The U.S. Army this week drove 34 of its heaviest M60 tanks onto a newly designed West German superhighway bridge and parked them there with the crews inside to help the local state government with a stress test of the bridge's strength.

The bridge, high over the Neckar River just south of Stuttgart, sagged by just the right amount -- 16-20 inches -- the West German engineers who designed it said. But it did not collapse.

What would have happened had it collapsed, however, is a question that is distressing Army headquaters today after pictures of the test showed up in the military newspaper Stars and Stripes.

"I was hoping none of you guys would call," an Army public relations man said in a telephone interview. "But to tell you the truth, when i saw those pictures I was thinking the same thing."

Officially, Army spokesman say they were assured by the autobahn authorities in Bahen-Wuerttemberg before the test was made that no danger existed since the bridge was designed to handle 2,200 tons and the 34 tanks, each worth about $890,000, weigh in together at 1,800 tons. The West Germans wanted to use tanks because there was no other way to get so much weight on the bridge with normal vehicles.

Asked why they did not use West German tanks, the Army said the nearest West German tank units were 180 miles away while a U.S. battalion was only a 25-mile clank down the autobahn. Furthermore, the local unit commander thought it would be good for community relations and also for training because the crews don't get a chance to drive their M60s along the heavily traveled autobahns very often at 35 miles an hour, as they might in wartime.

Behind the scenes, however, the episode caused a problem and some senior advisers, worried about the Army either looking foolish or even worse outcomes, argued against the move. Actually, sources say the unit involved had agreed with the state to do it 10 days before higher Army authorities found out about it.

The test took place on Monday but it was not until the previous Friday that Army headquarters learned about the plan and quickly moved to get legal releases from liability from the state, official West German assurrances that the exercise was safe, and U.S. Army engineers to make their own assessments.

Apparently some Army officials still believed that the test should not take place, but sources explain that it would have created a community relations problem at that point since the state was geared up for the test.

The half-mile long bridge is the only one of its kind. Because of the terrain it spans, the bridge is supported by three huge columns under the center span. But it uses suspension cables mounted underneath the bridge to support the two end spans. Although the bridge has been in service since December, it must undergo stress tests for safety after it has gone through the extremes of four season weather changes.

Twice this year, much smaller but still what seemed to be extremely rugged overpass bridges across the autobahns, collapsed when trucks ran into center pillars.