(MPa) 

Strain at max. stress Strain at 85% of 

max. stress 

Strain ductility 

ratio 

1 R35Am20 35.56 0.00245 0.0081 3.3 

2 R60Am20 35.7 0.00265 0.0092 3.5 

3 R100Am20 34.88 0.0024 0.0094 3.92 

4 S35Am20 35.6 0.00356 0.0075 2.07 

5 S60Am20 35.84 0.0037 0.0077 2.08 

6 S100Am20 34.64 0.0024 0.0097 4.04 

Table 8: Summary of values on the stress-strain curve for specimens with (ρS = 2%) 

S. No. Specimen name Max. stress 

(MPa) 

Strain at max. stress Strain at 85% of 

max stress 

Strain ductility 

ratio 

1 R30Am23 36.0 0.00367 0.0077 2.88 

2 R55Am23 35.8 0.0026 0.0097 3.5 

3 R85Am23 35.5 0.0024 0.0082 3.4 

4 S30Am23 36.2 0.0041 0.0073 2.07 

5 S55Am23 35.87 0.0026 0.0082 3.15 

6 S85Am23 35.4 0.00243 0.0083 3.4 

 

reinforcement had favorable inclination to delay the 

inclined crack propagation that was likely to ensue after 

enough disintegration of the core concrete had taken 

place. To the effect, comparatively extended yielding 

plateau was observed on the stress-strain curve (Figure 

6, 8 & 10) plotted for spirally tied columns. 

Clearly, the effect of reducing the spacing of the 

lateral reinforcement was more significant on enhancing 

the deformation capacity than improving the strength of 

confined concrete. Several studies (Rechart et. al, 1929; 

Saaticoiglu and Razvi, 1992; Mohammadreza and 

Mohammadehsan, 2017) mentioned that the conception 

of failure in confined concrete begins at the same level 

of load and strain to that of plain concrete.  

This implied that the confinement through the lateral 

reinforcement became effective after enough disintegration 

of core concrete had taken place. However, due to 

confinement the disintegration of core concrete was 

delayed and the concrete member was enabled to sustain 

maximum stress while undergoing degrading strain 

deformation.  

The stress-strain diagram plotted on (Figure 3 

through to 12) had confirmed the same observation. 

Prior to the attainment of the maximum stress, columns 

with different lateral reinforcement spacing had 

overlapping stress-strain relationship and exhibited their 

respective unique path on the descending branch of the 

stress strain curve.  

Thus, the effect of the confinement was observed on 

the descending branch of the stress strain diagram such 

that the lateral reinforcement became effective after the 

attainment of the maximum stress capacity of the core 

concrete. Hence, once the maximum stress was attained, 

the increment on the stress capacity was no more 

significant and the enhancement on deformation 

capacity, as a result of adequate confinement was more 

pronounced than the improvement on stress carrying 

capacity of the investigated columns. 

3.2. Buckling Mode of Longitudinal Reinforcement 

and Spacing of Lateral Reinforcement 

Several studies (Sheikh and Yeh, 1990; Rajput and 

Sharma, 2018) had mentioned that the disintegration of 

core concrete and buckling of the longitudinal bar were 

the major causes for failure of reinforced concrete column 

member subjected to higher loads. The buckling modes 

of longitudinal reinforcement for various representative 

column specimens were shown in Figure 13 through to 18. 

Invariably, Sheikh and Yeh (1990) and Sheikh and 

Toklucu (1993) mentioned that under applied axial load, 

unreasonably close spacing of lateral reinforcement 

tends to temporarily restrain buckling of the longitudinal 

reinforcement. At the same time the ensued confinement 

had restrained the expansion of the core concrete up to 

higher concrete strain deformation leading to sudden 

and localized buckling of longitudinal reinforcement 

(Sheikh and Yeh, 1990).  



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

28 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This case was more clearly observed on column 

specimens with 2.3% lateral reinforcement where the 

spacing of the lateral reinforcement was in such a way 

that s/dc was ≤ 0.35. Accordingly, in column specimen 

S30Am23, the closer spacing of lateral reinforcement 

and relatively higher amount of lateral reinforcement 

had prevented gradual buckling of longitudinal bar and 

limited the buckling to few spacing (Figure 14) of 

transverse bar (localized failure). Testifying the same 

observation, the stress-strain curve (Figure 12) behavior 

for column specimen S30Am23 had exhibited rapid 

strength degradation on the falling branch of the curve. 

On the other hand, in column specimens where 

amount of reinforcement was ≥1.6% and the spacing of 

transverse bar was in such a way that s/dc ratio was ≥0.5, 

buckling of longitudinal reinforcement at relatively 

lower concrete strain had reduced the stress and 

deformation capacity of confined concrete. As a result 

Figure 13: Undeformed shape of the reinforcement 

embedded in specimen S30Am23 

Figure 14: Deformed shape of the reinforcement 

embedded in specimen S30Am23 

Figure 15: Undeformed shape of the reinforcement 

embedded in specimen R60Am18 

Figure 16: Buckling mode of the reinforcement 

embedded in specimen R60Am18 

Figure 17: Undeformed shape of the reinforcement 

embedded in specimen R120Am16 

Figure 18: Buckling mode of the reinforcement 

embedded in specimen R120Am16 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

29 

the peak stress in these specimens was attained at strain 

less than 0.24% (Table 4-8) which was relatively lower 

strain compared to the strain of specimen with the same 

amount but, with closer spacing of lateral reinforcement.  

In these specimens, buckling of longitudinal 

reinforcement was due to the lack of support at 

relatively lower concrete strain of the core concrete. 

