The Friendship Heights post office building, built in upper Northwest Washington 3 1/2 years ago at a cost of $2.7 million, has developed cracks that the U.S. Postal Service says amount to structural damage.

There is a long, thin crack in the floor of the area of the building at Wisconsin Avenue and Upton Street NW where employes sort mail, and a 24-inch crack in one of the concrete pillars that hold up the beams supporting the first floor.

In addition, employes have been told by supervisors not to park in certain areas of the employe parking lot, since there are cracks in the concrete ceiling of a parking structure for postal vehicles beneath the employe lot and postal officials fear further damage.

U.S. Postal Service inspectors have checked the building and found some structural damage but "determined the cracks were not dangerous," said George Conrad, public information officer for the D.C. post offices.

However, the shop steward for the letter carriers at the station, George C. Hoggard, said he is concerned about the safety of employes and has warned them to stand near a wall or evacuate the building if they hear any cracking noises.

Francis Briscoe, acting area manager of the Friendship station, said most of the cracks began appearing this winter. There are no cracks in the section of the building where the public comes for services such as buying stamps.

The National Association of Letter Carriers Branch 142 met this week to hear employe concerns about the safety of the Friendship Heights structure. Postal employes said they were asked not to discuss publicly the Friendship post office.

Hoggard, a retired D.C. police officer, said the structure has had a variety of problems since it opened in 1978. The customer parking lot began sinking soon after the post office opened, he said, but that problem has been repaired.

But there are still several cracks and crevices--some three inches wide, according to Hoggard--in the employe parking lot, which forms the roof of the garage for postal service vehicles.

Areas of the employe lot in which workers have been asked not to park are covered with metal plates. Beneath the plates are cracks, Briscoe said.

"What concerns me is not the size of the cracks , but the rapid deterioration of that building. Not only the roof, but the entire facility is deteriorating," Hoggard said.

The Friendship post office handles about 250,000 pieces of mail per day for the Chevy Chase, Cleveland Park, Georgetown and Friendship Heights sections of upper Northwest.

The Postal Service says it will cost about $255,000 to repair the structural damage to the building. Who or what is responsible for the damage is unclear.

Nicholas Chacos, president of NGC Inc., the Bethesda construction company that built the post office, said he does not know why the building is cracking because he hasn't seen the cracks.

He said ruptures might be due to "expansion and contraction, a problem with the heat--how hot and cold the building gets. That has to do with the design."

Chacos said postal service inspectors monitored the construction work along the way. "As far as I know, we did the work properly," Chacos said.

Robert Wening, an architect who worked with Mills & Petticord, a firm no longer in existence that designed the building, also said he has not seen the cracks.

He said the kind of concrete covering that was placed over the first floor does tend to "flex under heavy loads . . . It doesn't mean any danger at all. It's just a surface-appearing thing."

He said the cracks in the concrete pillar may be "cosmetic, rather than structural. I don't think there's any danger of the building collapsing." He added that he did not think the cracks were due to flaws in the heating system.

The building has been ill fated since its opening in October 1978. Last summer, the front wheels of a crane broke through the surface of the employe parking lot, causing extensive damage. And last spring, a car jumped the curb near the corner of Wisconsin and Upton, and crashed into a front window in the customer service area.

Marvin Tievsky, president of the Friendship-Tenleytown Citizens Association, said some of the association's members have complained to him about slow service at the post office. He said the association is trying a get substation to serve the Friendship Heights area.