SPACE CENTER, Houston (AP) _ Two astronauts will become orbiting construction workers next month, manhandling a 45-foot-long beam during what they described Thursday as the most physically demanding spacewalk ever attempted.

Astronauts Jerry Ross and Sherwood Spring will construct the aluminum truss and a pyramid-shaped metal frame during two 61/2 -hour spacewalks designed to test techniques that may be used in building a permanent space station.

The seven-day mission aboard space shuttle Atlantis is scheduled to begin at the Kennedy Space Center on Nov. 27, the day before Thanksgiving, and end Dec. 4.

Mission commander Brewster Shaw said at a news conference that plans call for a landing in darkness on a concrete runway at Edwards Air Force Base, Calif. The mission plan may be changed, however, to accommodate a new launch for a Mexican satellite, Shaw said. That would mean a night launch from Kennedy and a midday landing at Edwards.

Others on the crew are astronauts Bryan D. O'Connor, pilot; Mary L. Cleave, mission specialist; and Rudolfo Neri Vela and Charles D. Walker, payload specialists.

Ross and Spring will have to connect about 60 pieces of aluminum beam together to construct the truss and the pyramid, then take the equipment apart and restow it.

The construction is an experiment to determine how well spacewalking astronauts are able to put together metal framework of the type that will be used to build a space station.

Ross said it is easy to manuever the beam pieces in the weightlessness of orbit, but connecting each piece is difficult.

The spacesuit gloves take strength to close when gripping the metal pieces and the repetition required for the construction is exhausting, he said.

''It requires much more enrgy than the other EVAs (spacewalks),'' said Ross. ''I expect we'll be really physically tired at the end of each day.''

After building the truss and the pyramid, each of the spacewalkers will experiment with moving the construction frames by hand.

Ross said it will be difficult to manuever the 45-foot beam, but the astronauts will try to move it in a controlled way to see if it would be possible for astronauts to put together large structures while in orbit.

Shaw said the ''bread-and-butter'' part of the mission will include the launch of three commercial satellites, including one owned by the Mexican government, and taking hundreds of photographs of Earth.

Vela will be the first astronaut from Mexico. He plans to observe the launch of the Morelos-B satellite, a craft that will be used for telephone and television communication in Mexico, and will conduct five experiments designed by Mexican scientists.