Int. J. Structural Engineering, Vol. 2, No. 1, 2011 35

Copyright © 2011 Inderscience Enterprises Ltd.

Experimental tests on short composite and internally reinforced concrete columns

Ahmed A. El-Barbary Faculty of Engineering, 8 Margeel Bek Street, Flat 1, Zamalek, Cairo, 11211 Egypt E-mail: structure_egypt@yahoo.com

Mahmoud T. Elmihilmy* and Wahba W. El-Tahan 7 Adel Abou Baker St., Flat 9, Zamalek, Cairo, 11211 Egypt Fax: 202-27354748 E-mail: elmihma@hotmail.com E-mail: mahmoud7@link.net *Corresponding author

Abstract: Experimental investigation of reinforced concrete composite columns behaviour was conducted. Eight rectangular reinforced concrete columns with different parameters were tested under axial loads and biaxial bending. Four columns were internally reinforced with I-section structural steel and the other four were heavily reinforced with reinforcement bars with an equivalent area to the I-section. The variables considered in the study were:

1 the eccentricity of the applied force 2 the ratio of Mx and My.

The main goal of this research was to compare the behaviour of the two groups. The results of the tests indicated that the experimental loads for internally reinforced columns were about 82% of that for the composite columns. A computer program was developed based on the ACI provisions to calculate the theoretical capacity of the tested columns. All the experimental failure loads were greater than the predicted theoretical values by an average value of 84%.

Keywords: reinforced concrete columns; experimental tests; composite columns; axial loading; biaxial bending; ACI code.

Reference to this paper should be made as follows: El-Barbary, A.A., Elmihilmy, M.T. and El-Tahan, W.W. (2011) ‘Experimental tests on short composite and internally reinforced concrete columns’, Int. J. Structural Engineering, Vol. 2, No. 1, pp.35–49.

Biographical notes: Ahmed A. El-Barbary is an Assistant Lecturer, Faculty of Engineering, 10th of Ramadan Institute, Egypt.

36 A.A. El-Barbary et al.

Mahmoud T. Elmihilmy is an Associate Professor, Structural Engineering Department, Faculty of Engineering, Cairo University, Egypt. He received his PhD from Auburn University, USA. His research interest includes reinforced concrete members including concrete long columns, deep beams and beams strengthened with fibre reinforced polymer (FRP).

Wahba W. El-Tahan is an Associate Professor, Structural Engineering Department, Faculty of Engineering, Cairo University, Egypt. He received his PhD from Ohio State University, USA. His research interest includes fracture mechanics, reinforced and pre-stressed concrete structures.

1 Introduction

Composite columns are becoming very effective structural element for high-rise buildings throughout the world due to several advantages such as:

1 significant saving in material and construction time

2 smaller cross section and higher strength to weight ratio than conventional reinforced concrete columns

3 inherent ductility and toughness that can be useful in resisting lateral loads

4 higher load carrying capacity due to the composite action of steel and concrete. Furthermore the confinement of the outer shell in case of in-filled columns increases the compressive strength of concrete.

Figure 1 Types of composite columns (a) internal I-beam + steel reinforcement and (b) external pipe + steel reinforcement

Structural steelI-beam

Circular stirrupsor spiral reinforcement

(a) (b)

Composite columns can also include concrete filled into fibre reinforced plastics (FRP) shell or into steel pipe as shown in Figure 1. The ACI-08 defines composite columns as compression members reinforced longitudinally with one of the following:

Experimental tests on short composite 37

a concrete surrounding structural steel core

b structural steel confining concrete core.

Experimental work on composite columns has been performed by several numbers of researchers (El-Noury and Chen, 1982; Cusson and Paultre, 1994; Hsu et al., 1995; Azizinamini and Ghosh, 1997; Cusson and Paultre, 1994; Diniz and Frangopol, 1997; Wang and Hsu, 1992; Cedolin et al., 2008). Muñoz and Hsu (1997a, 1997b) tested four square encased composite columns under biaxial loading. One of the specimens was a short column while the others were long. Each specimen consisted of structural steel I-shape section encased by concrete and additionally reinforced with four longitudinal steel corner bars. Shams and Sadeghvaziri (1997) tested concrete filled steel tube which showed excellent earthquake resistance. Taylor et al. (1998) presented the results of tests performed on composite columns subjected to axial load and uniaxial bending.

