Strength Enhancement of Steel Frame Filled with Concrete · 2018-01-19 · Strength Enhancement of...

© 2017. Fatema Sultana, Soumya Suhreed Das & Tushar Chowhan. This is a research/review paper, distributed under the terms of the Creative Commons Attribution-Noncommercial 3.0 Unported License http://creativecommons.org/ licenses/by-nc/3.0/), permitting all non commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Global Journal of Researches in Engineering: E Civil And Structural Engineering Volume 17 Issue 4 Version 1.0 Year 2017 Type: Double Blind Peer Reviewed International Research Journal Publisher: Global Journals Inc. (USA) Online ISSN: 2249-4596 & Print ISSN: 0975-5861

Strength Enhancement of Steel Frame Filled with Concrete

By Fatema Sultana, Soumya Suhreed Das & Tushar Chowhan Ahsanullah University of Science and Technology

Abstract- Reinforced concrete combines the benefit of steel and concrete for having both tensile and compressive strength, but due to the pressing necessity of modern lightweight constructions, further strength enhancements are necessary. This paper aims to design steel frames which will increase the compressive strength of composite concrete. Five different steel frames were designed in finite element analysis suit ABAQUS with angled bars and straight bars to check how the use of angular bars fair against straight bras under axial loads. From the analysis, it was revealed that introduction of angle section in reinforcement rather weakens the composite concrete strength. Steel frame with larger angles was relatively weaker than steel frame with smaller ones, but introduction of four angled bars with same amount of reinforcement as two angled bars derived improved the results. It has been observed that in ideal conditions, steel frame constructed with straight steel bars performs better under axial loads.

Keywords: ABAQUS, auxetic shapes, composite concrete, finite element analysis, and steel angles.

StrengthEnhancementofSteelFrameFilledwithConcrete

Strictly as per the compliance and regulations of:

GJRE-E Classification: FOR Code: 090599


Fatema Sultana α, Soumya Suhreed Das σ & Tushar Chowhan ρ

Author α:

Department of Civil Engineering, Ahsanullah University of Science and Technology.

e-mail: [email protected]

Author σ:

Lecturer, Department of Civil Engineering,

Stamford University Bangladesh.

e-mail: [email protected]

Author ρ:

Department of Civil Engineering, Dhaka University of Engineering & Technology (DUET). e-mail: [email protected]

Abstract-

Reinforced concrete combines the benefit of steel and concrete for having both tensile and

compressive strength, but due to the pressing necessity of modern lightweight constructions,

further strength enhancements are necessary. This paper aims to design steel frames which will

increase the compressive strength of composite concrete. Five different steel frames were

designed in finite element analysis suit ABAQUS with angled bars and straight bars to check

how the use of angular bars fair against straight bras under axial loads. From the analysis, it was

revealed that introduction of angle section in reinforcement rather weakens the composite

concrete strength. Steel frame with larger angles was relatively weaker than steel frame with

smaller ones, but introduction of four angled bars with same amount of reinforcement as two

angled bars derived improved the results. It has been observed that in ideal conditions, steel

frame constructed with straight steel bars performs better under axial

loads.

Keywords:

ABAQUS, auxetic shapes, composite concrete, finite element analysis, and steel

angles.

I.

Introduction

raditional reinforced concrete is a composite material which is made up of cement, aggregate

which contributes to the compressive strength and regular steel bars which contribute to tensile

strength. This work explored the possibilities of enhancing the strength of composite concrete by

introducing innovative frame design inside the concrete cylinders. As concrete is an irreplaceable

construction material which is used in a considerable amount for almost all structures, therefore

the effective use of the concrete and steel in order to create a perfect bond and utilize the optimal

strength of both materials is essential for durable and

economical construction (Binici, 2005)[1].

In this research, recent experiments and numerical studies have been compared to establish the

success and drawbacks of different metal frames in concrete in order to introduce a unique shape

for the purpose of enhancing composite concrete strength, and finite element analysis method

was performed with different adjustment to seek out the best performing model. This project's

aim is to replicate geometric benefits of auxetic structures in concrete reinforcement by

using

angular bars. The introduction of angles in the reinforcement

bars may allow the bars to deform

inward and as a result allow the concrete to hold the shape longer which may enhance the

strength development. Use of angular bars as a viable reinforcement system of concrete

structures much depends on the positivity of experimental results.

II.

