The weather was almost perfect for a birthday party. Saturday morning the sky was blue, offering a view of the Olympic Mountains still wearing their winter whites and of ferries steaming out of Coleman Dock just below.

She had to look her best. Like buying a new dress for the party, the Alaskan Way Viaduct got her lanes repainted, renewed dashed stripes on both road decks. Road crews hung over the edges in cherry pickers, inspecting her sides.

The city was invited. People lined up at white canopies on the ramp at Columbia Street. Transportation staff checked their guest lists. More than 120 people RSVPed.

The breeze was cold. Guests shivered under their bright yellow party hard hats and safety orange costume vests.

Our hosts were Dave Dye, deputy director for the Washington State Department of Transportation, and Ron Posthuma, assistant director for King County Department of Transportation. As they waited for the party to start, they kidded each other.

"We're just pretty faces in silly hats."

They closed the viaduct for the weekend for maintenance and a full inspection, which has been done every six months since the Nisqually earthquake.

They hadn't bought balloons or hired a clown for the party. This time, though, Mr. Bubbles was here. That's the name for the machine that washes the walls of the Battery Street Tunnel. It was busy scrubbing off a year's worth of grime from the yellow tile.

The viaduct is 55 years old on April 4. Don't say anything to her, but she is showing her age. Concrete is eroding, chipping, exposing rusty rebar. She has crow's feet and wrinkles, bandaged by crack gauges. These measure her aging, not in years, but by millimeters. Think of them as party favors that pop if you pull them apart.

She is sagging too. Bents 93 and 94, the pairs of columns between Columbia and Yesler streets, have sank five inches relative to the adjacent columns north and south. Pilings go 40 feet down, but it isn't enough to keep her straight.

"It's the fact that 95 hasn't moved - and 94 has - that is breaking the back of the viaduct," said Tom Madden, an engineer with the state.

They care about her, and the safety of the people who drive here everyday. They are spending $5 million to give these deeper pilings, drilling another 30 feet down and attaching new footings. They'll finish by May.

She is still wearing the same Battery Street Tunnel since it was built. Her hosts will spend $40 million to upgrade the tunnel, starting in 2009 with new sprinklers and communications. At the other end, they will remove clingy electrical transmission lines from her structure.

This is in anticipation for a big gift in 2010. Don't tell, but she will be getting a whole new south end. The state will spend $545 million to replace 40 percent of viaduct, from Holgate to King Street, with new ramps to surface streets, an overpass above the rail spur, and an underpass for cars and trucks to use while trains block the intersection.

Back at the party, hosts offered gifts to their guests. A five-gallon bucket was filled with palm-sized pieces of concrete. The viaduct didn't say whether she was insulted that no one took any.

Guests listened to Ken Juell, with the state department of transportation, tell This is Your Life, starting with the railroads which built trestles to the wharfs, and the yards of dirt hosed from Seattle's hills used as fill behind the seawall.

Madden dampened the festive mood by talking about viaduct's lack of reinforcement. She was built to tolerate one-tenth the force of gravity shaking sideways, but modern standards expect structures to withstand at least 0.3 gravities, some up to 0.7 against mega-quakes. Her footings, under compression beneath the structure, lack any rebar. During an earthquake, footings can undergo tension - and easily pull apart.

Madden sobered the guests by reminding them of liquefaction, the wet loose soil the viaduct is built on, which can turn to quicksand during an earthquake. Plus gribbles, salt water seaworms, have eaten into the seawall, five feet deep in places. If the seawall collapses, the loose fill will follow, bringing the viaduct down with it.

She's worked hard for many decades, and she's only has a three or four birthdays before she retires in 2012.

Dye, one of our hosts, bought a chocolate sheet cake with white frosting. It sat in a pink box next to a stack of paper dessert plates and a box of white plastic forks.

This surprise, however, wasn't for the viaduct's birthday. It was for Helen Sommers, the state representative for Ballard, Queen Anne and Magnolia, who is retiring after 36 years in the Washington Legislature.

Blue piping on the cake spelled out: "Congratulations on your retirement. Thanks for your leadership."

"Thank you for all your work, and support on the viaduct," Dye told her.

Sommers was speechless.

And so was the Alaskan Way Viaduct.

Matthew G. Miller may be reached via wseditor@robinsonnews.com