Even when dealing with large and unsubtle structure, small details often provide important information. The picture below shows the underside of a floor in a loft building constructed in the 1920s. (Some tenant decided that dark gray paint would look nice.) Given that New York is full of concrete-encased steel floor beams, the question comes up: is this a steel- or concrete-frame building?
The answer is in the chamfers. Ordinary practice in concrete construction was (and usually still is) to add chamfer strips to the forms so that concrete beams do not have sharp corners. For whatever reason – possibly because there was little overlap between reinforced-concrete construction companies and the companies that provided concrete floors for steel construction – chamfered corners were not normally used for the concrete encasement around steel beams. In this case, the carpenter who built the forms went one step further and used chamfers where the shallow filler beams meet the deeper girder and where the beams meet the slab underside.