Background of the Study

Numerous attempts have been made to increase concrete compressive strength with steel

confinements. Kang (2010)[5] states, steel confinement performance dictates the stress

development in reinforced concrete. Countless experiments have been performed to generate

computational models and establish an optimal system for steel frames filled with concrete

(Susantha, 2001)[2]. The ordinary hollow sections confine concrete from outside and require

corrosion proofing which may add up to cost. However, the inadequacy of the RC members

under dynamic (lateral) loading and uncontrolled crack development required a modified model

of the steel frame for an improved outcome (Moghaddam, 2010)[3]. However, Susantha (2001)[2]

indicated some shapes exhibits superior attributes than the other counterparts in terms of bond

strength and load carrying abilities. In the test of loading circular shaped column showed better

confinement results than octagonal or boxed shapes (Susantha, 2001)[2]. Tao’s (2015)[6] push out

a test with stainless and carbon steel on the bond behaviour between concrete and steel frames

which unveiled that the stainless steel exhibited weaker bond strength. It has been concluded that

the most effective method of increasing bonding strength is through welding internal rings on the

inner face of the steel tube, adding shear studs and the use of expansive concrete. Susantha

(2001)[2] also claims that

width to thickness ratio and strength of steel controls the post-peak

behaviour of the composite members through strategic placement of the steel fibres, cracking can

be reduced to a tolerable level. Tabak (2007)[7] maintains that by saying the volume of fibres in

concrete needs to be well controlled and when the volume of fibre is between 1.0-1.5% and l/d

ratio is close to 80 the workability of RC mix deteriorates drastically. This final report

investigated possible auxetic shapes that may be viable for building steel frames and conduct

experiments. Spagnoli (2015)[4] experimented with two-dimensional Auxetic elementary reentrant

cell which can

be applied

to

steel

frames.

Attempts were

T

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made to use re-entrant hexagonal design to construct

Auxetic braided composite rods to enhance the toughness, shear

and better fracture behaviour

(Subramani, 2015)[6].

III.

Methodology

In order to design optimized steel frames, FEM analysis suit ABAQUS was adopted as it affords

the

convenience of replicating the physical model of steel framed concrete cylinder and test the

model's

performance under loading in a computer without actually having to construct the model

in a lab. In this

article, five models have been designed and tested using ABAQUS suit. The first

model consists of two

angular bars with six degrees angles and the second model has been

designed with inverted angular bars

using same angles, third model featured three degrees

angular bars, fourth model consists of four six degrees angled bars with same steel material as

the two angled

bars model and lastly fifth model was designed with two straight bars with same

section size as the first model. In

order to perform the analysis, all three steel frames have been

placed in the concrete specimens of 200 mm

height and 100 mm diameter. Then analysis

performed

and data collected in the form of Microsoft Excel Tables. From the raw data, different

graphs were created and

checked to see if the graphs show realistic trends.

Before testing the model in ABAQUS a simplified 2D model was designed using AutoCAD

2014

platform to specify the dimensions and angles for FEM suit 3D design. Figure 2.1.1 and

2.1.2 illustrates 2

angular bars with six degrees angles model and the inverted bars model

respectively. Other models are also

represented accordingly.

(a) (b)

Fig. 2.1.1: (a) Two six degrees angled bars plan and elevation view, (b) Two six degrees inverted angled bars plan and elevation view.

Fig. 2.1.2: (a) Two three degrees angled bars plan and elevation view. (b) Two straight bars plan and elevation view.



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a)

Design of two six degrees angular bars in ABAQUS

Here, two bars with 6-degree angles were proposed to be tested in a 0.1m diameter concrete

cylinder. Stress-strain diagrams have been prepared to

understand the composite model's performance under

loading. For this model, 6-degree angle was chosen

after trial and error method in ABAQUS. Before selecting

6 degrees, different angular bars have been tested in

ABAQUS, however, different errors arose from ABAQUS with larger angles and meshing

became extremely

complex which may not have derived desired outcomes.

Fig.

2.2.1:

Two six degrees angular bars cut concrete and two six degrees angular bars

ABAQUS model.

b)

Design of Four Angled Bars in ABAQUS

A frame with four square bars with the same angle and same steel volume were developed in

order to check which frame performs better under load.

Fig.

2.2.2: Four six degrees angular bars cut concrete and four six degrees angular bars cut

concrete.

c)

Design of two six degrees angled inverted bars in ABAQUS

The inverted frame designed to check the performance of the composite concrete when the steel

bars are angled outward. The design steps are very similar to the first six degrees angular bar

model. The only difference for this model is that the bars have to be rotated outward.



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Fig. 2.2.3: Two inverted bars ABAQUS model and two inverted bars cut concrete.

d) Design of Two Three Degrees Angular Bars in ABAQUS

The three degrees angular bar model was designed to check how gradually increasing the angle

in

the reinforcement bars impact the strength development of the composite concrete. The steps to design the three-degree angular bars specimen were identical to six-degree angular bar model.

