Elevations of structural steel on drawings are usually measured above a given reference point, or:

The correct answer is that elevations of structural steel on drawings are usually measured above the foundation. This reference point is crucial as it provides a consistent baseline for all measurements throughout the construction process, ensuring that various structural elements are correctly positioned relative to one another.

In the context of construction, measuring above the foundation helps account for any variations in ground levels that could occur due to soil compaction or other site conditions. This approach ensures that everything is built on a solid and stable base, which is essential for the integrity and safety of the structure.

The other options, such as above ground level, above sea level, and above concrete level, may not provide the same level of consistency and practicality in a construction environment where the precise location of structural elements must align with the foundational framework.