What happens when labor unions real ize they no longer have unlimited access to the public kitty?

Sometimes they get . . . reasonable.

Witness the deal Mayor Bloomberg struck last month with the Building and Construction Trades Council in hopes of keeping $5.3 billion in city building projects above water.

The agreement would save $300 million over four years — mostly through union work-rule concessions.

Among other things, the union OK’d eight rather than seven-hour workdays, flexible scheduling, overtime caps and a no-strike pledge.

The rationale seems breathtakingly simple: Without givebacks, in this economy, the projects risked stalling completely.

Alas, the principles of basic economics still don’t seem to apply to New York’s public-sector unions:

* The state’s Public Employee Federation contract provides for 3 percent annual raises despite a $27 billion 2½-year budget deficit — and the union’s faced no credible threat of layoffs.

* The MTA is worse than flat broke — yet transit workers were recently awarded 11 percent raises over the next three years plus health-care sweeteners.

* The city’s teachers union has reportedly been expecting up to 4 percent annual raises from current contract talks — even though top salaries have already risen 43 percent in the last eight years, and the city faces a $5 billion budget hole next year.

Such is these unions’ political power that they’re accustomed to living large no matter what — never mind that they’re driving New York into bankruptcy.

Unlike their hardhat cousins, they’re not likely to feel the pinch any sooner.