FLEXURAL BEHAVIOUR OF STIFFENED MODIFIED COLD-FORMED STEEL SECTIONS EXPERIMENTAL STUDY

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 6, Issue 9, Sep 2015, pp , Article ID: IJCIET_06_09_010 Available online at ISSN Print: and ISSN Online: 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, National Institute of Technology, Srinagar Prof. (Dr.) A. R. Dar Professor, Department of Civil Engineering, National Institute of Technology, Srinagar 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 editor@iaeme.com

2 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 coldformed 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 coldformed 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 editor@iaeme.com

3 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 editor@iaeme.com

4 Flexural Behaviour of Stiffened Modified Cold-Formed Steel Sections Experimental Study 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 kn beyond which lateral torsional buckling commenced (unrestrained compression flange) and subsequent failure was observed. The average stiffness in the linear region was kn/m. The ultimate failure load was kn editor@iaeme.com

5 Syed Mohammad, Mir Faizan Ul Haq, Mufti Minaam Mehmood and Prof. (Dr.) A. R. Dar Table 1 ISMB 150 Load (kn) Deflection(mm) Mid Span 1/3 rd Span 2/3 rd Span Figure 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 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 editor@iaeme.com

6 Flexural Behaviour of Stiffened Modified Cold-Formed Steel Sections Experimental Study Table 2. The beam behaved linearly till kn where corresponding deflection was mm, beyond which signs of local buckling in one of the lips were observed. The average stiffness in the linear region was kn/m. Fig.5 shows the loaddeflection curve of the beam. Table 2 Unstiffened section Load (kn) Deflection (mm) Mid span 1/3rd span 2/3rd span Figure 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 editor@iaeme.com

7 Syed Mohammad, Mir Faizan Ul Haq, Mufti Minaam Mehmood and Prof. (Dr.) A. R. Dar 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 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 kn/m. Table 3 Stiffened section Load (kn) Deflection (mm) Mid span 1/3rd span 2/3rd span Figure 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 editor@iaeme.com

8 Flexural Behaviour of Stiffened Modified Cold-Formed Steel Sections Experimental Study stiffeners. The detailed behaviour of the sections has been discussed in the following section 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 kn. The average stiffness in the linear region is kn/m. The load-deflection curve shows a predominantly linear behaviour till local buckling as shown in Fig. 7. Table 4 Unstiffened section Deflection (mm) Load (kn) Mid span 1/3 rd span 2/3 rd span Figure editor@iaeme.com

9 Syed Mohammad, Mir Faizan Ul Haq, Mufti Minaam Mehmood and Prof. (Dr.) A. R. Dar 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 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 loaddeflection curve is shown in Fig. 8. Table 5 Stiffened Section Deflection (mm) Load (kn) Mid span 1/3rd span 2/3rd span Figure editor@iaeme.com

10 Flexural Behaviour of Stiffened Modified Cold-Formed Steel Sections Experimental Study 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 kn/m to kn/m. 4. For samples 4 & 5, increase in stiffness was 10.03% from kn/m to kn/m. 5. The failure mode shifted from local to global after stiffening from Sample 2 to sample 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 editor@iaeme.com

11 Syed Mohammad, Mir Faizan Ul Haq, Mufti Minaam Mehmood and Prof. (Dr.) A. R. Dar 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 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 editor@iaeme.com

12 Flexural Behaviour of Stiffened Modified Cold-Formed Steel Sections Experimental Study 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, 4 th edition. [2] IS , 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: , ISSN Print: [4] British Standard BS5950-5: 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 [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 [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: , 29(11), November [9] Stone and LaBoube (2005), Behavior of cold-formed steel built-up I-sections, Thin-Walled Structures 43 (2005) [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 [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 editor@iaeme.com