Pittsburgh's economy grew progressively more dependent on the steel industry from the late 1800s to the mid 1900s. When that industry collapsed in the 1970s-1980s Pittsburgh was hit hard. The second largest industry in Pittsburgh was glass manufacturing. That industry has also essentially collapsed. Pittsburgh has lost 54.8% of it's population since 1950. This is the 5th highest percentage of any major American city. Pittsburgh seems to be stabilizing recently and has even shown some signs of growth in a few areas. This revival is still very new and tenuous and is focused in just a few areas of the city. Pittsburgh has historically had strong neighborhoods. It's geography isolates pockets of development with barriers of hills and rivers. I think this has contributed to the fact that decline and stability have been very uneven in the city. Allegheny county which Pittsburgh is in, has lost 24.9% of it's population since 1960. Within the county decline has also been very uneven. Some suburbs have continued to grow while others like Braddock have lost far greater percentages of residents than Pittsburgh. I think that Pittsburgh's strong neighborhoods have helped the city survive and they are still one of the most appealing things about the city.
Pittsburgh very quickly demolished steel mills as they closed in the 70s and 80s. This has left the city without many tangible reminders of the industry that is still a big part of Pittsburgh's identity. Some of the former mill sites like the Southside works have been successfully redeveloped. Others like Hazleton are still empty. Large sections of the city were butchered by urban redevelopment projects in the mid 1900s. The Northside was the most severely impacted by renewal and freeway projects but large areas on the East and South sides survived unscathed. Many neighborhoods in Pittsburgh like the Southside and Lawrenceville are stable or thriving. In others though abandoned buildings and demolition of historic structures is still common.