Performance of Infilled Cold Formed Steel Channel Section Beams

Performance of Infilled Cold Formed Steel Channel Section Beams

Jyothi K. N.

Post Graduation Student, Department of Civil Structural Engineering, Sri Venkateshwara College of Engineering,

Bangalore, Karnataka State, India

Valsa Ipe .T

Professor, Department of Civil Engineering,

Sri Venkateshwara College of Engineering, Bangalore, Karnataka State, India.

AbstractThe overall view of this paper is to study the behavior of cold formed steel channel section beams with an infill of concrete. The flexural behavior of cold formed steel infilled and hollow beams is investigated by conducting experimental study using two point load test [1]. Totally twelve beams are tested, out of which six are infilled with concrete and remaining six are hollow sections. Depth to thickness ratio is varied from 45 to 75 for all sections. For comparison, theoretical values are calculated based on Rigid Plastic Analysis for infilled beams and Australian/ New Zealand code 4600-2005 for hollow sections. Tremendous increase in load carrying capacity of infilled sections is observed. Using same amount of steel, high strength was attained by filling the sections with concrete. Experimentally it is observed that the load carrying capacity for infilled beams is not affected with a flat width ratio up to 71 for depth of section.

KeywordsFlexural;Infilled beams;Hollow beams;Flat width ratio;Depth to thickness ratio;

1. INTRODUCTION

   Composite construction refers to two load-carrying structural members that are integrally connected and deflect as a single unit. Slab and beam constructions are commonly used in bridges and buildings. Slab beam bonding is possible through the use of shear connectors welded. When the flange is in compression, the bond between the shear connector and the slab is assumed to be perfect. Cold formed steel members are cold formed in rolls or press brakes from flat steel which is not thicker than 12.5mm. Cold formed steel goods are made by working of steel sheet using stamping rolling or pressing to deform the sheet into a useable product [1, 9].

   CFS sections are composed of thin plate elements which are highly prone to local buckling and this necessitates more sophisticated analysis and design procedures. In addition, more theoretical and experimental studies being conducted to capture the complex behavior when used in combination with concrete. The composite construction consists of providing monolithic action between prefabricated units like steel beams so that the two will act as one unit [2]. At the initial stages there will be natural bond between concrete and the steel. Shear connectors are provided to help the steel and concrete element to act in a composite manner ignoring the contribution made by the inherent natural bond towards this effect. Here the flexural behavior of cold formed steel channel section beams with an infill of concrete and hollow beams are investigated by conducting experimental study using two point load test.

2. INFILLED AND HOLLOW BEAMS

   The theoretical studies related to concrete in-filled beams and hollow channel sections are carried out based on the rigid plastic analysis and Australian /New-Zealand code (AS/NZS 4600-2005) respectively [3]. The ultimate loads are found out from the known moment carrying capacity.

   For analysis of infilled beams the following considerations are made.

   Fy = yield strength of the steel l = lip length

   d = web of the section

   b = bottom flange

   t = thickness of the steel r = radius

   E = Youngs modulus of elasticity=2×10^5MPa L = span of the beam.

   For a beam,

   Pc = force due to concrete Pb = bond force

   bc = width of the concrete

   Nc and Ns = distances of neutral axis from the end of compression face for concrete and steel.

   M = moment carrying capacity of the composite beam

   = the reduction factor d = the depth of the web l = lip length

   t = thickness of steel

   fy = yield stress of the steel

   fck = characteristic strength of concrete.

   The equilibrium of forces in concrete, the distance from the neutral axis is obtained from the equation.

   where = reduction factor= 0.85-0.007(fck-28)

   The equilibrium of forces in the steel, the distance from the neutral axis is obtained from the equation.

   The moment of forces on the top fibre, the moment carrying capacity M of the composite sections can be determined from expression.

   s c ck c

   M= t f y (d2+dbc-2N 2)-0.4252N 2f b -t2fy

   For complete interaction between steel and concrete

   N c = Ns = N and

   For the analysis of hollow beams Australian/ New Zealand code (AS/NZS 4600-2005) [4] is referred for determining the moment carrying capacities. The following considerations are made for a hollow beam

   1. Based on the initiation of yielding and

   2. Nominal section moment capacity

      Fy = yield strength of the steel l = lip length

      d = web of the section b = bottom flange

      t = thickness of the steel r = radius

      E = Youngs modulus of elasticity=2×10^5MPa L = span of the beam.

      To locate the neutral axis using effective width calculation 6 steps are carried namely

      1. Determining the section capacity: The effective width (be) of stiffened elements is known fro1m. the equations For <=0.673: be= b

   For >=0.673: be=b

   where b= flat width of element =effective width factor

   iii) Plate slenderness factor , For <=0.673: be= b

   For >=0.673: be= b

   1. Effective width for capacity : This can be determined as below

      1. The section member capacity, the effective width

      2. For <=-0.236

   2. The moment of inertia (Ix) and the section modulus(Sx) :

   3. The bending moment or the moment carrying capacity: M = S x f y

   4. Ultimate load P(kN):

3. EXPERIMENTAL STUDY

   Twelve specimens of two series are considered. Six hollow sections and six infilled sections. These specimens represent channel sections of length 1800mm [5]. Following two materials are used

   1. Steel: The tension test has been conducted on the specimen to know the properties of steel sheet for conducting the tests. From the results of the tension test, the yield strength Fy of steel was found to be 205.2N/mm2.

