SANTA CLARA — The San Francisco 49ers began propping up the skeleton of their new stadium on Monday, the most visible sign of progress yet.

Crews working on the $1.2 billion Santa Clara stadium have been drilling 3,000 holes five stories deep and filling them with concrete as a base to support 14,000 massive steel beams. It’s been pivotal work, but not exactly visible.

That changed at the crack of dawn Monday, when workers operating four giant specialty cranes shipped in from Austria began installing the first steel bars that will make up the outline of the stadium next to Great America. The beams were delivered from Utah, Idaho, Texas and elsewhere, with the biggest piece towering 90 feet high and weighing 18 tons, more than all the 49ers players put together.

“This is the frame of the house,” said Niners project executive Jack Hill. “The fans will see, as they drive by, (that) this will go up very quickly.”

Following the glitzy groundbreaking on April 19, the steel erection serves as the first construction milestone on the path toward the NFL stadium’s planned opening in summer 2014.

A month from now, crews will start installing a giant metal deck on the beams and cover it with concrete to create the surfaces fans will walk on during games. In the fall, they plan to start putting in the stands, aisles and walls — made up of precast concrete sections being manufactured outside Sacramento — before bolting in the seats.

So far, the biggest problem for the stadium hasn’t been construction — that’s gone along on schedule, Hill says — but rather a Santa Clara County oversight board’s decision last month to take $30 million from the project and give it to schools.

On Monday, state Sen. Elaine Alquist, D-Santa Clara, said she would introduce a bill that would restore the redevelopment tax money to the stadium, as voters approved in 2010. Though cities and counties across California have been grappling over similar disputes since the state scrapped redevelopment agencies, Alquist argues the stadium deserves special treatment because of its potential economic benefit.

“This is not about taking funding away from schools,” Alquist said. “It’s about generating tax revenue and thousands of jobs through one of the largest construction projects in the region in decades.”

SB 1245 will hit the floor on Aug. 6 and must be approved by the Legislature by the end of August, and signed by Gov. Jerry Brown in September.

The bill is a backup plan in case the Niners lose an ongoing civil court case to win back the money. A Sacramento judge has already frozen the money and indicated he may rule in favor of the team at an Aug. 17 court hearing.

Contact Mike Rosenberg at 408-920-5705. Follow him at twitter.com/rosenberg17.