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Romans had a basic understanding of proportions and general vibes of design, but they couldn't really create calculations to verify their design as we do today. For instance, the designers of the Pantheon understood they needed to lighten the concrete as it went up and had novel techniques of doing so that we use today, but they didn't really know.

The state of the art for understanding basic stresses didn't become known until the 19th century. The design of the Eiffel Tower was impressive, in part, because that structure has a calc book defending it. A lot of early railroad bridges were still designed based on ratios and vibes.

That said, not all loads were understood by that time. The Tacoma-Narrows Bridge collapsed in part because of a gross misunderstanding of wind loads combined with the ability to design structures light enough that wind loads controlled.