Ijciet 06 09_010

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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
© IAEME Publication
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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