What subfield are you applying to for your PhD? History? Theory? Computation? Building science?

Academic admissions committees will be looking for evidence of your ability to do academic research. As long as you're providing the required writing samples, it probably wouldn't hurt to send a very selective (i.e., brief) set of work samples in a concise portfolio--specifically, the ones illustrating claims made in your app essay. It may not get looked at, or it may. But I would not send a full portfolio.

You can always call/email and ask the admissions contact, too.

when i applied two years ago each of the programs (history/theory) that i applied to required portfolios of design work in addition to writing samples...

I applied environmental behavior studies. and my porfolio is a 10 page double sided. I guess design work shows applicants' understanding of spaces, which could be of great importance for research...hopefully.

thanks I'll include my portfolio
ps. I'm still wondering how committee decide on "academic and research potential"...

Phase,

You indicate potential by describing how curious you are about a research question, or set of questions. You are in no way held responsible for the subject matter proposed in your application essay, but it should convey how much you crave to find things out about relevant topics. Because that's what you will be doing in a doctoral program.

For example: in my application essay to do the PhD in urban planning, I wrote that my interest in learning about the history and impact of codes on the built environment was rooted in my constant frustration with them as a practicing architect. Why were they there? Who wrote them? (Surprise! Often, it's architects.) What purpose do they serve? Et cetera. Now, I did end up doing this kind of research, but I wasn't bound to it by my application.

So, a curiosity sufficient to endure systemmatic research methods is part of this "potential." Another part is writing ability: writers improve with experience, of course, but to be able to write clearly at the application stage is crucial. (Self-evident? You bet. But, still, many graduate students can't write their way out of the proverbial paper bag.)

Good luck!

yeah, i think that you're writing samples and cv are the primary vehicles of demonstrating "academic and research potential"... ideally the writing samples that you submit will have some relation to the areas that you hope to investigate in your phd studies... these could either be papers that you've presented at conferences or published in journals or just papers that you've previously written for school... for example, my samples included one conference paper and 2-3 short essays that i had written during my masters studies...

thanks all, great insight.
since I don't have much research experience and no paper or excellent writing samples, I guess I'd hope my essay could get me into the door...