Khayat et al. (2001) studied heavily reinforced columns cast with normal strength concrete with reinforcing steel ratio of 3.6% where steel bars distributed around the column perimeter. The authors concluded that it is recommended to distribute the vertical bars on the outer perimeter of the column and the inside core of the column to facilitate concrete casting. Doval et al. (2001) studied circular columns filled with concrete in FRP shells. This structural system improves the ultimate resistance and confinement of the concrete core.

2 Test program

The experimental program consists of eight short reinforced concrete columns. The columns were divided into two groups. The first group is composite columns and is called (C) group. The second group is columns with reinforcement bars and is called (R) group. The parameters investigated in theses tests are:

1 eccentricity of the applied load (e/t = 0.1 to e/t = 0.72).

2 ratio of Mx to My (θ = 32° to θ = 65°). Table 1 Details of tested columns

Column group

Cross-sec. (mm)

Height* (mm) Outer (rft) Inner (rft) Lateral ties

(R) 250 × 250 1,600 8 φ 12 2φ32 + 4φ18 8 φ 10/m\ (C) 250 × 250 1,600 8 φ 12 B.F.I.B No. 100 8 φ 10/m\

Note: *not including corbels.

Table 2 Properties of the structural steel (B.F.I.B)

Dimensions B.F.I.B no.

Area (cm2)

Weight (kg/m) H

(mm) b

(mm) s

(mm) T

(mm)

Ix (cm4)

Iy (cm4)

fy N/mm2

100 26 20.4 100 100 6.0 10.0 450.0 167.0 316.6

All columns were 1,600 mm long with square cross section (250 mm × 250 mm) to ensure short column behaviour. Group C was reinforced with 100 mm structural steel

38 A.A. El-Barbary et al.

(B.F.I.B) as shown in Table 1. The properties of the I-beam are shown in Table 2. An equivalent area of reinforcement steel was placed in group R. Extra stirrups were provided at column ends to avoid local failure. Corbels in columns C-3, C-4, R-3, and R-4 are provided with sufficient reinforcement to transfer the bending moments to the middle section of the columns without failing prematurely. The reinforcement details of the tested specimens are shown in Figure 2.

Figure 2 Reinforcement details of the tested specimens

4 Φ 18

4 Φ 18

2 Φ

18

2 Φ

32

2 Φ

18

3 Φ

12

2 Φ

12

3 Φ

12

4 Φ 18

8 Φ 12

2 Φ 328 Φ 12

3 Φ

12

2 Φ

12

3 Φ

12

3 Φ

12

2 Φ

12

3 Φ

12

2 Φ

32

2 Φ

18

2 Φ

18

3 Φ

12

3 Φ

12

5050

125

1255

10@ 125mm

C1&C2

10@ 125mm

R1&R220

0

C3&C4 R3&R4

2500

1600

4 Φ 18

550

550

550

250 300 250 300

1550

1600

1600

1550

1550

250

250

200

250

250

125

5050

125

125

I-Bea

m

I- Bea

m

125

d=10@ 50mm4

2

4

2

4

2

4

1

3

2

550

d=10@ 125mm

d=10@ 50mm

d=10@ 125mm

2 2

3

1

4 4

22

44

Experimental tests on short composite 39

The mechanical properties of the reinforcement bars were obtained from the results of the tension tests carried out on three steel bars specimens as shown in Table 3. Table 3 Mechanical properties of the reinforcing steel

No.

