My major was civil engineering in my undegraduate study and I finished MS in structural engineering at University of Pittsburgh without thesis. Currently I am planning to start my PhD career in the U.S. or Canada and this decision is adamant. I already got conditional admission from one university in the U.S. which ranking around 90. Conditional admission means I don`t even have a chance to apply for financial support. All other professors either told me their fundings are extremely limitted or no plan for admitting new PhD members.
I am so frustrated that I`m now thinking about initiate my self-funded PhD study. I know it`s gonna be a massive spend and my family can afford me that, but I am hesitating about the way other students treat me. Am I debasing myself by doing this? What if I couldn`t get any offer till the deadline. Dose it worth it that I spend tens of thousands of dollars to attend a bottom ranking university?
Can someone please help me and give me some advise regarding what I should do?
Thank you very much!

Don't self-fund. Particularly not in a STEM subject and particularly not in N America. Take a year out, work on whatever you think are the current weaknesses in your application and reapply. As far as I know, it's a real issue in N America to either self-fund or attend a low-ranked programme and can have serious limitations on your future prospects, so really not worth rushing into if another year applying might leave you in a much better place.

I agree with bewildered. But if you're still considering it, also factor into the financial calculations the 'opportunity cost' i.e. not only is there the financial cost from the tuition and your living expenses, you are also losing out on whatever salary you would be getting by doing something else instead.

Edited to add... you don't mention what your long term career plans are. If you want to be an academic engineer, then a PhD may be useful. If, however, you want to work in industry, check that a PhD would really enhance your career prospects - you may be better off without one.

I am a self-funded PhD in the US paying for my expenses with a combination of loans, my job as a tutor, and personal money just for tuition purposes. It is possible to do so but expect a lot of long days and longer nights and be ready to put in a lot more time and effort as a fully funded PhD student does. If you have enough strive and ambition to be able to attain your PhD than it shouldn't be much of a problem. Hopefully this helps!