What is the difference between purlins and secondary beams?

Answer 1

It's easiest to think of them like this:

Each sub-discipline of structural engineering have their own terms.

For the building industry, generally;

Purlins are located on the roof of a structure. A purlin usually do not support any other beams, just loads from the roof deck, which makes it secondary.

Girts are located on the walls of a structure. Girts usually does not support any other beams, just loads from walls, which makes it secondary.

Secondary beams support deck framing in general (usually used for floor beams). Secondary beams shift the loads to primary beams, which as you know can happen anywhere in the structure.

Primary beams are the larger beams in the structure that support secondary beams (think girders). Girders are usually supported by columns.

Purlins and Girts are secondary beams that are located in specific location. The industry just calls them that to make it easier. Imagine writing sentences like secondary roof beam "A" frames into secondary roof beam "B." This could easily be reduced to purlin "A" to purlin "B."

You will find slightly different naming conventions for structural components in all structural engineering industries (Offshore vs Onshore Industrial, Transportation, commercial building, residential, etc.)