With an average of 324 inches of snowfall each year, Snoqualmie Pass is no stranger to closures prompted by avalanche danger.

So far this winter, there have been 21 closures — 15 of them centered in an area about six miles east of the summit.

It is there, near the banks of Lake Keechelus, that state transportation officials had planned to build a bigger snowshed to replace a smaller one constructed in the early 1950s.

But under a plan announced last week, the state will instead build a pair of bridges similar to the one built decades ago for the westbound lanes just west of the summit.

An engineering review showed bridges should cost about the same, and would eliminate $650,000 a year that would have been used to maintain fire and related safety systems planned for the new snowshed, according to Don Whitehouse, regional administrator for the state Department of Transportation.

Both the bridges and the expanded snowshed would cost about the same to construct — about $71 million, according to the transportation department.

The bridges will take traffic up and over a series of paths designed to direct snow, along with rock and wood debris carried by avalanches, between the piers and toward Lake Keechelus, transportation officials say.

At their highest points, the elevated roadways will be about 70 feet above the ground. That’s lower than the bridge west of Snoqualmie Pass, which reaches about 200 feet.

The bridges are part of an estimated $353 million upgrade, including the construction of additional lanes, of Interstate 90 between Hyak and the Keechelus Dam.

Work to begin tearing down the existing snowshed, which was opened in 1951 and covers only the westbound lanes, is expected to start in August. The eastbound bridge is expected to be completed by 2015, while the westbound bridge is projected to be done two years later.

Reducing closures on Snoqualmie Pass has been a priority for the state. At an elevation of 3,033 feet, Snoqualmie is the busiest mountain pass in the Pacific Northwest. It sees an average of some 27,000 cars, trucks and buses each weekday and nearly twice that many on busy weekend days.