flexural behaviour of stiffened modified cold-formed steel...

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 6, Issue 9, Sep 2015, pp. 104-115, Article ID: IJCIET_06_09_010

Available online at

http://www.iaeme.com/IJCIET/issues.asp?JTypeIJCIET&VType=6&IType=9

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

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FLEXURAL BEHAVIOUR OF STIFFENED

MODIFIED COLD-FORMED STEEL

SECTIONS–EXPERIMENTAL STUDY

Syed Mohammad

Graduate Student, Department of Civil Engineering,

National Institute of Technology, Srinagar

Mir Faizan Ul Haq

Post Graduate Student, Department of Civil Engineering,

Indian Institute of Technology, Kanpur

Mufti Minaam Mehmood

Graduate Student, Department of Civil Engineering,


Prof. (Dr.) A. R. Dar

Professor, Department of Civil Engineering,


ABSTRACT

The present study deals with the enhancement of the flexural capacity of

cold formed steel beams using stiffeners. Beams with two back-to-back lipped

channel sections were tested with and without stiffeners. Four such beams

were considered with depth 150 mm and thickness of sheets 1mm and 2mm.

ISMB 150 was also tested and was used as a yardstick for comparison with

equivalent cold-formed sections. All the sections were subjected to “Four

point flexural test” to study their behaviour in pure bending. From this study it

was found that strength and stiffness of sections made out of 2mm sheets can

be substantially enhanced using stiffeners whereas for 1mm sheets the

enhancement was not so profound, primarily due to very high propensity for

local buckling. Moreover, beams of 2mm sheets exhibited stiffness comparable

to ISMB 150 in the initial loading stage.

Key words: Cold-Formed Sections, Flexural Strength, Buckling, Stiffeners.

Cite this Article: Syed Mohammad, Mir Faizan Ul Haq, Mufti Minaam

Mehmood and Prof. (Dr.) A. R. Dar. Flexural Behaviour of Stiffened Modified

Cold-Formed Steel Sections–Experimental Study. International Journal of

Civil Engineering and Technology, 6(9), 2015, pp. 104-115.

http://www.iaeme.com/IJCIET/issues.asp?JTypeIJCIET&VType=6&IType=9

Flexural Behaviour of Stiffened Modified Cold-Formed Steel Sections–Experimental Study


1. INTRODUCTION

Cold-formed steel construction is gaining popularity with civil engineers for use as

primary and secondary load bearing structural components. This is because they have

higher strength-weight ratio than hot-rolled steel sections, are easy to handle, erect

and transport, and facilitate quick pre-fab construction. Moreover, they can be formed

to close tolerances into any viable shape giving designer a greater freedom of choice.

But these members are thin with their ultimate strengths being governed by buckling. [1]

Hence, there is a need to fully evaluate the performance of light gauge cold-formed

steel sections so as to come up with a precise method of analysis and design. Two

theories are commonly used for the same-the “Effective Width Concept” and the

newer and more robust one, the “Direct Strength Concept”. Indian codes [2]

still

follow the former while the latter is being increasingly adopted by other countries of

the world. [3]

IS 801 stipulates the analysis and design guidelines for assemblies such

as built-up compression and flexural members based on effective width concept. The

specifications of light-gauge steel sections have been covered in IS 811.

Pioneering work in this field was done by America Iron and Steel Institute

(A.I.S.I.) in 1930’s. Presently, American, Australian and European researchers are at

the forefront of research in light gauge steel construction.

Kakade et al (2014) studied the various design methods for cold-formed light

gauge steel sections. Their study revealed that the design strengths predicted by both,

American as well as Indian standards are on the conservative side. [4]

Goswami, Arlekar and Murty studied the limitations of available Indian hot-rolled

I-sections and found out that, besides other limitations, the Indian hot-rolled I-sections

are inadequate for use in tall structures in high seismic regions. [5]

Liping Wang, Ben Young (2014) investigated the structural behaviour of cold-

formed steel stiffened cross-sections subjected to bending. The stiffeners were

employed to the web of plain channel and lipped channel sections to improve the

flexural strength of cold-formed steel sections that are prone to local buckling and

distortional buckling. [6]

Cheng Yu and Weiming Yan (2010) proposed a modified design method based on

the Effective Width Method for determining the buckling strength of typical cold-

formed steel sections subjected to bending. Comparison with experimental results

indicated that the proposed method was reasonably accurate in calculating the flexural

strength of standard C and Z sections. [7]

Kulkarni et al (2014) did a comparative study of the Indian and British standards

for the design of cold-formed flexural members. The former (IS-801) is based on

effective width method while the latter (BS-5950) is based on direct strength method.

