Structural Defficiency

KansasCitySkywalkCollapse-01

From Wikipedia

I started my working career as an engineer right out of college, working for the university from which I had just graduated. And quickly I began to hear engineering horror stories. One was the case of the reversed viewport.

The University of Texas Austin campus had a new accelerator building where they intended to do research in (relatively) high-energy physics. It was a linear accelerator where charged particles were shot down an evacuated tube under the force some very high voltage differences. In order to do something with these high-speed particles you have to insert something in their path inside the evacuated tube, and to do that you have to work with a port, an engineered hole, in the side of the tube. That’s where the problem came in.

The carefully designed port was essentially a hole in the tube wall, and there was a vacuum flange built into the port. The vacuum flange allowed researchers to bolt a matching flange to the port, tighten the bolts, and obtain a vacuum-tight seal.

Came the time the researchers needed to add a new device to the port, and that device needed to have a matching vacuum flange. No problem. They sent over engineering drawings of the vacuum flange to the office where I worked in the Physics building. This was before I came to work, so others told me the story later, and it was funny. Sort of.

The engineers in the physics building designed the required device with the matching flange. Then somebody had the whole business constructed in the machine shop and sent it over to the accelerator lab. It did not fit. The bolt holes did not line up. You wanted to say WTF?

An investigation ensued. Yes, the engineers received the correct drawings of the vacuum flange. Yes, the new device was designed correctly according to the engineering drawings. Yes, the flange was constructed according to the engineering drawings. No, the damn thing did not fit. And then they revealed the source of the problem.

When the accelerator was constructed the machinist who produced the vacuum flange reversed the drawing. He constructed a mirror image of the port design.

No problem. The accelerator lab guys figured they could just make the matching flanges to conform to the as-built. And they did. And everything worked just fine, because it really didn’t matter whether the flange was left-handed or right-handed.

But they never changed the drawings. When they asked the engineers in the physics building to build a new device they never told them about the change. Nobody talked to nobody, and everybody just went along doing their work and drawing their pay.

A few years later lightening struck again. This time I was working for an engineering company, and we got the job to design and build a bracket to hold a gold-coated mirror for the McDonald Observatory. No problem. My boss had done this before. He had the drawings from the previous bracket he had designed. He gave me the drawings and told me to make a new design, only use a 45-degree angle.

I did that, and we had it built, and we shipped the finished product out to the observatory. There was a tight schedule. The astronomer had reserved only a small window of time on the telescope. The astronomer came out from Langley to the observatory in West Texas. We got a phone call. The damn thing did not fit.

My boss was non-plussed. What was the problem? We built the first one, and there was no problem. There should be no problem with the new one.

Actually, there was a problem with the first one. Only, at the observatory the engineers had spotted the problem immediately and had their machinists produce a correct version of the bracket. And they never told anybody. They didn’t tell the NASA guy from Langley when he came out the first time to use the telescope, and they never told my boss. And we spent government research bucks building yet another wrong bracket. And it was not cheap.

And this kind of stuff can be deadly:

The Hyatt Regency hotel walkway collapse occurred at the Hyatt Regency Kansas City in Kansas City, Missouri, United States on Friday, July 17, 1981. Two vertically contiguous walkways collapsed onto a tea dance being held in the hotel’s lobby. The falling walkways killed 114 and injured a further 216 people. At the time, it was the deadliest structural collapse in U.S. history, not surpassed until the collapse of the south tower of the World Trade Center in 2001.

This was another of those situations. The original designer, Gillum-Colaco International Inc., produced a workable design. You may need to visualize this:

  • There was a second-floor walkway, a third-floor walkway and a fourth-floor walkway, all suspended from the lobby ceiling structure by steel rods.
  • The forth-floor walkway was suspended directly by steel rods attached to the building structure above.
  • The second-floor walkway was suspended directly below the fourth-floor walkway, on the same rods. See the drawing.

kc_walk_proposed

The rods attached to the upper building structure ran straight down, passing through the supports for the fourth-floor walkway to the second-floor walkway. The rods were designed to hold the load of both walkways. The fourth-floor walkway was supported by the rods by threaded nuts, seen in the drawing. To install the nuts, the rods had to be threaded their length up to the fourth-floor walkway.

First some notes about the strength of threaded fasteners. An SAE (Society of Automotive Engineers) machine screw will, under tensile load, pull in two before the threads will strip, provided at least six threads are engaged. If the nut engages at least six threads there would be no danger of the threads stripping under any load the screw will sustain. Threading a load-bearing bar, like a screw, produces some weaknesses over an unthreaded bar. Strengths of threaded fasteners are ultimately determined by putting samples in a testing machine and pulling until the rod separates.

The problem came when the builder, Havens Steel Company, did not want to thread all this much steel rod. They proposed an alternative.

kc_walk_actual

Now Havens needed to thread only a few inches of steel rod. Everything should work the same, right? No. Look at the drawing at the top of the page, and follow along with this explanation:

Due to the addition of another rod, the load on the nut connecting the fourth floor segment was increased. The original load for each hangar rod was to be 90kN, but the alteration increased the load to 181kN. The box beams were welded horizontally and therefore could not hold the weight of two walkways. During the collapse, the box beam split and the bottom rod pulled through the box beam resulting in the collapse.

For non-engineers out there, 90kN is 90,000 Newtons, about 20,183 pounds. Multiple rods supported each section of the walkway, so the design was safe for the expected load.

Look at the drawing at the top of the page again. There is something else wrong with the altered design. Offsetting the down rod and the up rod produces a shear force on the beam that was not intended by the original designers. GCI, Inc. surrendered their professional engineering expertise to Havens, a company lacking any such credentials. And people died.

All this came to mind a few days ago when a former college roommate sent me this photo from Jakarta.

IndonesianArchitecture

Here is what Byron Black had to say in his note:

Daddy makes enquiries. An architect friend hollers out laughing down the telephone line. “Oh that place! They hired these whiz-bang architects from Paris; the geniuses decided they’d go for a supple, light structure. Cut back on the reinforcing rods.
“Then the owner changed her cute little mind and added fifteen more floors to the original design. Wanted to sell more units.
“Nobody thought to tell these guys that the buildings they’d drawn up, and the precise angle they’d set them at, were going to pick up the wind and sail back and forth. Even when the contractor was building them, the workers all bundled up and scared shitless from the swaying, the architects never found out.
The owner added 15 additional floors. And never told the architects. And never changed the design to strengthen the lower structure to support the additional load.
I’m thinking there must be a Take An Engineer Out To Lunch Day sometime soon. Do that. And have a good talk.
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