Really fantastic submission to the jam. Some of these levels are hard but I think after this. I am something of a building engineer myself!
Something that would be cool, would be to add short keys to concrete and steel beams. I thought it was really hard to get a solution in 30 seconds. Some of the earlier levels were doable but the later ones I can't imagine how to do it in 30 sec only. Giving concrete and steel beams dedicated keys would speed up my skyscrapers.
Also not sure if I should still be winning this one XD