Hyatt Regency Kansas City Walkways, 1981: A Connection Detail Changed During Construction

What happened

On July 17, 1981, at about 7:05 p.m., two suspended walkways in the atrium of the Hyatt Regency Hotel in Kansas City, Missouri, collapsed. The National Bureau of Standards (NBS) report states that 113 people died and 186 were injured, the most devastating structural collapse in terms of loss of life and injuries in the United States up to that time.

What the NBS investigation found

• Three walkways spanned the atrium at the second, third and fourth floors. The second-floor walkway was suspended from the fourth-floor walkway directly above it, which was in turn suspended from the roof framing by hanger rods. The second and fourth-floor walkways fell.
• The most probable cause was insufficient load capacity of the box beam-hanger rod connections. The failure appears to have started at the connection on the east end of the fourth-floor walkway's middle box beam.
• Two factors contributed: the original connection design was inadequate, and it was the same for all three walkways; and a change in hanger rod arrangement during construction roughly doubled the load on the fourth-floor connections. The contract drawings showed continuous rods from the roof through the fourth-floor box beams to the second floor. As built, one set of rods ran from the roof to the fourth floor and a second set ran from the fourth floor to the second floor.
• At the time of collapse the maximum load on a fourth-floor connection was only 31% of the ultimate capacity expected of a connection designed under the Kansas City Building Code. Even the original arrangement would have given only about 60% of that expected capacity. With the change, the walkways had minimal capacity to resist their own weight from the day they were built.

Apply the lesson

• Who reviews a change a fabricator proposes to a connection detail, and does that review include recalculating the load path?
• Draw the load path for a suspended or stacked system. Check how much load each connection carries in the as-built arrangement, not only in the drawing.
• Check whether the structural engineer of record reviewed and approved each connection change in writing.

Engineering lesson: a small change to a detail can change the load path. Every change to a connection needs an independent check of the forces it carries.

Why a "small" change doubled the load

This illustration follows the NBS description of the two arrangements. It uses no numbers from the report beyond what is stated there.

1. Original design: a single continuous hanger rod passes from the roof through the fourth-floor box beam down to the second-floor box beam. Each walkway hangs from the rod through its own beam, so the fourth-floor beam supports only the fourth-floor walkway load at that connection. Call that load P.
2. As built: one rod runs from the roof to the fourth-floor beam, and a second rod runs from the fourth-floor beam down to the second-floor beam. The fourth-floor beam now carries its own load and also the load of the second-floor walkway hanging from it.
3. The fourth-floor connection therefore carries about 2P instead of P. The NBS report describes this as a change that essentially doubled the load on those connections.

The report adds that even the original arrangement would have given only about 60% of the capacity expected under the Kansas City Building Code, so the change made a weak connection much weaker.

Sources

• NBS: Investigation of the Kansas City Hyatt Regency Walkways Collapse (NBSIR 82-2465), hosted by NIST