The present study deals with the experimental test of the cold-formed steel sections under axial compression load to determine the buckling mode, deflection, stress, strain and various parameter. The test is done in a loading frame of 50-ton capacity with proper conditions.
2.1 Cold-formed built-up section
The column was cast with grade S35 and yield stress of 240 N/mm2. The dimension of the column was 100 x 40 x 2 mm face-to-face built-up plain channel section was selected as shown in figure 1 for the investigation based on an initial study that was undertaken to determine that is prone to local buckling. The column height was chosen 1230mm. And GFRP will be very effective in strengthening the given section.
Figure 2 shows the images of the casted specimen.
2.2 GFRP (Glass Fiber Reinforced Polymer) and Adhesive
High strength, high modules E-CR glass fiber (termed as M123) produced by Advantex was used along with PERPOL 100 Orthophthalic Non-thixotropic polyester resin (adhesive)for external strengthening purpose. The M123 chopped strand mat is made with medium size strands of Advantex glass roving with a composition of 54% SiO2 - 15%Al2O3 - 12%CaO. It has a nominal tensile strength and a tensile modulus of 1950 Mpa. Typical thickness of GFRP composite produced by a single layer of fibre witted with resin is 1mm. Figure 3 shows the Glass fiber reinforced polymer (GFRP) wrapped specimen
2.3 Fabrication of test specimens
The face-to-face built-up columns were bolted to a M10 thick grade 240 at both ends to achieve pinned conditions. Then the test columns surface was prepared to achieve a good bond between steel and GFRP. Solvent cleaning and grinding are normally used for this purpose. However, grinding was adopted in this research as it produces a high-energy surface, which has the ability to attract materials (GFRP and adhesive) to the substrate surface. Consequently, perfect bonding between steel and GFRP was achieved. For a layer of the strengthened case, the strengthened column was allowed to remain for about 60 mins before the application of the fiber sheet. All strengthened columns were allowed to cure for 3 days at room temperature before the testing.
2.4 Test set up:
Column of regular cold-formed steel and glass fiber reinforced cold-formed steel which is likely to fail at ultimate loads were tested using 500kN capacity of the universal testing machine. The machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. The strain measurements are done with an extensometer
The test process involves placing the test specimen in the testing machine and slowly extending it until it fractures. During this process, the elongation of the gauge section is recorded against the applied force. The data is manipulated so that it is not specific to the geometry of the test sample. The elongation measurement is used to calculate the engineering strain, ε, using the following equation:
STRAIN = CHANGE IN LENGTH (ΔL)/ ORIGINAL LENGTH (L0)
where ΔL is the change in gauge length, L0 is the initial gauge length, and L is the final length. The force measurement is calculated using the following equation:
STRESS = LOAD (F)/AREA (A)
where F is the tensile force and A is the nominal cross-section of the specimen. The machine does these calculations as the force increases so that the data points can be graphed into a stress-strain curve
2.5 Test procedure
The test specimen was fixed to the loading frame of 50-ton capacity as shown in figure 4. The alignments were properly checked and the deflection gauges & strain gauges were fixed the in necessary locations. Axial load was applied with the help of hydraulic jack. To measure the axial strain in the face of the built-up columns, dial gauges were installed. The lateral displacement of tests columns was measured using LVDT were placed at the mid-position in pursuit of capturing the local buckling. In addition, the symmetric loading arrangement of the setups prevented any eccentric loading on the test column. Initially, the test column was loaded to about the expected failure load, and then the loading rate was adopted to clearly examine the local buckling formation of the column. Necessary readings are taken from the proving ring, LVDT & the strain gauges. Graphs are plotted from the results obtained.
Figure 5 shows the fixing of dial gauges and the loading position
2.6 Buckling Mode Images: Figure 6 shows the buckling and flange deflection of the specimen. Figure 7 shows the Overall deflection and buckling of GFRP wrapped column