Cobb Commissioners Tuesday unanimously approved an $805,000 contract with Los Angeles-based AECOM Technical Services to design and engineer a bridge over Interstate 285, that will connect the Braves stadium and mixed use development with the Galleria area.

The cost rose more than $50,000 over initial estimates. Cobb Transportation Director Faye DiMassimo said contingency funds were built into the contract because drilling might be delayed as a result of the Interstate 75 managed lane project that is being constructed in the same area.

The design is being funded by $600,000 in Federal Transit Administration funds that are annually appropriated to the county; the rest will be covered by county SPLOST revenue.

County officials want to build a double-decker bridge, with each span measuring 1,100 feet. The top span of the bridge will carry pedestrians, and the lower deck a circulator bus.

The county wants the bridge completed in time for the Braves first pitch in 2017. To that end, the county is offering a 2-percent bonus if the engineering firm has its work done in a time frame that allows construction bids to be opened by October.

That’s the latest plan in the ever-evolving project. It remains unclear how much bridge construction will cost, or how the county will fund it.

The AJC first reported in November that the county will be responsible for funding at least half of the bridge’s cost, despite Commission Chairman Tim Lee’s repeated assurances that no local tax money would be used for its construction. That information was revealed in an application the county submitted for the project with the Atlanta Regional Commission. The newspaper reviewed hundreds of pages of emails, reports, artist renderings and conceptual drawings for that story.

The county has said it has a “conceptual” cost estimate of between $6-9 million. But when asked if the construction cost could be higher, DiMassimo said: “Potentially.”

A Request for Proposal for bridge design says a variety of funding is anticipated to pay for bridge construction, including federal transit funds, special purpose sales tax receipts, “and/or other local funding.”