Their study revealed that both the design concepts gave nearly the same design

strength while being highly conservative. [8]

Stone and LaBoube (2005) studied the behaviour of cold-formed steel built-up I-

sections to assess the design provisions of the North American Specification for the

Design of Cold-Formed Steel Structural Members. Their investigation also revealed

that design specifications were conservative in predicting the ultimate capacity of the

built-up sections. [9]

2. METHODOLOGY

This section expatiates on the beams used, cross-sections considered, loading

equipment, testing and instrumentation. The study involved fabrication and testing of

Syed Mohammad, Mir Faizan Ul Haq, Mufti Minaam Mehmood and Prof. (Dr.) A. R. Dar


five samples, four of cold formed and one hot rolled section for comparison purpose.

Two sections were made out of 2mm steel sheets using press-brakes. Other two

sections were made out of 1mm steel sheets. Mild steel, with yield strength 250 MPa,

was used for fabrication. The beams consisted of two back-to-back channel sections

with lips at the ends to act as local stiffeners. Depth of all sections was 150mm and

effective span of 2.1m.

For further stiffening, angle sections of the same nature were used in the

maximum moment zone to strengthen against buckling. ISMB 150 of same span was

also tested similarly for a comparative study of cold formed and hot rolled sections’

relative strength and stiffness. The samples were subjected to “four-point flexural

test”. The middle section subjected to maximum moment in the zone of “pure

bending” (only bending and no shear) was studied. The loading arrangement is as

shown in Fig 1.

Figure 1 Four point flexural test

Fig.2 shows the different cross sections considered. Double lines represent

stiffeners used in the “middle third”, maximum moment region. Sample 1 tested was

ISMB 150 and rest of the samples are as shown. Stiffeners used in sections were of

the same nature as the beams. A slight extension of stiffeners was provided beyond

the middle third region upto 350 mm on both the sides to ensure failure in the middle

zone.

Figure 2 Cross Sections of beams



Bolting and welding was employed for jointing and spacing specifications were

taken from IS 801. Moreover, bearing stiffeners were also provided at the loading and

reaction points to prevent any web crippling or shear buckling. Loading was carried

out on loading frame by means of a hydraulic jack. Dial-gauges were set up at the

mid-span and one-third span from both ends to measure deflection. The loading frame

is shown Fig. 3.

Figure 3 Testing Arrangement

3. RESULTS AND DISCUSSION

The load-deflection curves for various sections have been plotted to study the strength

and stiffness characteristics. Comparative curves of stiffened and unstiffened sections

have been plotted to quantify the enhancement of strength and stiffness after

stiffening. Failure modes have also been discussed. Finally, the strength-weight ratios

for various sections have been tabulated to select the most optimum section and a

combined curve also plotted for all the sections.

3.1 Sample 1 (ISMB 150)

The hot rolled beam was tested on the loading frame. The load and corresponding

deflection values are tabulated in Table 1. Fig. 4 shows the load deflection curve. For

simplicity only mid span values have been plotted.

The curve was linear upto 83.74 kN beyond which lateral torsional buckling

commenced (unrestrained compression flange) and subsequent failure was observed.

The average stiffness in the linear region was 6.184 103

kN/m. The ultimate failure

load was 67.31 kN.



Table 1 ISMB 150

Figure 4

3.2 Lipped Sections (2mm thick sheets)

Lipped sections made from 2mm thick sheets were fabricated and tested with and

without stiffeners. The detailed behaviour of the sections has been discussed in the

following section.

3.2.1 Sample 2 (Unstiffened-lipped section)

The I-Section made of two ‘Lipped Channels (2 mm thick) placed back to back’ was

tested as described earlier. The relevant values of load and deflection are tabulated in

Load (kN) Deflection(mm)

Mid Span 1/3rd

Span 2/3rd

Span

0 0 0 0

1.325 0.21 0.19 0.17

2.65 0.42 0.38 0.34

3.975 0.64 0.58 0.55

5.3 0.89 0.78 0.7

10.865 1.81 1.52 1.53

18.55 3.07 2.64 2.7

23.85 3.92 3.4 3.39

29.15 4.85 4.2 4.1

37.1 5.91 5.14 5.2

47.7 7.6 6.59 6.7

58.3 9.22 8 8.12

63.6 10.07 8.74 8.72

68.9 10.95 9.48 9.51

74.2 11.82 10.23 10.25

79.5 12.65 10.94 10.96

83.74 14 12.02 12.22

67.31 15.08 12.54 12.7

67.31 15.96 12.48 12.8



Table 2. The beam behaved linearly till 43.06 kN where corresponding deflection was

12.49 mm, beyond which signs of local buckling in one of the lips were observed. The

average stiffness in the linear region was 3.425 103

kN/m. Fig.5 shows the load-

deflection curve of the beam.