Fig. 2.2.4: Two three degrees angular bars cut concrete and two three degrees angular bars ABAQUS model.

e) Design of Two Three Degrees Angular Bars in ABAQUS

The three degrees angular bar model was designed to check how gradually increasing the angle in

the reinforcement bars impact the strength development of the composite concrete. The steps to design the three-degree angular bars specimen were identical to six-degree angular bar model.

Fig. 2.2.5: Two three degrees angular bars cut concrete and two three degrees angular bars ABAQUS model.



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IV. Result

To perform a comprehensive analysis and compare the strength development in all five models,

displacement and forces over the set time intervals were collected. Forces were graphed against displacement to find the yield strength of the composite models. Stress-strain graphs were also developed for all five composite frames separately. The stress-strain curve allowed the

opportunity to investigate the steel frames behaviour and the trends.

Ideally, for the first set of steel frames, the more angular bars would exhibit more force resistance. The

six degrees angular bars would be most efficient, then the three degrees angular would be the second highest in terms of strength development and lastly the least effective would be the frame with two straight bars. For the second set of models with inward and outward

angles, the bars with outward angles should be weaker than inwardly angular bars. Lastly, four angular bar design should exhibit similar strength development as the two angular bars, because same amount of steel have been used in both models.

a) For two angled bars with 6 degrees angle

Fig.

2.2.6:

Force and displacement graph and stress strain curve of two 6 degrees

angled bars.

For the concrete cylinder for this particular model expands below the midpoint following the

deformation of the bars which also occurs below the midpoint which is illustrated below:

Fig. 2.2.7: (a) Deformed 2 six degrees inverted angular bars concrete cylinder, (b) six degrees inverted angular bar. (c) Two 6 degrees inverted angled deformed bars.



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b)

For Two Angled Bars with 3 degrees Angle

Fig.

2.2.8:

Force and displacement graph and stress strain curve of two 3 degrees angled bars.

(a) Deformed 2 three degrees bars concrete cylinder with

shadowed region illustrating the displacement, (b) two three degrees angular bar.

(c) two three degrees angled

deformed bars.



c) For two straight bars

Force and displacement graph of two straight bars and Stress-Strain graph of two straight bars.

Fig. 2.2.9:

Fig. 2.2.10:

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Fig.

2.2.11:

(a) Deformed 2 straight bars concrete cylinder with shadowed

region illustrating the displacement, (b) Two three degrees angular bar. (c)

Two three degrees angled deformed bars.



d) For Four six degrees angled bars

Fig. 2.2.12: Force and displacement graph of four 6 degrees angled bars and Stress-Strain graph of four 6 degrees angled bars.

Fig. 2.2.13: (a) Deformed 4 angular bars concrete cylinder with shadowed region illustrating the displacement, (b) Four six degrees angular bar. (c) Four six degrees angled deformed bars.

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e)

Comparison of Six Degrees Angled Bars, Three Degrees Angled Bars and Straight Bars

The comparison of the test results did not support the hypothesis in result section that inducing

more angles into the reinforcement bars would increase the composite concrete’s strength. The

assessment of the stress-strain graph clearly illustrates the opposite is true. The acuter the angle,

the weaker is the composite strength of concrete. This may be due to the fact that

the

compressive cylinders were tested under axial loading. The stress-strain diagram below clarifies

that composite frame with two straight bars generates higher compressive stress. This

comparison does show that introducing

angles in reinforcement bars impacted in the strength

development of concrete. However, it did not improve the compressive strength of the reinforced

concrete cylinder.



Fig. 3.1: Stress-Strain comparison graph of two straight bars, two angled bars at 6 degrees and two angled bars at 3 degrees.

A comparative yield stress, strength and displacement developments are presented in Figure 3.2.The straight bars yields at 52 MPa after displacing 5.4 mm from the top surface. The steel bars with 3 degrees angles generate similar yield stress about 52 MPa, whereas an increase of angle to 6 degrees derives the

weaker yield stress of 49.5 MPa at a lesser displacement of 4.5 mm. This may be attributed to the fact that straight bars are stronger in resisting axial movement as the straight bars counteract the forces due to loading with a direct linear reaction force.

Fig. 3.2: Yield strength comparison table for all models.

