Beams that support interior walkways in the new Municipal Center at 14th and U streets NW could collapse without warning, due in part to "gross" design flaws, according to a new engineering report commissioned by the D.C. Department of Public Works.

Carl C. Hansen, a consulting engineer from Silver Spring, concluded in his April 18 report that repairs the city made to the beams in 1984 are inadequate and that new, permanent supports must be designed to make the walkways safe.

"I'm not trying to be an alarmist, but we just can't afford to have something happen like the Hyatt Regency in Kansas City," Hansen said in an interview yesterday.

A total of 112 persons died when skywalks at the Missouri hotel collapsed almost five years ago.

The report found a "gross deficiency" in the design of beams that support four walkways on the third through sixth floors of the Municipal Center.

It characterized the problems on the third and sixth floors as the most serious.

Hansen recommended that temporary posts be installed immediately to brace the beams. Workers hired by McLaughlin Construction Management Inc., which is supervising the $41.7 million project for the District, began erecting scaffold-like supports from the sixth floor to the basement the day after the report was issued. Dick Janec, MCM's construction executive, said the work was finished last Wednesday.

Hansen said the city, whose employes began moving into the building in November, is fortunate that the walkways have held their loads so far. Six of the building's eight floors are occupied, and the walkways have been allowed to stand without any temporary supports since late February.

Hansen's report, a copy of which was obtained by The Washington Post, conflicts with the conclusions of engineers from VVKR Inc., an Alexandria firm that designed the building. Public Works Director John E. Touchstone said in a statement that his department will follow Hansen's recommendations "to be absolutely certain the walkways are safe."

"Everybody will sleep better this way," said one project official, who asked not to be identified.

VVKR officials declined yesterday to comment on Hansen's report.

Richard T. Ball, a structural engineer with VVKR, said several months ago that if the original design had called for more steel reinforcements in the beams, the beams "probably" wouldn't have caused any problem. But he characterized the possible design misstep as minor and said the walkways, as reinforced, are safe.

Tara Hamilton, a spokeswoman for the Public Works Department, said project managers have yet to determine how much it will cost to fix the beams or whether the problem will extend the project beyond its current July 12 completion date.

Construction of the Frank D. Reeves Center, described by Mayor Marion Barry as one of his administration's boldest public works efforts, is already at least 12 months behind schedule and $1.5 million over the projected cost because of a variety of problems.

City officials promised that the project would provide a catalyst for development in the crime-ridden Shaw neighborhood by the end of 1984.

But according to project documents, some contractors fell far behind schedule and had to redo substantial portions of their work. District officials, according to some contractors, failed to provide the project with the supervision and direction it needed.

In February, after a year of wavering, District officials said they were confident they had resolved all the structural problems associated with the project.

The beams under the walkways first became a cause of concern in late 1984, when workers from the Sherman R. Smoot Corp., which did the building's concrete work, noticed that the concrete beams under the walkways had cracked at the corners on every floor.

At the city's direction, VVKR designed reinforcements for the beams that involved installing high-strength rods perpendicular to the cracks. Temporary shorings also were erected under the beams at that time.

Six months after the repairs were made, C.M. Tipnis, a vice president with Smoot, said in a letter that he feared "sudden failure" of the beams.

Tests conducted early this year seemed to show that Tipnis' fears were unfounded, according to city officials. When the temporary supports were removed, the cracks widened only very slightly and project managers announced that the problem was solved.

Hansen said in an interview, however, that he felt "a good deal of doubt about the adequacy of the remedy they had made." He said "the most dangerous situation is on the sixth floor," because workers failed to install a sliding plate that would allow the beam to move slightly where it joins another beam. He said the plate would have reduced stress on the beam under the walkway.

In his report, Hansen said he was also worried about the third floor because only three high-strength rods were installed as reinforcements.

An engineer hired by The Washington Post said last year that the only immediate solution to the problem that he could think of -- erecting another pillar in the middle of the walkways -- would destroy the beauty of the building's interior design.

But Hansen said it might be possible to erect a new column behind a partition, where it would be less noticeable. Horace Jones, head of construction and engineering for the Department of Public Works, instructed VVKR to design new supports for the beams in a letter dated Friday.

The Municipal Center is one of a number of public works projects that have been hamstrung by construction or design flaws. Officials of the D.C. public schools have complained that the Public Works Department has repeatedly selected contractors for school jobs who took too long to do the work and failed to perform up to standards. In two instances, new roofs on school buildings developed severe leaks.