Jerry Ross, top, and Lee Morin tackle the girder
The girder will form a backbone for the platform.
Shuttle Atlantis astronauts Jerry Ross, 54, and Lee Morin, 49, spent more than seven hours fixing the 13-metre-lomg (44 feet) girder in place.
Retired Air Force colonel Ross said he hoped people would be inspired by a couple of spacewalking grandfathers.
"As long as you have a mind to, and the determination, you can do just about anything," he said.
The two astronauts had 54 bolts to loosen or tighten on the girder, as well as many cable connectors, clamps and latches to tackle.
The girder will have a railcar attached to haul cargo. It will be the centre of a framework that will eventually extend 107 metres (356 ft) and support nearly an acre of solar panels and radiators.
A previous spacewalk had bolted down the girder's struts and laid some cables. Two more spacewalks will be required to fully install the girder - one on Sunday, the other on Tuesday.
Record flight
"A big congratulations to the Silver Team," astronaut Steven Smith radioed from Atlantis when the men had finished their part of the mission.
Ross is the American space agency's (Nasa) most experienced spacewalker with eight walks to his credit. He is on a record seventh space flight.
Morin is a Navy doctor on his first space trip.
The men both have two grandchildren, but Morin has been a grandfather longer - for eight years.
Atlantis and its seven crew members are scheduled to return to Earth on 19 April.