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Steel Frames Yield Stress (Mpa) Yield Strength (KN) Displacements (mm)

Two Bars at 60 Angles

Two Bars Inverted at 60

Angles

Two Bars at30 Angles

Two Straight Bars

49.5 388 4.5

34 270 2.8

52 405 4.8

52 415 5.4

54.5 428 5.2Four Bars at 60 Angles

The compressive strength analysis of angular bar frame against the straight bar frame did not

produce positive results. The purpose of using the angular bars was to introduce the benefits of

Auxetic frames into the

composite concrete. Figure 3.3 illustrates the stress-strain relationship

found by Ahmad (2015) in his study of the crash worthiness of auxetic foam filled tubes.



Fig. 3.3: Stress-strain relationship of conventional and Auxetic foams.

Angular bars may be used in circumstances where using angles in bars is an advantage, as the yield strength of angular reinforced bars is marginally lesser compared to the straight reinforced bars when the angles are controlled to a limit. However, in ideal conditions, straight bars would be a better choice as straight reinforcement bars in composite concrete cylinders have produced higher yield strength.

f) Comparison between Inward Angled Bars and Outward Angled Bars

The comparison is presented below with graphical representations.

Fig. 3.4: Stress-Strain comparison between Two 6 degrees angled bars and Four 6 degrees angled bars.

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There was a considerable difference at strength development of the two steel frames. The inward

angular frame yielded at 49.5 MPa after 4.5 mm

displacement but the inverted angled bars steel

frame yielded at only 34 MPa after 2.8 mm displacement.



Fig. 3.5: Stress-Strain comparison between Two 6 degrees angled bars and Four 6 degrees angled bars.

The yield stress of the two models was 54.5 MPa at 5.2 mm displacement and 49.5 MPa at 4.8 mm displacement for the steel frame with four bars and steel frame with two bars respectively from Figure 3.5, which shows the four bars model resist 5 MPa more axial stress and endure further displacement before yielding than the counterpart. The four bars steel frame providesmore confinement than the two bars model which allows the axial load to be divided into four bars instead of two bars in the two bars design. This does follow the argument of Kang (2010)[5], lateral confinement regulates the stress development and ductility of the composite concrete.

V. Conclusion

Five steel frames with angular bars were designed to take advantage of the angular geometryunder loading. Three angular bar frames were designed to check the effect on strength development with the increase of angle on the bars. Another model was designed with reversed angled bars to see if an introduction of outward angle enhances the strength. The third model was developed using four angled bars and checked to see the strength development with moreconfinement. The comparison among the models with three-degree angles and six-degree angles and straight

g) Comparison between Two Angled Bars and Four Angled BarsThe comparison is shown below with graphical representations.

bars revealed that steel frame designed with straight

bars is better under axial loading. Even though the steel frames with angled bars did not enhance the compressive strength of composite concrete under axial loading, same frames could be tested under lateral loading to check, if the proposed steel frames yields better results. Lateral transverse reinforcement allowsthe steel frame to be more ductile. Stirrups could be used on the outer surface of the angular barsin the form of transverse reinforcement. This may increase the strength of the composite concrete. More research could be performed to see if any other geometries could be incorporated into reinforcement steel from the auxetic frames.

References Références Referencias

1. BINICI, B. 2005. An analytical model for stress_______-strain behavior of confined concrete. Engineering Structures, 27, 1040-1051.

2. SUSANTHA, K. A. S., GE, H. & USAMI, T. 2001. Uniaxial stress–strain relationship of concrete confined by various shaped steel tubes. Engineering Structures, 23, 1331-1347.

3. MOGHADDAM, H., SAMADI, M., PILAKOUTAS, K. & MOHEBBI, S. 2010. Axial compressive behavior of concrete actively confined by metal strips; part A: experimental study. Materials and Structures, 43, 1369-1381.

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4.

PAGNOLI, A., BRIGHENTI, R., LANFRANCHI, M. & SONCINI, F. 2015. On the

Auxetic Behaviour of Metamaterials with Re-entrant Cell Structures. Procedia

Engineering, 109, 410-417.

5.

HAN, T. H., YOON, K. Y. & KANG, Y. J. 2010. Compressive strength of circular

hollow reinforced



concrete confined by an internal steel tube. Construction and Building Materials, 24, 1690-1699.

6. SUBRAMANI, P., RANA, S., GHIASSI, B.,FANGUEIRO, R., OLIVEIRA, D. V., LOURENCO, P. B. & XAVIER, J. 2016. Development and characterization of novel auxetic structures based on re-entrant hexagon design produced from braided composites. Composites Part B: Engineering, 93, 132-142.

7. YAZıCı, Ş., İNAN, G. & TABAK, V. 2007. Effect of aspect ratio and volume fraction of steel fiber on the mechanical properties of SFRC. Construction and Building Materials, 21, 1250-1253.

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