Nor can they afford more blue-chip bonds, which won't pay much, as long as the Federal Reserve keeps offering cheap money to an economy that's in no mood to party.
So the city has turned to edgier investors, like Joshua Harris, the Wharton-grad billionaire who leads the group that bought Philadelphia's 76ers a couple of years back (and, more recently, the NHL's New Jersey Devils).
Since May 31, Harris' Apollo Global Management has been pumping $50 million in city pension dollars, through the new Apollo Franklin Investment Fund L.P., to buy "stressed" corporate loans, short-term financing, junk-rated debt, and other high-interest lending acts.
Isn't that extra-risky? It's the opposite, insists Sumit Handa, the pension's chief investment officer and architect of Philadelphia's new-look portfolio. He says Apollo's loans are well-collateralized - secured by real estate and other property Apollo can seize and sell if the borrowers fall behind.
Also, Handa says Apollo is letting the city collect profits when they become available - instead of waiting 10 years for investments to "mature," like those high-fee buyouts, venture, and real estate funds the city bought in the 1990s and 2000s.
"It's working out great," so far, says Francis Bielli, the pension system's executive director. In its early days Apollo Franklin is generating almost 1 percentage point a month on its investments, he says.
A similar fund run by Kohlberg Kravis Roberts has grown $50 million into $56 million in 13 months, according to the monthly report circulated at the pension board's meeting Thursday.
Under Handa, the pension system has narrowed the gap by which its investment performance lags its profit benchmarks, says city finance director Rob Dubow.
Dubow praises the system's new $150 million Independence Fund, run by city employees who buy and short stocks, without paying any fees to outside managers.
During the last year Independence has returned 10.6 percent. Like the Kohlberg and Apollo funds, that's better than the system's 7.95 percent annual target, though also way short of the 20.2 percent the pension system's stock investments returned this year.
But in months when the market fell, Independence still made money. With investments like that, it's easier for the city to protect existing assets, keep cutting checks, and wait while stocks rebound, Bielli says.
Maybe $50 million sounds like a lot. But it's just over 1 percent of the city's total pension investments - less than a half-percent of its pension liability - and a rounding error amid the total $113 billion in other people's money that Apollo manages.
Will this help make Philly pensions solvent? This sort of thing has worked well - for Harris, who is worth more than $2 billion, thanks to past fees and investments, according to the Forbes list of richest Americans.
Let's hope Philadelphia's new investments play better than Harris' weak Sixers. This city's finances can't afford a rebuilding year.