Complex sentence: When ground was broken for the Wrigley Building in 1920, there were no major office buildings north of the Chicago River and the Michigan Avenue Bridge, which spans the river just south of the building, was still under construction.
Split into simpler sentences: When ground was broken for the Wrigley Building in 1920, there were no major office buildings north of the Chicago River. The Michigan Avenue Bridge, which spans the river just south of the building, was still under construction.