Ever hear of the "Philadelphia Plan"? It was rolled out 50 years ago as a national model to integrate the building trade unions. That few people today know about it helps explain why this city five decades later remains a bastion of inequality when it comes to black and brown members of skilled labor organizations.

Reacting to protests by the NAACP, the Urban League, and other civil rights groups in the 1950s and 1960s, Washington's response was to include a fair-employment clause, advocating voluntary steps to increase minority hiring, in all federal contracts.

The policy failed miserably, which prompted African American protesters to shut down federal construction sites in Philadelphia. The Kennedy administration responded by creating the Philadelphia Plan, which required prospective contractors to specify the number of nonwhite workers they planned to hire.

But the comptroller general of the United States ruled the Philadelphia Plan illegal, saying its hiring goals too closely resembled quotas. President Richard Nixon, who courted the black vote in 1968, resurrected the Philadelphia Plan. But he abandoned it in 1970, apparently to appease white construction workers who had staged a hard-hat riot in New York in support of his Vietnam War strategy.

Prior to the Philadelphia Plan, minority membership in the city's ironworkers, plumbers, steamfitters and pipefitters, sheet-metal workers, electrical workers, and elevator constructor unions was estimated at less than 1 percent. By 1980, the unions claimed it had climbed to 12 percent. But it was 10 percent in 2008, the last time the unions made a disclosure.

That's terrible in a city that is 44 percent black and 13 percent Hispanic and that has had three African American mayors since 1984. It seems unreal that neither Philadelphia's mayors nor any of its Councils have effectively used the city's leverage to get the building trades to do more to increase diversity.

Instead, the unions flex their muscle in cases like the Zoning Board's recent rejection of a proposal to build apartments in a vacant West Poplar warehouse zoned for industrial use. That decision also killed an innovative agreement by the developers, Post Bros., to make minority workers 50 percent of the construction crew.

Architecture critic Inga Saffron pointed out in a recent article that two of the board's five members are building trade union officials. It's not out of the question that they may have acted to protect union jobs. A Post Bros. construction site was picketed by goons in 2012 because work was given to nonunion contractors.

Mayor Kenney's Rebuild initiative includes a commitment to hire 45 percent minority workers and 50 percent city residents on each project to renovate 150 parks, recreation centers, and libraries. To accomplish that goal, he has gotten the building trades to agree to launch a "pre-apprenticeship" program to train minority workers.

That's a good start. But this city's history tells us it's going to take more than a "pre-apprenticeship" program to change the unions' membership rolls to better reflect the city's diversity.

A good next step would be for the Zoning Board to reconsider a decision that has little to do with making the best use of a vacant building and everything to do with keeping the building trade unions happy.