   2. 2. Concrete: The concrete of grade M20 has been prepared

      using 53 grade ordinary Portland cement, coarse aggregates of 20mm size, natural river sand and portable water and used as an infill.

      1. A. Preparation and Procedure Of Testing Infilled And

   By knowing the compression flange width, the Neutral Axis (Ycg) can be located assuming that the web is fully effective.

   1. Stiffened elements with stress gradients : The stresses are calculated using the equations

      1. Stress ratio,

      2. Plate buckling coefficient,

   k =4+2(1+)3+2(1-)

   Hollow Specimens :

   The sheet was procured and it was cut into desired dimension and fabricated into desired dimensions as shown in Table 1. which is bent in cold condition [6]. The sheets were bent into desired shapes and lips were also bent using the press brake machine. The sections were painted externally for infilled beams and internally for the hollow beams. The specimens were kept for drying after painting. The concrete was poured from the open side. Both the sides were closed by steel sheet so that the concrete poured does not spill off out. The concrete poured beams were covere with wet gunny bags for 28 days. Shear connectors [7] are welded at a spacing of 150mm c/c for 600mm at both the ends and at the centre portion of the beam, spacing given is 300mm.

   DESIGNATION	DESCRIPTION
   INFILLED BEAMS
   AI 1	1	250mm x 90mm
   AI 2	2	250mm x 110mm
   AI 3	3	250mm x 120mm
   AI 4	4	250mm x 130mm
   AI 5	5	250mm x 140mm
   AI 6	6	250mm x 150mm
   HOLLOW BEAMS
   AH 1	1	250mm x 90mm
   AH2	2	250mm x 110mm
   AH3	3	250mm x 120mm
   AH4	4	250mm x 130mm
   AH5	5	250mm x 140mm
   AH6	6	250mm x 150mm

4. EXPERIMENTAL RESULTS

   90

   80

   70

   60

   50

   40

   30

   20

   10

   Load deflection behavior of infilled and hollow beams are compared and discussed as shown in Fig .2 and Fig 3.There is linearity in the curves of in filled sections almost up to 55kN, later the curve followed a non-linear path up to the failure point. Whereas, the curves of hollow sections AH1, AH2, AH3 followed a linear path almost up to 15kN and then showed a non-linear path. The sections AH4, AH5, AH6 showed a linear path up to the failure point since these sections failed to resist the loads.

   Load kN

   Table . 1 Designation and Description of Specimen

   A loading frame of 100 tons capacity was used for testing the beams. Two point loads were applied at varying distances from the supports using a beam to provide a complete flexure area or bending area in the centre portion of the beam. Two roller supports were provided 50mm apart from the ends of the beam as shown in Fig 1. Load was applied gradually using a hydraulic jack till the failure of the beams [8]. The axial deformation is measured using the LVDTs (Linear Variable Displacement Transducers. Strain gauges of 350 , 1.9 gauge factor strains were used. In this investigation strain gauges were attached to the composite beams and not for hollow beams.

   P/2 P/2

   Span L

   Fig 1. Placing of the supports and the loads

   -50

   0

   -10 0

   Deflection mm

   50

   AI1

   AI2

   AI3

   AI4 AI5 AI6

   Fig 2. Load-Deflection curve for all the infilled beams

   Load kN

   The curves of infilled sections were highly stiff than the curves of hollow sections. The average ductility index for all the infilled sections is 5.33 as shown in Fig 4.The composite interaction between the steel and the concrete increased the ductility index.

   25

   20

   15

   10

   5

   0

   AH1

   AH2 AH3 AH4 AH5

   AH6

   -5 0 5 10

   -5

   Deflection mm

   Fig 3. Load-deflection curve for all hollow beams

   8

   6.71

   6

   5.66

   4

   2

   0

   5.01 5.03

   4.9

   4.72

   1. Ultimate loads

      Fig 4 . Ductility of infilled beam

      Fig 5. Experimental ultimate loads on hollow section- flat width ratio curve

      INFILLED BEAMS
      Name	PthI in kN	PexpI in kN	Pexp/Pth
      AI 1	45.2	60	1.327
      AI 2	48.53	70	1.4424
      AI 3	50.13	73.3	1.462
      AI 4	51.7	77	1.489
      AI5	53.25	77.3	1.451
      AI 6	54.77	79.2	1.446
      HOLLOW BEAMS
      Name	PthH in kN		PexpH in kN	Pexp/Pth
      AH 1	13.5		18.5	1.37
      AH 2	13.09		19.2	1.4667
      AH3	15.93		19	1.1927
      AH 4	16.09		18	1.118
      AH5	17.45		17.6	1.008
      AH 6	18.18		16	0.88

      Table 2. Theoritical and Experimental Ultimate Loads

      It is observed from Table 2. that on an average, the ratio of experimental to theoretical loads (Pexp/Pth) was found to be 1.43 for infilled sections, whereas for hollow beams the ratio (Pexp/Pth) is 1.36 for sections AH1, AH2, AH3, but as the depth increased for the sections AH4, AH5, AH6 the ratio of experimental to theoretical decreased on an average of 1.

