for sufficient rentable space to make the building profitable. What shattered that restriction was the invention of first the iron- and then the steel frame building, in which the building's load was carried by an internal metal frame skeleton, which the masonry – and later glass – simply hung off of without carrying any weight. Although first used in Chicago, the steel-framed skyscraper caught on most quickly in New York City in the 1880s, and from there spread to most other American cities in the 1890s and 1900s. The apparent lack of a height limitation of this type of building set off a fervent debate over whether their height should be restricted by law, with proponents and opponents of height limits bringing out numerous arguments in favor of their position. The question of height limits also had a profound implication for the nature of downtown itself: would it continue to be a concentrated core, or as it grew, would height limits force it to spread out into a larger area. In the short run, the proponents of height limits were successful in their efforts. By the 1910s, most of the largest and medium-sized cities had height limits in effect, with New York – despite several concerted efforts to enact them – Philadelphia, Detroit, Pittsburgh and Minneapolis being notable holdouts. Zoning Ultimately, though, it would not be height limits per se that restricted skyscrapers, but comprehensive zoning laws which would set up separate requirements for different parts of a city, and would regulate not only height, but also a building's volume, the percentage of the lot used, and the amount of light the building blocked, and would also encourage setbacks to reduce a building's bulk by allowing additional height per foot of setback – the exact amount depending on what zone the building was in. New York City was the first to do this, with the 1916 Zoning Resolution, which was prompted in good part by the construction of the Equitable Building in 1915, a 40-story building with straight sides and no setbacks, which raised fears of the downtown area becoming a maze of dark streets that never saw the sun. What was worse, at least to real estate interests, the building dumped 1.2 million square feet (111,000 m2) of office space on what was a sluggish real estate market. To many in the real estate industry, the zoning law was