Dia (mm)

Avg. diam (mm)

Cross section (mm2)

Gage length (mm)

Yield stress (N/mm2)

Ultimate stress

(N/mm2)

Elong. (%)

1 12 12.345 119.7 120 317.476 531.355 27.5 2 18 17.950 253.1 180 336.683 502.653 29.4 3 32 32.55 832.1 320 288.416 450.650 31.6

The eccentricities of the applied load vary from 20 mm to 180 mm in the (X) and (Y) directions, as shown in Table 4. For specimens C-1, C-2, R-1, and R-2 the eccentricity was relatively small such that the eccentricity is located inside the column section. The load eccentricities were achieved by placing the specimens directly on the lower bearing plate of the testing machine as shown in Figure 3(a). For specimens (C-3, C-4, R-3, and R-4) the eccentricity was outside the cross section and two short cantilevers were constructed in two perpendicular directions as shown in Figure 3(b).

Figure 3 Application of the eccentric loading for columns with small and big eccentricities (a) eccentricity for columns (C-1, C-2, R-1, and R-2) and (b) eccentricity for columns (C-3, C-4, R-3, and R-4)

(P)

(P/2)

(P/2)

(P)"C.Gof piston"Applied load

250

550

300ex

550

300

250

ey

ex

C.G of column

ey

C.G of piston

250

250

(a) (b)

Table 4 Eccentricity of the applied load

Column no. ex (mm) ey (mm) e (mm) θ°

R-1 C-1 43 20 47.4 65.0 R-2 C-2 73.5 51.5 89.7 55.0 R-3 C-3 100 160 188.7 32.0 R-4 C-4 180 180 254.6 45.0

40 A.A. El-Barbary et al.

Normal weight concrete was used in casting all specimens. The concrete was produced using the locally available materials. For each column, three concrete cubes and three cylinders were prepared from each mix. The concrete compressive strength of the columns at the time of testing was determined by testing the corresponding cylinders and cubes. The concrete compressive strength of the tested cylinders is shown in Table 5. Table 5 Concrete compressive strength of the tested cylinders

Specimen no. Cylinder strength fc’ (N/mm2)

C1 21.5 R1 19.8 C2 22.1 R2 17 C3 19.2 R3 18.2 C4 22.4 R4 22.1 Average 22.07

3 Test setup

All specimens were tested in a hydraulically operated 5,000 kN capacity testing machine and were loaded to failure as shown in Figure 4. Concrete strains were measured by a 200 mm long mechanical demec gauges. The demec points were arranged at the mid-height of the column. Moreover, an electrical strain gauge, with a gauge length of 60 mm, was glued to the concrete after cleaning and smoothing its surface. Strains of the longitudinal reinforcement and the steel I-beam section were measured by electric strain gauges. The strain gauges were mounted on at the mid-height of the column before casting concrete. The locations of the strain gages are shown in Figure 5. Linear variable differential transformers (LVDTs) were used to measure lateral deformations of specimen. Two LVDTs were erected at mid-height of the columns to measure lateral deformations. The strain gauges and the LVDTs data were collected using strain indicator device (data logger).

The test procedure can be summarised in the following steps:

1 The specimen was placed between the machine heads.

2 The axis of applied load was centred with machine axis and the specimen was displaced to achieve the required eccentricity.

3 To avoid local failure due to stress concentration at both ends (cap and base) for columns exposed to small eccentricity (C1, C2, R1 and R2), steel bearing plates were provided.

4 All electrical strain gauges wires and LVDTs were connected to the data logger.

5 The initial readings of steel strains, concrete strains, all demec strain gauge dials and LVDTs data were taken before the beginning of the test.

Experimental tests on short composite 41

6 At every stage of loading, cracks were observed and marked, if any. To facilitate the detection of cracks in concrete, the surfaces of the columns were white painted.

7 The reading of all demec points, dial gauges and electrical strain gauges were recorded in order to calculate strains in concrete and steel.

8 Specimens were loaded using a load control system. The load was incrementally applied with an initial rate of 100 kN/min for columns subjected to small eccentricity loading and 50 kN/min for columns subjected to big eccentricity loading. Before failure (at a load of 70% of the failure ultimate load), the rate of loading was reduced to one half of the aforementioned loading rates.

