Yes - I find it hard to imagine I-beams couldn’t be found embedded in a geological layer somewhere.

Very few large things make it into the fossil record ( https://news.ycombinator.com/item?id=16837795 ), and a small fraction of those are discovered. It's possible, but I-beams are rare compared to plastic particles, which are basically carpeting the seabeds, rivers and beaches now.

"microplastics everywhere". http://www.bbc.co.uk/news/science-environment-43363545

"Microplastic pollution in oceans is far worse than feared, say scientists", "pervasive and persistent nature" https://www.theguardian.com/environment/2018/mar/12/micropla...

"The deep sea is a major sink for microplastic debris" http://rsos.royalsocietypublishing.org/content/royopensci/1/...

We don't know what the effect of millions of years on this debris will be, but I am suggesting that due to the fact of the human species as a geological force, most of the rocks from this era will be a bit unusual in composition.

I-beams would dissolve into rust very quickly.

Your best bet for metals might be gold as it is not typically reactive with water or oxygen. And as a reminder, we do sometimes find solid blobs of gold... :-)

Agreed about the gold. That came to mind also.

I also agree that the I-beams would rust away, as would rebar embedded in concrete. Would they remain as identifiable shapes or would the material migrate?

"remain as identifiable shapes" is basically fossilisation, which does happen. And it likely applies to I-beams as much as it does to coke cans, plastic bottles and cigarette buts. Except that the latter are going to be much more numerous.