For months, the six unions that operate at the center have been in negotiations with its new management - SMG, the Conshohocken arena and convention center management company that replaced city-appointed administrators Dec. 1.
The carpenters are the only union on strike.
At issue is a revised customer-service agreement with modified work rules that make it easier for customers to do business at the center, said John McNichol, a Harrisburg lobbyist named president and chief executive of the Convention Center.
The modified rules allow individual exhibitors to do more work on their booths themselves, depending on the size and complexity of the display.
As the carpenters see it, the more work exhibitors do, the less there is for union workers.
"I'm trying to protect my members and not lose jobs," said Edward Coryell Sr., secretary-treasurer of the Metropolitan Regional Council of Carpenters, which includes Local 8, the workers assigned to the Convention Center.
"They want this crazy stuff," Coryell said. "They want exhibitors to use power tools, which we are not going to allow. They want to double the size of the booths exhibitors can erect on their own. It's currently 300 square feet."
Management contends that the more work exhibitors can do themselves, the more likely it is that the center will gain a reputation as an easy place to do business, said Gregory J. Fox, chairman of the Convention Center's board of directors.
The carpenters are hurting their own membership and tens of thousands of hotel and restaurant jobs that depend on the center's events, Fox said.
His position is endorsed by Mayor Nutter.
"The reality is, the more customer-friendly the Convention Center is, the more customers you will have, the more work hours you'll have, and the more work you'll have for the various trades over there," Nutter said Thursday.
"Clearly there's a need for reform, for further reform of work rules in order for the Pennsylvania Convention Center to be truly competitive, bring more conventions in."
The picketing was the latest chapter in a long book of problems between the center's management and its union workforce.
Management has long contended that union inflexibility on work rules has added cost and complications to running conventions, costing the center bookings.
Unions maintain that the producers that set up the conventions on behalf of the attendees overcharge labor costs, pocketing the difference and blaming it on the unions.
More than 10 years ago, a customer-service agreement was set up to resolve the issue, and a contractor, Elliott Lewis Convention Services L.L.C., was hired to handle the labor billing.
The agreement was rarely enforced by either side. When SMG was appointed, both labor and management expressed optimism that long-standing disputes could be resolved.
Besides the customer-service agreement, the center is negotiating collective-bargaining pacts with four of the six unions, including the carpenters. Contracts had been extended until April 30.
Talks with the carpenters fell apart Thursday morning after a tentative pact had been reached, Coryell said. He said a convention center board subcommittee rejected it.
The Laborers International Union Local 332 and the International Brotherhood of Teamsters Local 107 signed contract extensions until May 6, said Pete Peterson, a spokesman working for Fox and the Convention Center board.
The remaining union, the International Brotherhood of Electrical Workers Local 98, directed by John Dougherty, has promised to sign an extension but had not signed it as of Thursday.
Agreements for two other unions, Ironworkers Local 405, and Local 8 of International Alliance of Theatrical Stage Employees, have not yet expired.
Events scheduled in the building over the next week include the Broad Street Run and meetings of the American Cancer Society and Commonwealth Connections.
Inquirer staff writer Troy Graham contributed to this article.