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Concrete In Australia : September 2009
Concrete in Australia Vol 35 No 3 41 for columns derived from the detailing requirements of the ties. An evaluation of the studies by Ghazi (2001) and Zaina (2005) indicate a level I10 = 5.6 for a 50 MPa column fabricated to the minimum detailing requirements of the standard. Maintaining a similar level of ductility for HSC columns dictates that more stringent design rules are required for HSC columns than for conventional strength columns. Taking I10 ≥ 5.6 as a sufficient level of ductility in non-seismic regions gives an effective confining pressure requirement of kff er c = ¢ 001 . (9) Note that higher values are required for seismic regions where the ductility index requirement as obtained from AS1170.4 is such that >3. 2 APPLICATION Design for confinement to the core of columns is required in a section that fails in a primary compression mode and is subjected to high stress. at is in dominantly compression regions where plastic hinges are required to form. Research by Mendis & Kovacic (1999) has shown that where the axial force on a section is less than 03 . ¢fA cg , no special provisions are needed to obtain a sufficiently ductile section for non-seismic design over and above those detailed for restraint of the longitudinal reinforcement. In addition, no additional provision for tie or helix reinforcement is required over and above that required for restraint of the longitudinal reinforcement where the bending stress in the section is less than 60% of the capacity of the section. e application of AS3600-2009 clause 10.7.3 is summarised in the M-N interaction plot shown in Figure 4. In the unhatched regions, the section stresses, or confinement demands to ensure an adequate level of ductility, are sufficiently low that no additional attention is needed other than limits on the spacing of the tie reinforcement. is limit is that the spacing of the ties or helix reinforcement shall not exceed the lesser of 0.8 times the depth of the section in the direction of the bending being considered and 300 mm. For design action effects on a section that lie within the hatched region, ties or helix reinforcement is provided such that the minimum effective confinement pressures are provided on the section at the strength limit state and with the maximum tie spacing the lesser of s ≤ 0.6D and 300 mm, where D is measured with consideration to the axis of bending being considered. Additionally, to ensure that fitments are placed at a sufficient length along the member from the centre of the expected plastic hinge, the fitments are required to extend a minimum length measured each side of the maximum moment bounded by the lesser of (Figure 5): (i) 1.2 times the dimension of the cross-section measured normal to the axis of bending being considered, and (ii) the distance to the end of the member. 3 DESIGN EXAMPLES 3.1 Example 1: High axial load with low moment In this first example, two columns from a 20 storey building having the floor plan shown in Figure 6 are designed and detailed for a case of gravity loading. e column considered is B2 at Level 1 of the structure, which is subject to high axial load and low moment. For the columns we shall take ¢ = fc 80MPa,fsyf. =500MPaand A B C D E 1 2 3 4 5 6 8.0m 8.0m 8.0m 8.0m 8.0m 8.0m 8.0m 8.0m 8.0m Figure 6. Floor plan for design examples 3.1 and 3.2.