Conformably, the mode of buckling of the longitudinal 

reinforcement (Figure 18) for column specimen 

R120Am16 (s/dc ≥ 0.5) had clearly demonstrated that 

buckling of longitudinal reinforcement was appeared 

over the length where no lateral bars support was 

provided. Thus, inadequate support for longitudinal bar 

failed to delay severe buckling of longitudinal 

reinforcement leading to reduction of load carrying 

capacity of RC column member.  

In contrast buckling of longitudinal reinforcement 

that was observed in Figure-16 range over several 

spacing of lateral reinforcement and hence, gradual 

buckling of longitudinal reinforcement. Consequently, 

column specimens in Group 2 - 4, where the ratio of the 

spacing to core concrete dimension (s/dc) was in the 

range of 0.4 − 0.5, had exhibited enhanced overall 

stress -strain curve behavior for confined concrete. 

Accordingly detailed specimens in Group-2 (Figure 5 

and 6), Group-3 (Figure 7 and 8) and Group-4 (Figure 9 

and 10) had exhibited stress-strain curve with 

considerable yielding plateau, particularly spirally tied 

specimens had extended yielding plateau after the 

attainment of the maximum stress. 

3.3. Strain ductility 

In this study, the strain ductility ratio was calculated 

from the ratio of the strain corresponding to 85% of the 

peak stress on the descending branch to the strain at 

peak stress (Table 4-8) for each investigated RC column 

specimen. As it could be observed the conclusion based 

on the strain ductility alone would lead to 

misinterpretation of the analysis result. For instance, 

specimen S60Am20 and S100Am20 in Group-4 have 

2.1 and 4 strain ductility, respectively. 

Thus, specimen S100Am20 with higher strain 

ductility expected to have higher stress sustaining 

performance and better stress-strain relationship 

compared to specimen S60Am20. However, specimen 

S60Am20 has 3.5% higher stress capacity and the strain 

at peak stress in specimen S60Am20 was 54% higher 

than that of specimen S100Am20. In addition, at their 

respective strain at 85% of the peak stress on the 

descending branch both specimens possess nearly equal 

strain capacity. Thus, the higher strain ductility 

observed on specimen S100Am20 did not necessarily to 

mean the specimen had better stress and deformation 

capacity. 

4. Conclusion and Recommendation  

Axially loaded confined RC column specimens with 

s/dc ratio in the range of 0.4–0.5 and amount of 

transverse reinforcement in the range of 1.6% – 2% had 

exhibited better non-linear stress-strain curve behavior. 

Accordingly, confined RC columns had enhanced stress 

and deformation capacity. The stress-strain for these 

columns had exhibited yielding plateau observed after 

the attainment of the maximum stress such that the 

column had the capacity of sustaining maximum stress 

while undergoing strain deformation.  

Accordingly, detailed spirally tied square RC columns 

had 2.6% and 40% higher stress and deformation 

capacity than comparable tested RC column specimens. 

Whereas, detailed hoop tied RC columns had 2.3% and 

12% higher stress and deformation capacity than 

comparable tested RC column specimens. Clearly, in 

both cases the enhancement on deformation capacity, due 

to adequate confinement was more considerable than the 

improvement on stress carrying capacity of the confined 

concrete. It was also noted that the rectangular spiral 

configuration of the lateral reinforcement had more 

significant enhancement on the axial deformation 

capacity compared to the enhancement due to rectangular 

hoop tie configuration.  

Lower amount of lateral reinforcement (≤ 1.4%) 

failed to provide sufficient confinement to core concrete 

as a result; the peak stress was attained at comparatively 

low strain. On the other hand, column specimens with 

higher amount of lateral reinforcement (≥ 2.3%) where 

the spacing was provided in such way s/dc ratio was ≤ 

0.35 had experienced over confinement that hindered 

gradual buckling of longitudinal reinforcement. As a 

result, sudden and localized buckling of longitudinal 

reinforcement had caused rapid strength degradation of 

confined concrete as such exhibited on the falling 

branch of stress-strain diagram of accordingly detailed 

column specimens.  

Whereas, in RC column members where tie spacing 

exceeded half (s/dc ≥ 0.5) of the core dimension, the 

peak stress capacity of confined concrete was attained at 



Thomas Belete et al.                                                                                                        Ethiop.J.Sci.Sustain.Dev., Vol. 8 (2), 2021 

30 

early stage of concrete strain (nearly similar to plain 

concrete). In this case the major cause for the immediate 

reduction of stress capacity that was observed after the 

attainment of the peak stress was due to the lack of 

adequate lateral ties that could delay the buckling of 

longitudinal bars.  

By contrast, in column specimens with amount of 

transverse reinforcement in the range of 1.6%–2%, 

where the spacing was in such a way s/dc ratio was in 

the range of 0.4-0.5, the buckling of the longitudinal 

reinforcement had taken over comparatively extended 

length and hence accounted for gradual loss of the load 

carrying capacity of accordingly detailed column 

specimens. 

Rectangular spiral configuration of lateral 

reinforcement could be considered as a better alternative 

particularly, where deformation capacity of RC column 

is the main concern. In either (hoop or spiral) case of 

lateral reinforcement configuration, provided that 

adequate amount of lateral reinforcement ratio, the 

spacing of lateral reinforcement ratio should not exceed 

half of the core dimension (s/dc≤0.5) particularly where 

deformation capacity is the main importance. This 

requirement is consistent with the requirement of 

Ethiopian building code for RC column where medium 

and high ductility class column is to be constructed.  

On the other hand, for axially loaded RC column 

with ≥ 2% lateral reinforcement, spacing of lateral 

reinforcement in such a way that s/dc is ≤0.35 of core 

dimension should be avoided. RC columns detailed with 

such closer spacing had experienced over confinement 

that had caused sudden and localized buckling of 

longitudinal reinforcement, which was not a desirable 

behavior for load carrying RC columns. 

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