Table 2 Unstiffened section

Figure 5

3.2.2 Sample 3 (Stiffened-lipped section)

While testing Sample 2, as stated above it was observed that the lips of the section

buckled. Thus, sample 3 was tested with lips of the compression flange stiffened. The

Load (kN)

Deflection (mm)

Mid span 1/3rd span 2/3rd span

0 0 0 0

3.3125 1 0.89 0.99

6.625 1.97 1.92 1.93

9.9375 3 2.93 2.9

13.25 3.99 3.85 3.82

16.5625 4.86 4.62 4.66

19.875 5.82 5.71 5.6

23.1875 6.69 6.4 6.42

26.5 7.57 7.22 7.25

29.8125 8.53 8.16 8.14

33.125 9.43 9.05 9

36.4375 10.44 9.89 9.92

39.75 11.41 10.65 10.82

43.0625 12.49 11.74 11.83



test results are tabulated in Table 3. A significant increase in strength was observed in

comparison to sample 2. The load deflection curve is shown below (Fig. 6).

The behaviour was linear up to a load of 60 kN, beyond which the sample

showing signs of yielding and entered the plastic zone, evident from the flattening of

the curve at load of 67.84 kN. The beam also showed signs of distortional buckling.

Further loading failed to produce any reaction from the beam with only deflection

increasing. The average stiffness in the linear region was 5.8 103 kN/m.

Table 3 Stiffened section

Figure 6

3.3 Lipped Sections (1mm thick sheets)

In order to improve the strength-weight ratio, thickness of the sheets was reduced to

1mm. Lipped sections made from 1mm thick sheets were also tested with and without

Load (kN) Deflection (mm)


0 0 0 0

10.6 1.85 1.63 1.64

21.2 3.59 3.2 3.18

31.8 5.41 4.79 4.8

42.4 7.28 6.32 6.45

45.05 7.72 6.79 6.84

50.35 8.61 7.57 7.64

54.325 9.34 8.33 8.29

56.975 9.95 8.72 8.84

58.3 10.2 10.11 10.06

63.6 12.14 11.92 11.76

64.925 12.82 12.5 12.36

67.575 14.92 14.24 14.13

67.84 15.82 14.97 14.89

68.105 16.68 15.78 15.63

67.84 18.58 17.36 17.23

67.84 20.7 19 18.97

67.84 21.7 19.76 19.7



stiffeners. The detailed behaviour of the sections has been discussed in the following

section.

3.3.1 Sample 4 (Unstiffened-lipped section)

The sample was tested similarly and the test results have been tabulated in Table 4.

Due to lower sheet thickness, the sample showed signs of local buckling of the

compression flange at comparatively smaller load of 13.125 kN. The average stiffness

in the linear region is 3.09 103

kN/m. The load-deflection curve shows a

predominantly linear behaviour till local buckling as shown in Fig. 7.

Table 4 Unstiffened section

Figure 7

Load (kN)

Deflection (mm)

Mid span 1/3rd

span 2/3rd

span

0 0 0 0

0.625 0.2 0.17 0.18

1.25 0.39 0.35 0.36

1.875 0.58 0.55 0.54

2.5 0.75 0.73 0.72

3.125 0.96 0.94 0.93

3.75 1.17 1.13 1.14

4.375 1.41 1.29 1.37

5.625 1.77 1.68 1.72

6.25 1.99 1.9 1.93

6.875 2.2 2.05 2.13

7.5 2.44 2.28 2.35

8.125 2.64 2.49 2.55

8.75 2.89 2.76 2.8

9.375 3.15 3.09 3.05

10.625 3.63 3.51 3.53

11.25 3.88 3.75 3.78

11.875 4.18 4.02 4.11

12.5 4.48 4.37 4.41

13.125 5.04 4.92 5.01



3.3.2 Sample 5 (Stiffened-lipped section)

A sample similar to sample 4 was stiffened using a channel shaped section to stiffen

the lips and the compression flange as shown earlier (Fig. 2). A nearly linear curve

upto 14kN was seen and the average stiffness in the linear region is 3.4 103

kN/m.

The new sample did not show any substantial increase in strength and stiffness

and failed in more-or-less the same fashion, with local buckling of the compression

flange in the middle third region. The results have been tabulated in Table 5 and load-

deflection curve is shown in Fig. 8.

