CHICAGO (CBS) — Musicians with the Chicago Symphony Orchestra are walking the picket lines on Monday, after going out on strike.

The Chicago Federation of Musicians, which represents the CSO musicians, said they walked off the job Sunday and would not return until an agreement is reached. They said they simply want the Chicago Symphony Orchestra Association to give them what they’ve earned.

The musicians plan to picket outside Orchestra Hall every day from 8 a.m. to 8 p.m. until they have a new contract.

“This is really about the future of the orchestra,” said Steve Lester, a double bass player.

Lester said, for about a decade now, the orchestra’s musicians have not received competitive contracts.

“The kind of contracts we’ve been able to win have been slipping,” he said.

Musicians and the association have been negotiating for 11 months. The previous contract expired in September, but was extended until March 10 to facilitate continued negotiations. The musicians voted to authorize a strike last month.

A major sticking point in contract negotiations has been pensions. Musicians currently receive a defined benefit plan, meaning their pension levels are guaranteed.

“It has to be funded,” Lester said. “We think the association can afford that funding, they don’t.”

The union said the association is seeking to switch to a defined contribution plan.

“They just put in a set amount and whatever happens to that money is on you,” said Cynthia Yeh, a principal percussionist with CSO.

Yeh said she’s been with the orchestra for 12 years, and she signed up for the job with certain benefits in mind.

“Now 12 years later to say, ‘Oh, actually, by the way, that’s not going to happen,’” she said.

The CSO board of trustees released a statement stating it “would be irresponsible for the board to continue to authorize a pension program that jeopardizes the orchestra’s future.”

The association called the musicians’ contract demands “unreasonable and detrimental to a sustainable future for the CSO.”

“We are disappointed by the union’s choice to disrupt the CSO season now underway, and we are available to return to negotiations when they are ready,” CSO Association President Jeff Alexander stated. “We have worked tirelessly to be responsive to the musicians’ many proposals during negotiations and have responded with a package that provides exceptional benefits, offers salary increases, improves working conditions and protects their retirement benefits.”