Figure 4 Test setup of the specimens

42 A.A. El-Barbary et al.

Figure 5 Arrangement of electrical strain gages

C

A

B

D

E

B

A

C

E

D Type (C) Type (R)

4 Analysis of the experimental results

The results of the failure loads are summarised in Table 6. The relationships between compression and tension strain readings for steel and concrete at column mid-height and the applied load are shown in Figure 6.

Figure 6 Concrete and steel strains for the tested specimens

Experimental tests on short composite 43

Table 6 Experimental results of the tested specimens

Specimen fc’ (N/mm2) Load at first tension crack

(kN)

Experimental failure load

(kN)

Load level R = Pu/ (f'c.Ac)

C1 21.5 2,300 2,350 1.749 R1 19.8 2,000 2,025 1.636 C2 22.1 600 1,300 0.941 R2 17 500 920 0.866 C3 19.2 50 550 0.458 R3 18.2 50 410 0.360 C4 22.4 50 450 0.321 R4 22.1 50 425 0.308

Figure 7 Cracking patterns for the tested specimens

Specimen (C1) Specimen (R1)

Specimen (C2) Specimen (R2)

44 A.A. El-Barbary et al.

Figure 7 Cracking patterns for the tested specimens (continued)

Specimens (C3 and R3) Specimen (R3) Specimen (C3)

Specimens (C4 and R4) Specimen (R4) Specimen (C4)

Experimental tests on short composite 45

The failure mode of specimens C1 and R1 was compression brittle failure starting at the middle third of the column height at the extreme compression apex as shown in Figure 7. This was accompanied by a dramatic loss of load carrying capacity and spalling of small portions of concrete cover and buckling of the compression reinforcement. No cracks appear in the compression faces until the failure load for both specimens (C1and R1), while the tension cracks appear at 98% and 90% of failure load for specimen C1 and R1, respectively.

The mode of failure of specimen C2 is a balanced failure (as expected from the theoretical analysis using interaction diagrams). The failure started at the middle third of the column height at the extreme compression apex. This was accompanied by a dramatic loss of load carrying capacity and spalling of small portions of concrete cover and buckling of the compression reinforcement. Tension cracks appear at an earlier stage of loading about 50% of the failure load. These cracks were horizontal due to the presence of the vertical tensile stresses. The mode of failure of specimen R2 is different than specimen C2. A local failure was occurred for specimen R2 and failed prematurely as shown in Figure 7.

The failure mode of specimens C3 and R3 was a tension failure. It is noticed that the development of cracks at the tension apex for both specimens started to propagate at an early stage of loading (9% and 12% of the failure load for specimens C3 and R3, respectively). These cracks were horizontal due to the presence of vertical tensile stresses produced by the biaxial bending moments. The extension of the cracks continued on the tension side and extended towards the compression side, while no cracks appear in the compression faces until the failure load. Also, no cracks were developed in the base of column or in the two short cantilevers due to the presence of adequate reinforcement and bigger cross sections.

The failure mode of specimens C4 and R4 was a tension failure, where the steel bars at tension zone was yielded before concrete crushing. The development of the tension cracks started at an early stage of loading (11% and 12% of the failure load for specimen C4 and R4, respectively). The lateral deformation of specimen R4 is greater than that of specimen C4.The maximum recorded compression strain for both specimens are greater than 0.003. Although the concrete compressive strength of specimen C4 is less than specimen R4, the failure load of specimen of specimen C4 is higher than that of specimen R4.

The findings of the test results can be summarised in the following:

1 Failure experimental loads of the internally reinforced columns were less than that of the composite columns, as shown in Figure 8. It can be concluded from figure that the failure loads for columns with internal reinforcement are about 18% less than that of the composite columns. Thus, the capacity of R group is about 82% of that of group C.

2 Concrete compression strains for both composite columns and the internally reinforced columns were similar as shown in Figure 6. This reflects the similarity of the behaviour of both types in the compression zone of concrete column.

