Forty members of the Chicago Teachers Union are slated Monday to consider a “serious offer” for a new four-year contract, a step that’s making another teachers strike look less likely.

Should that group of teachers, social workers and other school staffers vote for Chicago Public Schools’ latest proposal that makes them pay more for retirement but offers 2.75 to 3 percent raises in years two through four, then that tentative contract agreement goes before the House of Delegates, as the union’s governing body is known.

“The Big Bargaining Team will tell us if there’s a [tentative agreement],” union vice president Jesse Sharkey said.

But if the Big Bargaining Team shoots down the offer, CPS and the CTU then enter a final legal stage of negotiations that starts a 105-day clock ticking toward a strike.

Teachers are being offered raises of 2.75 percent next school year and then 3 percent for each of the next two years, the Sun-Times has learned. They’ll also continue to receive “step and lane” raises for experience and added education through June 2019.

But CPS wants teachers to pay their entire 9 percent pension contribution, including the 7 percent the district agreed decades ago to fund, so veteran teachers will start paying 3.5 percent more toward their pension starting in July and then the whole 7 percent as of July 2017. CPS is also asking the CTU’s 27,000 members to pay part of their health insurance premium, beginning with 0.8 percent next year and another 0.7 percent the next year.

The school district is also looking for at least 1,500 teachers to retire, offering a $1,500 bonus per year of service, according to a written summary of the offer. If fewer teachers retire, the contract gets reopened on July 1 for 30 days of negotiations and mediations before proceeding to fact-finding.

And CPS can only open new charter schools after it closes existing ones, according to its latest offer.

A Chicago Public Schools spokeswoman said she couldn’t discuss specifics of the latest contract offer. No information was immediately available about how many CPS teachers and paraprofessionals retire each year.

A Chicago Teachers Union spokeswoman could not be reached for comment Sunday.

If the latest offer is rejected, the face-finding phase will be conducted by a professional arbitrator who has already settled high-profile disputes between the city and the police union, and between the state and prison employees.

The parties have jointly chosen Steven M. Bierig as a fact-finder. He’s been tasked with listening to both sides and, after two months’ consideration, issuing a ruling that could, in theory, become the new teacher contract.

He will not hear from the two sides Monday; this break is intended to give the union a chance to consider the city’s latest offer. But he has been retained in case his services are needed.

If either side rejects his findings, the state-required process ends, allowing teachers to walk off the job about a month later — or mid-May. In 2012, CPS immediately shot down the proposal by fact-finder Edwin Benn, and a contract wasn’t reached until after teachers had walked picket lines for seven school days.

In 2013, Bierig, a former management labor lawyer, agreed with Chicago Police that the city owed $1 million in disputed overtime to the 3,100 police officers who worked at the 2012 NATO summit. In 2012, he also ruled in favor of Lake County Sheriff’s officers who balked at intensifying their existing fitness and drug testing, and for Illinois prison workers who complained facilities were closed by former Gov. Pat Quinn without negotiations.

A graduate of Chicago Kent College of Law, Bierig was admitted to the Illinois bar in 1984. He practices in north suburban Highland Park and charges $1,350 per day, according to the Illinois Labor Relations Board.

He did not respond to requests for an interview.

The parties had reached enough of an agreement Thursday on what union president Karen Lewis called a “serious offer” to seek approval from the next layer of management Monday.

CTU’s House of Delegates meets Wednesday, but Sharkey said it’s unlikely to be asked to vote on any agreement so soon.

The parties have been trying for well over a year to work out a new contract to replace the one that expired last June, a process that reset after CEO Forrest Claypool was installed in late July.

The CTU has published a timeline on its web site identifying May 16 as the first possible strike day but hinting that it might wait for the start of the new school year to take any action.

State law requires several steps from the teachers before they can walk out. At least 75 percent of membership must agree to strike — a vote taken in December far surpassed that threshold.

They must seek help from a mediator for a “reasonable period,” help that’s been at the bargaining table since August. If mediation fails, they must go through 105 days of fact-finding: 60 days to hear and review the facts, 15 days to consider the findings, then a 30-day cooling-off period.

How long that “reasonable period” lasts can be disputed. Initially the CTU wanted to start fact-funding in late November, but ultimately both sides agreed it would start Feb. 1.

Union officials have said they’re loath to strike, but will if they believe they must.

Contributing: Mitch Dudek