Dallas City Hall was designed by I.M. Pei and opened in 1978. Its design reflects an inverted pyramid, with its exterior walls sloping upwards at a 34-degree angle, allowing both for additional office space for workers on the top floors as well as a shaded perimeter around the building for visitors and office workers during the day. The building was constructed with a concrete mix intended to evoke the natural colors of the Dallas-Ft. Worth area geography. The gravity-defying design so unnerved then-Mayor Erik Jonsson that Pei added a series of non-loadbearing oval columns to the perimeter of the building to serve as reassurance of the building structural integrity.