3 The usage of steel caps which surround the top and bottom of the specimens with small eccentricities provide good confinement to specimens for both types (composite and internally reinforced specimens) and prevent premature cracks to be developed.

46 A.A. El-Barbary et al.

4 Maximum concrete compression strain values increased with the increase of the applied eccentricity on both types.

5 The recorded concrete strains measured using mechanical strain gauges were approximately linear with respect to the neutral axis distance. Thus the assumption of plane section before bending remains plane after bending is valid just before concrete crushing.

6 Horizontal tension cracks and vertical compression cracks appear early for specimens with large eccentricities for both types than for specimen with small eccentricities. This because the developed tensile stresses on the tension face of specimens with large eccentricities exceeded concrete tensile strength.

7 The failure modes of all specimens can be classified into three basic modes: • sudden compression failure such as specimen C1 and R1 • balanced failure such as specimen C2 • tension failure such as C3, R3, C4 and R4.

Figure 8 Experimental failure loads for the tested specimens

5 Theoretical capacity of composite columns

The ACI-08 (ACI Committee, 2008) states that the strength of a composite member shall be computed for the same limiting conditions applicable to ordinary reinforced concrete members.

To determine the capacity of the tested columns that were subjected to axial loads and moments, interaction diagrams have to be constructed (Lloyd and Rangan, 1996; Lee and Pan, 2001; Furlong et al., 2004; Sousa and Caldas, 2005; Bajaj and Mendis, 2005). A

0

500

1000

1500

2000

2500

R1 C1 R2 C2 R3 C3 R4 C4

Composite columns

Columns with Reinforcement

Expe

rimen

tal L

oad

(kN

)

Specimen

Experimental tests on short composite 47

computer program was developed to carry out all the necessary calculations (El-Barbary, 2007). The program is based on the strain compatibility method, equilibrium of forces and the provisions of the ACI-08. The neutral axis distance and inclination was assumed and the corresponding forces in the steel bars, structural steel and concrete are evaluated as shown in Figure 9. A trial and adjustment procedure is performed by changing the neutral axis inclination and/or position until equilibrium is achieved and a point on the failure surface is obtained.

Figure 9 Calculations of the internal forces for composite columns

X

Y

s1F

s2F

s3F

s4F

s5F

s6F

0.0030.85 fc'

Cy(i)

L

L (i)

N.A Cy

Cx

Lppo

s (i)

β

ey(i)

ex(i)

IiF

Table 7 summarises the predicted ultimate loads for all tested specimens. The average ratio between the experimental values and the calculated theoretical values (Pth) is 1.84. This clearly indicates that code values are conservative. Also, it was found that the ratio of the ultimate capacity using the ACI procedure for both internally and composite columns is about 95% indicating similar performance of both types. Table 7 Experimental and theoretical loads for the tested columns

Column Experimental failure load ‘PEXP’ (kN)

Theoretical load ‘Pth’ (kN)

EXP

th

PP

C1 2,350 1,170 2.01 R1 2,025 1,161 1.74 C2 1,300 626 2.08 R2 920 574 1.60 C3 550 286 1.93 R3 410 286 1.44 C4 450 237 1.90 R4 425 210 2.02 Average 1.84

48 A.A. El-Barbary et al.

6 Conclusions

The behaviour of reinforced concrete composite columns and heavily reinforced columns was studied. To investigate such behaviour, several variables were included in the study such as: amount of eccentricity and ratio of Mx and My. To observe the effect of these variables, experimental investigation on eight rectangular columns was performed. The inertia and area of steel section used in the composite column were almost similar to those used in the internally reinforced columns with steel bars. Concrete and steel strains were recorded in addition to the lateral displacement at each load increment. Crack patterns, the mode of failure and the mechanical behaviour are also observed for each column. The average ratio of the ultimate load and experimental failure load of internally reinforced columns (R-type) and composite (C-type) is about 82%. Theoretical investigation of the tested columns was performed using strain compatibility method. The results of the study indicated that the capacity of both the composite columns and internally reinforced columns using the ACI-08 procedure is conservative.

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