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The 27,000-pound beam, crammed with wiring and conduits, is the first of seven segments that will eventually become a 356-foot spine for the station that will support four giant solar power arrays, heat radiators and new laboratory modules.

Astronauts from the space shuttle Atlantis, which docked with the station on Wednesday, completed one of their mission's most difficult tasks by attaching the truss to the station's Destiny science module. Small problems encountered in installing the girder stretched the spacewalk by a pair of astronauts to 7 hours 48 minutes, about an hour longer than planned.

The operation started early today when Dr. Ellen Ochoa used the station's 58-foot robot arm to lift the truss gingerly from the Atlantis's cargo bay and maneuver it into position on the science module, where a clamp temporarily captured it.

To everyone's relief, the multijointed, Canadian-built arm worked perfectly during the four-hour operation. One of the joints had malfunctioned during earlier operations and engineers had to devise new software instructions for the arm.

"We're going to put in for your iron workers' card here soon," said the veteran astronaut Jerry L. Ross from inside the shuttle. Mr. Ross, who has taken seven spacewalks in his 22-year astronaut career, is to team up with Capt. Lee M. E. Morin of the Navy for two of the mission's four outside excursions.

Colonel Walheim, making his first spaceflight and first spacewalk, was in awe of the view from 240 miles up and paused occasionally between jobs to marvel.

"Wow, look at that sunrise," he said. "The moon's just coming over the horizon at the same time as the sunrise. Spectacular."

Mr. Ross said at one point, "Welcome to spacewalking, buddy."

After bolting down the $600 million truss, the astronauts hooked up power and coolant cables to turn on heaters, computers and other devices. The spacewalkers had trouble opening a tray on the truss and had to pull on it together to free it.

Some of the hoses and cables the astronauts were working with were stiff, twisted and hard to maneuver, extending the time of the work. The spacewalkers were told to skip some low-priority jobs and end their long workday.

On the second spacewalk on Saturday the other team of astronauts, Mr. Ross and Captain Morin, is to continue installing the truss. They will deploy and attach two large tripod braces on the other side of the beam to complete its mechanical attachment to the station and continue mating electrical and fluid lines.

The 14.5-foot-wide beam contains 475,000 parts, including four computers, 664 feet of ammonia coolant lines, 10 miles of electrical cabling, 971 electrical connectors and an array of heaters, switch boxes and other components.