Ph.D

Need to complete your M.Tech in Structures. It will be preferable if you join as faculty first in some of engineering colleges or design and construction company. Then decide about your PhD.

No, you are required to have a higher percentage. Minimum 65% in UG and PG. However if you have found your potential supervisor already from an Australian University who is interested in thesis, then may be he/she can help. Hope this helps. Thanks.

Hello, aim for a higher first class, there are no cut offs but to be a researcher you must show excellent academic performance and a strong research orientation. Keep checking university websites for updates and latest admission requirements, this will be helpful when you are preparing. Just remember that your profile will be evaluated holistically and will not be focused only on your grades when you are applying to universities in US.
Regards

The eligibility criteria for both M.Phil and Ph.D varies for every institute. However the basic requirement is a Masters degree or an equivalent degree in the relevant discipline. However the cut off percentages vary. Some of the institutes conduct entrance exams and some of them require atleast two Research Publications for the Ph.D programs. Some also require work experience of a couple of years.

In india, the admission for doctoral programmes hv different criteria as per the qualification. Take a look at IIM requirement here.
http://www.iimahd.ernet.in/programmes/fpm/how-to-apply/eligibility-criteria.html

You may explore with IIHMR University .Their PH D cour is good.