      The Fig. 5 clearly indicates the decreasing of the ultimate loads as flat width ratio is increased. The sections AH1, AH2, AH3 had increase in ultimate loads since the flat width ratio was less than 60. Whereas the ultimate loads of sections AH4, AH5, AH6 decreased because the flat width ratio was more than 60.

      It is observed from Fig 6 that as the flat width ratio is increased there is an increase in the ultimate loads of infilled to ultimate load of hollow section beams.

      Fig. 6. Experimental loads of infilled and hollow sections- flat-width ratio curve.

   2. Strength To Weight Ratio

      Strength to weight ratio of the beams decreases with the increase in depth of the steel as shown in Fig 7. On an average the strength to weight ratio is found to be 4.73 for all the infilled sections.

      6

      5

      4

      3

      2

      1

      0

      5.29 5.15 4.95 4.86

      4.1 4.03

      Fig 7. Strength to weight ratio of infilled beams

      In general, it is observed that the moment carrying capacity and the rotational capacity increase with increase in depth of the section. Moment-curvature relation for infilled beams is shown in Fig 5 to Fig 11.

      10

      8

      6

      4

      2

      0

      AI1

      -0.5 0 0.5 1

      x 10-5 Radians

      20

      15

      10

      Moment kN-m

      Moment kN-m

      Fig 6. Moment curvature curve of AI1

      AI2

      0

      x 10-5 radians

      1.5

      1

      0.5

      5

      0

      Moment kN-m

      Fig 7. Moment- curvature curve of AI2

      20

      15

      10

      5

      0

      AI3

      0 5

      x 10-5 radians

      Moment kN-m

      Fig 8. Moment- curvature curve of AI3

      20

      10

      0

      AI4

      0 0.5 1

      x 10-5 radians

      Fig 9. Moment- curvature curve of AI4

      20

      10

      0

      AI5

      0 0.1 0.2

      x 10-5 radians

      Moment kN-m

      Moment kN-m

      Fig 10 Moment- curvature of AI5

      30

      20

      10

      AI6

      0

      -20 0

      20 40

      x 10-5 radians

      Fig 11. Moment- curvature curve of AI6

   3. Failure Patterns

   Failure was by buckling of the infilled beams where concrete in filled was easily separated from the steel section. Bending was also observed for all the infilled sections as seen in plate 1 and plate 2.It was noticed that in case of hollow beams, local buckling occurred in the compression zone. As seen in the plate 3 and plate 4 here occurred inward and outward buckling of the hollow beams and therefore it was end by the overall buckling.

   Plate 1. Buckling of steel in compression zone

   Plate 2.Separation of concrete and steel

   ACKNOWLEDGEMENT

   The authors Jyothi .K. N and Valsa Ipe T would like to thank the authorities of Sri Venkateshwara College of Engineering, Bangalore, for all the cooperation and encouragement.

   Plate 3. Buckling of steel inwards

   Plate 4. Buckling of steel outwards and bending

5. CONCLUSIONS

The analytical results and the experimental results were observed and found that rigid plastic analysis can be adopted for determining the flexural strength. The experimental load carried by infilled beams with shear connectors showed tremendous increase in load carrying capacity. On an average it was found to be 74.09% more than the hollow beams. The load carrying capacity and the deflections are considerably poor with respect to hollow sections. Thus hollow sections showed poor performance on the application of load. The ratio of ultimate loads of infilled beams to hollow beams increased with increase in flat width ratio. The ultimate loads of hollow sections increased where the flat width ratio of depth portion is less than 60 and ultimate loads decreased when the flat width ratio of depth portion is more than 60.The average ductility index of infilled beams was found to be 5.33.The strength to weight ratio of infilled beams on an average was

4.7 for all infilled beams. The moment carrying capacity and curvature increased with depth of section. The failure of infilled beams was by cracking of concrete, separation of steel and concrete. The failure of hollow beams was by inward and outward buckling and bending of the specimens.

REFERENCES

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2. Chitra R Thenmozhi (2011) vol12, Studies on prefabricated cage reinforced steel concrete composite beams Asian Journal of Civil Engineering.

3. Valsa Ipe T H ,Sharada Bai, K Manjula Vani, Merchant Mohd Zafar Iqbal(2013) vol 14

4. Australian/ New Zealand code- AS/NZS 4600-2005.

5. Khandaker M Anwar Hossain (2003) Experimental & Theoretical Behaviour of Thin Walled Composite Filled Beams ,Electronic Journal of Structural Engineering, #, 117-138.

6. IS801-1975, Code of practice for use of cold formed light gauge steel structural members in general building construction.

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9. Richard P Nyugen (1988),Strength of Composite Cold formed steel concrete beams.