Complex sentence: When completed in 1899, the massive edifice was the largest office building and first building incorporating a steel frame in Washington; the steel frame supports floors and interior constructions, but the outer walls, five feet thick at their base, are still self-supporting.
Split into simpler sentences: When completed in 1899, the massive edifice was the largest office building and first building incorporating a steel frame in Washington. The steel frame supports floors and interior constructions, but the outer walls, five feet thick at their base, are still self-supporting.