CHAT WITH A MAYOR

A city simply cannot survive without it, says Pittsburgh Mayor Richard Caliguiri. He's talking about the importance of kept-up roads, bridges, sewer and water lines, and even local housing, which in his view make up a city's ''critical base,'' its infrastructure.

Pittsburgh residents know firsthand all about the importance of that working network. Many have become adept at fashioning detours, as various downtown streets and area bridges have been shut down for repairs.

Only Venice has more bridges than this hilly city, which sits on three major rivers. But most bridges in and around Pittsburgh were built about the same time - the 1920s - and with a design life of about 50 years, shortened by steady neglect. Most of them hit big problems in the 1970s.

As Mayor Caliguiri recalls in an interview in his City Hall office, city engineers advised him almost weekly after he was elected in 1977 to either close or limit the weight on yet another bridge.

Roads and underground water and sewer lines had been similarly neglected, he says. So city and business leaders here put their heads together to set priorities and map a plan of action. So far, more than two dozen bridges have been repaired or rebuilt, and about half the local streets have been resurfaced. And the mayor says all roads - generally considered to have a 10- to 15-year life - will be checked every 12 years and the needed action taken. Under a recent city law, upkeep and management of the city's water supply, some of it running through century-old pipes, is to be turned over to a new water authority , which can borrow money at cheaper rates and set water rates that more realistically reflect costs.

''You can't just build all these things and then forget about them,'' insists Caliguiri, a former parks supervisor and City Council president who has been in government 27 years. ''These aren't glamorous projects, but the minute people can't get water out of their spigots or flush the toilet, they're going to be banging my door down.''

The key problem, he admits, is financing. Most funds for such base repairs have come from federal and state sources, but levels are down and dollars are more readily available for new projects than for upkeep. ''New roads should not be a priority over roads built 50 years ago that now need rehabilitation,'' says the mayor. ''We should take stock of what we've already built and take care of it.''