Table 5 Stiffened Section

Figure 8

Load (kN)

Deflection (mm)


0 0 0 0

1.25 0.26 0.15 0.17

2.5 0.64 0.45 0.5

3.75 1.01 0.76 0.83

5 1.44 1.15 1.19

6.25 1.86 1.48 1.55

7.5 2.29 1.87 1.92

8.75 2.75 2.26 2.31

10 3.13 2.63 2.65

11.25 3.63 3.05 3.08

12.75 4.3 3.62 3.67

13.125 4.55 3.76 3.85

13.75 4.87 4.05 4.12

14.375 5.24 4.35 4.45

15 6.16 4.97 5.05

15.625 7.95 5.46 5.57



3.4 Effect of stiffening

The effect of stiffening was substantial in “sample 2 and sample 3” but poor in

“sample 4 and sample 5” and can be summarized as:

1. Increase in strength (load carrying capacity) from “Sample 2 to Sample 3” was a

significant 57.91%.

2. Increase in strength (load carrying capacity) from” Sample 4 to Sample 5” was

19.04%.

3. For samples 2 & 3, the increase in stiffness was 69.34% from 3.425 103 kN/m to

5.8 103 kN/m.

4. For samples 4 & 5, increase in stiffness was 10.03% from 3.09 103

kN/m to 3.4

103 kN/m.

5. The failure mode shifted from local to global after stiffening from “Sample 2 to

sample 3”.

6. Failure mode was same for “Sample 4 and Sample 5”.

The effect on strength and stiffening has been shown graphically in Fig. 9 and Fig. 10.

The slopes of curves give stiffness of the sections.

Figure 9

Figure 10



3.5 Combined curves and strength-weight ratios

Combined curves have been plotted to give an idea of the relative stiffness and

strengths of all the sections tested. Weights, strength-weight ratios, theoretical

capacities and final deflections have been tabulated in table 6 and combined plots

shown in fig.11.

Stiffness of sample 1 and that sample 3 was found to be comparable in the initial

stages of loading with former failing at load a of 83.74 kN and latter at 68kN.

Theoretical capacities have been calculated using analysis procedure followed by IS

801 (Effective Width Concept).

Table 6 Combined results

Figure 11 Combined plots.



4. CONCLUSIONS

Sample 2 responded well to stiffening with strength increasing by 57.91% and

stiffness increasing by 69.34%. Sample 3 was structurally and economically the most

efficient one with strength-weight ratio of 2.52 in comparison to 2.39 for ISMB-150.

The use of 1 mm thick sheets is not recommended for fabrication of beams as they

have high susceptibility to buckling. And even after stiffening, strength and stiffness

were not improved substantially (only 19.04% and 10.03% respectively).

A comparison of the theoretical capacity (calculated using IS Codes) and the

actual capacity found experimentally shows clearly that the design standards are

highly conservative. There is thus, a dire need of revision in the current standards.

With proper stiffening arrangements and mass-production, cold-formed steel sections

can replace hot-rolled sections for lightly loaded structures, thereby reducing quantity

of steel used and economizing construction projects.

REFERENCES

[1] Wei-Wen Yu, Roger A. LaBoube. Cold-Formed Steel Design, 4th edition.

[2] IS 801-1975, Code of practice for use of cold-formed light gauge steel structural

members in general building construction.

[3] S.A.Kakade, B.A.Bhandarkar, S.K.Sonar. Study of various design methods for

cold-formed light gauge steel sections for compressive strength, International

Journal of Research in Engineering and Technology, 3. ISSN Online: 2319-1163,

ISSN Print: 2321-7308. 2014

[4] British Standard BS5950-5: 1998. Structural use of steel work in building, Part 5.

Code practice for design of cold-formed thin gauge sections.

[5] Rupen Goswami, Jaswant. N. Arlekar, C.V. R. Murthy. Limitations of available

Indian Hot-Rolled I-Sections for use in Seismic Steel MRFs, 2005

[6] Liping Wang, Ben Young. Design of cold-formed steel channels with stiffened

webs subjected to bending, Thin-Walled Structures 85, 2014, pp.81–92.

[7] Cheng Yu, Weiming Yan 2010, Effective Width Method for determining

distortional buckling strength of cold-formed steel flexural C and Z sections,

Thin-Walled Structures 49 (2011) pp.233–238.

[8] R.B. Kulkarni, Shweta.B. Khidrapure (2014), Parametric study and comparison

of Indian standard code with British standard code for the design of light gauge

cold formed flexural members, IJETR, ISSN: 2321-0869, 29(11), November

2014.

[9] Stone and LaBoube (2005), Behavior of cold-formed steel built-up I-sections,

Thin-Walled Structures 43 (2005) 1805–1817.

[10] N. Umamaheswari and Dhanya Mary Alexander. A State of The Art Report on

Fatigue Behaviour of Steel Structures Strengthened with Fibre-Reinforced

Polymer Composites. International Journal of Civil Engineering and Technology,

5(3), 2014, pp. 301 – 307.

[11] Vima Velayudhan Ithikkat and Dipu V S. Analytical Studies on Concrete Filled

Steel Tubes. International Journal of Civil Engineering and Technology, 5(12),

2014, pp. 99-106.

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