The use of Cold-Formed Steel system is significantly increasing in the countries where traditional structural solutions have always dominated the construction sector. This is due to recognized technical, structural and economic competitiveness of such systems. The CFS members obtained by cold rolling are produced by pressing or bending steel sheaths with thickness ranging between 0.40.7mm. These members provide several advantages such as lightness of structural systems, high quality of end products, flexibility due to wide variety of shapes and sectional dimensions, short execution period and minimization of environmental impacts.
In steel structures located in seismic area, factors like premature local buckling and low out of plain stiffness are regarded as main structural deficiencies. The use of Cold-Formed Steel structural systems in such area improves the energy dissipation property of the structure which accounts for the seismic resistance. These sections have a high potential for premature failures because of their thin walled nature. To
reduce the chances of failures ample ductility should be imparted to the sections.
Advantages of Cold Rolled Steel
• Some of the advantages of cold rolled sections as compared to the hot rolled counterparts are as follows:
• Cross-sectional shapes are formed to close tolerances and these can be consistently repeated for as long as required
• Cold rolling can be employed to produce any desired shape to any desired length
• Pre-galvanized or pre-coated metals can be formed so that high resistance to corrosion, besides an attractive surface finish can be achieved.
• All conventional connection methods i.e., riveting, bolting, welding, etc can be employed.
• High strength to weight ratio can be achieved in cold rolled sections.
• They are usually light in weight, easy to transport and erect.
Case Studies Experimental work on CFS elements for earthquake resilient moment frame buildings.
Experimental investigation on the use of thin walled cold-formed sections as energy dissipative elements for earthquake resistant moment frame for multi-storey buildings were carried out. In multi-story moment frames use of CFS often led to premature local buckling and low out of plane stiffness. To overcome these deficiencies the following experimental investigation was carried out. The tests were performed on 6 bolted beams to column connections with two different beam thicknesses 3mm and 4mm respectively. The test involved step-bystep development curved flange sections and beams with different types out of plane stiffeners in the connection region; the three configurations of the stiffener connections being: without stiffener, minimum stiffener and optimum stiffener. The test specimens were subjected to cyclic loading through hinge connections at the beam end. From the results obtained through the experiment it as concluded that:
1. The premature buckling due to discontinuity in the flange connection can be delayed by using combination of vertical or horizontal out of plane stiffeners.
2. Ductility factor was increased by 75%, moment strength by 35% and energy dissipation by 240%.
Damage Control systems
In order to control rocking behaviour of cold formed steel in multi-storey shear walls an innovative damage control system was proposed. The system involved hold-down equipped with a fuse function and was placed on the foundation of multi-storey shear walls. This HDF (hold-down fuse) reduces the damage to both structural and non-structural members in the cold-formed steel buildings.
To understand the basic behaviour of proposed controlling racking systems in an earthquake, shake table tests were performed using one storey, one span, real
size specimens. The specimen consisted of steel sheet shear wall with a height and width of 3000 and 6000mm respectively and steel bracings with height and width of 3000 and 1500mm respectively.
Two steel shear walls were set parallel to the shake direction. Figure shows the controlled-rocking system with multi-storey shear walls for cold-formed fuse frames. In this system, a fuse made of a steel plate which was placed between the racking frames and the foundation. It was connected to a channel of the upper frame and an anchor bolt supported by the foundation. In conventional cold-formed steel structures a frame is rigidly connected to the foundation by steel hold-downs. Therefore the building dissipates the earthquake energy mainly through plastic deformation of shear walls or bracing members in upper frames. Whereas, the newly proposed HDF’s dissipate most of the seismic energy and as a result member and walls within the frame sustain less damage.
Shake table tests of a cold-formed steel shear panel
Several investigators have tested CFS shear walls for static cyclic loadings and the results were evaluated. But, there is very limited literature available for the performance of CFS shear walls under dynamic loadings. To account for this, a shake table test of a full scale two storey single bay structure was conducted. They were separated by 3.9m centre to centre in the out of plane direction.
The shear panel was 2.8m wide and the storey height was 3m. Both the storeys were identical but the bottom storey was subjected to greater loads. The exterior columns were built-up by channel sections and were welded to the steel anchors and bolted to the slab.
The slab was a heavy reinforced one, 200mm thick and 4.40mm2 in area weighing 93kN. This slab acted as a rigid diaphragm. The specimen design was done considering the location of Seattle, Washington using the data guidelines from TI 809-07 (III). The seismic base shear for the shake table was estimated to be 96.1kN per frame. However the frames were designed for a base shear of 96.4kN each. Shake table specimen was instrumented with accelerometers, displacement gauges and strain gauges.
Through the test results it was seen that the thin steel straps used a cross bracing in CFS buildings are very tough and ductile. After the local buckling, the CFS built-up exterior columns performed well. The columns contribution to the shear capacity of structure was minimum but dependable throughout earth-quake simulation. When the bracings failed to provide strength and stiffness to the earthquake responses, the columns provided energy dissipation.
The CFS structure provided to be a very affective and dependable structural system for seismic loads and provide the bracing member is prevented from fracture.
Seismic behaviour of sheathed CFS shear walls cladded by gypsum and fiber cement boards.
Experiments are carried out to investigate the seismic response of steel sheathed CFS shear walls using gypsum and fibre cement board claddings. The experiment was carried out on six such specimens of various configurations and was tested under cyclic loading. With the introduction of claddings, it imposed additional forces on the members in the load path towards the foundation which resulted in the increases of shear strength of walls. Also use of single and double sided steel sheathing on CFS shear walls increased their shear strength and energy dissipation capacity. It was concluded that the shear strength and secant stiffness of CFS steel sheathed walls can be increased by 30% to 32% for single sided and 61% to 80% for double sided claddidng.
Use of CFS products are important and effective in the constructions carried out especially in seismic areas; lightweight structural systems, high quality end products, flexibility, short execution period, minimum impacts on the environment are some of the important properties of CFS systems which make them suitable for seismic areas.
CFS systems are under research and some of the research works discussed above yield the following conclusions:
• With the use of curved flange beams in earthquake resistant moment frame multi-story buildings resulted in the increase of ductility (upto75%) and moment capacity (upto35%) and also
hysteretic energy dissipation capacity (upto240%).
• By providing single and double sided gypsum and fiber cement cladding CFS steel sheathed walls, the shear strength and energy dissipation capacity can be increased by
31% and 32% for single sided cladding and up to 80% and 67% for double sided cladding
• With the use of vertical/horizontal out of plane stiffeners the premature buckling due to discontinuity in the flange is delayed.
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Figure 1: a) Different flange shape layouts; b) 3-D simulated view of curved flanged beams.
Figure 2: Experimental set-up.
Figure 3: Hold-down steel fuse
Figure 4: a) Different flange shape layouts; b) 3-D simulated view of curved flanged beams
Figure 6: Plan cross-sections of the specimens
Figure 7: Framing details and screw arrangement of the test specimen