rscassar, sounds like you're struggling with the concept of ductility as whole.
Ductility is just controlled damage, allowing the formation of controlled mechanisms (flexural usually) to dissapate seismic energy and to ultimately allow for more economic designs (which arguably need to be demolished due to the damage we are accepting in adopting ductility to lower the overall base shear). But think of it as achieving the life safety requirements, potentially at a lower cost overall than adopting an elastic design at the expense of some more onerous detailing.
The reality is as well in some countries such as NZ we have to use ductility, as we are dealing with seismic loads of more than 100% of the building weight otherwise, that becomes uneconomic. Often even though the overall amount of reinforcement for a ductil designmight be similar, the forces on the foundations after consideration of overstrneght might be considerably lower than designing for an elastic or nominally ductile design.
One aspect that designers seem to struggle with is that even at in an elastic design, if a big enough event comes along then you'll still form your mechanism and won't neccessarily have the remainder of the structure protected from say a shear failure in the case of a cantilevered wall system.
In a real seismic code (not any AS code really) aspects related to capacity design ensure these mechanisms can form and behave in a dependable manner and that the rest of the structure is protected. You're designing certain critical regions to actually yield, and protecting the remainder so you do not form any undesirable mecahnisms. I'll admit your code has gotten better, but it still has some really weird rules on some things from an outside perspective compared to more modern/developed seismic design codes.
Quite frankly some of the rules adopted in Australia and the way people seem to then manipulate them is a joke. Not a funny one. Designers seem to expend a lot of effort (as evidenced in these forums and my own personel dealing with Australian designers) in miscontruing and talking themselves out of doing what just needs to be done. Sure you can be smart about design, but it's a fine line between being smart and dumb as far as seismic design goes, the earthquake will find you out one way or another.
Most likely in my opinion they do this because they have not been educated with seismic design at the forefront of their design education, they simply lack the knowledge and experience to understand the concepts involved, but also old school thinking in the contruction industry and the inability to flex to take on board the evolving requirements with respect to seismic design. That old school we've never had to do it this way before attitude from the contractor and design side of things.
Hopefully at Australian Universities there is now a new breed of engineers being trained and coming through that do have the theoretical knowledge to know why things need to be done this way due to the increased focus on the seismic design side of things in your concrete code, and they won't be so manupulative and blases with the rules as a result.
You really need a serious earthquake that results in widespread damage and potentially loss of life to change your industries way of thinking about seismic design. NZ has been throuh this and learned (and continues to learn) the lesson. We still have our issues with the quality of designers, no easy fix to this I guess.
If mu is taken of 2 or 3 and it results in a higher reinforcement requirement and a lower structural capacity. ???.
If you're ending up with more longitudinal reinforcement then you're doing something wrong? If you're talking about more transverse reinforcement then yes, you'll have this as aspects like confinement in axial members and antibuckling requires more onerous requirements when dealing with ductility and potential for overstrength forces. I suspect because your seismic loads are so low, that the decision to go with a ductile design isn't being forced on you like it is in other countries as the only means of actually producing a building that makes economic sense.
