Complex sentence: The Senate is located in the Chile Congress, whose building was built under the last years of the Augusto Pinochet regime, and sits in the port city of Valparaíso, replacing the old National Congress, located in downtown of capital Santiago.
Split into simpler sentences: The Senate is located in the Chile Congress building, which was built during the last years of the Pinochet regime and stands in the port city of Valparaíso. This new bulding replaced the old National Congress, located in downtown of capital Santiago.