A critical change in the original design of the Hyatt Regency's skywalks doubled the stress on the part of the walks that pulled apart during the collapse, according to an investigation by the Kansas City Star.

City records -- in combination with visual examination by two experts and photographic evidence -- reveal that, at some point not reflected in the records, a change was made that doubled the stress on three steel "box beams" supporting the fourth-floor skywalk.

It was those beams that tore downward and away from their ceiling-anchored moorings, and both that walkway and a second-story walkway hanging below plummeted to the hotel lobby.

However, one of the experts -- a structural engineer hired by the Star -- cautioned that it is not yet possible to determine whether that tearing failure was the primary cause of the collapse, or merely one in a chain of structural failures. But it is clearly significant, he said.

The collapse Friday night killed 111 people and injured 188 more, at least 29 of them critically. At the time of the collapse, about 1,500 people were attending a tea dance in the hotel lobby -- and many were standing on the skywalks watching the dancing below, some of them either dancing or swaying to the music, according to witnesses.

The original design plans were revealed today when city officials made Hyatt construction records available for inspection, including the specifications for the project and the construction plans.

Those records and the altered design, as constructed, were studied by Wayne Lischka, a structural engineer retained by the newspaper. The Star consulted another engineer who had viewed the wreckage after the accident and who later was shown close-up photos of the damaged sky bridges. He independently corroborated Lischka's conclusions.

Once the deisgn change was made, Lischka explained, the box beams beneath the top walkway were required not only to support its weight but also that of the second-floor sky bridge, 30 feet below. Each sky bridge has been calculated to weigh about 65,000 pounds -- or almost the weight of a loaded trailer truck.

After examining the original design blueprints on file at City Hall and close-up photographs of the skywalk debris, Lishka determined that the box beams under the top sky bridge tore away from nuts that were on the lower ends of steel suspension rods that still remain anchored in the ceiling.

Whether the failure was the result of poor design, faulty construction or flawed materials cannot be determined without access to the site of the accident and a more intensive investigation.

The original architects' plan -- approved by city officials in the fall of 1978 -- called for six suspension rods, anchored in the ceiling, three to each side, to hold both bridges aloft.

The base construction of the walkways looks something like a ladder with three rungs -- with long steel I-beams making the sides of the ladder and shorter steel beams making the rungs. The shorter "rung" beams, which ran the width of the walkways, were attached to long steel rods that were in turn affixed to the ceiling of the lobby.

It was the point where these long rods attached to the "rung" beams that the tearing took place. Under the original plans, the rods would have gone from the ceiling, through the fourth-floor walkway and continued to the side beams at the bottom of the second-floor walkway.

That was changed sometime during construction so that one rod connected the top walkway to the ceiling, then another rod connected that walkway to the lower one.

The three box beams were formed from two steel "C-channels" welded together at top and bottom to form a rectangular, hollow beam.

Yet, when the skywalks were constructed, the critical change was made. Instead of having each suspension rod extend continuously from the ceiling through box beams supporting both walkways, the six rods dropped from the ceiling and ran through only the box beams underneath the width of the highest walkway.

Inches from the first rods, and through those same box beams, six other rods were added -- suspending the lower walkway from the higher skywalk, rather than from the ceiling.

Such a change meant that two separate support rods were installed inches apart on each of the three critical box beams.

That change, said Lischka and the other structural engineer who has contributed to the Star's inquiry, doubled the stress on the three steel box beams and nuts beneath the higher walkway. CAPTION: Illustrations 1 through 3, The stress on the top skywalk in the Kansas City hotel was doubled by a change in the original design. The artist's drawings show how the box beams under the top skywalk could have torn away from nuts that were on the lower end of steel suspension rods. Under the original plan filed with the city, six long suspension rods were to hold up both of the skywalks that collapsed. Weight was to be distributed evenly. Instead, the support system was built using six short rods out of the ceiling to hold the top skywalk. From that skywalk, six other rods were placed through separate holes in the box beams, suspending the lower skywalk from the top one. sSketches from the Kansas City Star; Picture, Red Cross worker Bettie Miller praying at memorial mass for 111 killed in collapse. UPI