The Monadnock Building (historically the Monadnock Block; pronounced ) is a 16-story skyscraper located at 53 West Jackson Boulevard in the south Loop area of Chicago. The north half of the building was designed by the firm of Burnham & Root and built starting in 1891. The tallest load-bearing brick building ever constructed, it employed the first portal system of wind bracing in America. Its decorative staircases represent the first structural use of aluminum in building construction. The south half, constructed in 1893, was designed by Holabird & Roche and is similar in color and profile to the original, but the design is more traditionally ornate. When completed, it was the largest office building in the world. The success of the building was the catalyst for an important new business center at the southern end of the Loop.