On 02/10/13 18:42, Arnold Reinhold wrote:
On 1 Oct 2013 23:48 Jerry Leichter wrote:

The larger the construction project, the tighter the limits on this stuff.  I used to work with a former structural 
engineer, and he repeated some of the "bad example" stories they are taught.  A famous case a number of years 
back involved a hotel in, I believe, Kansas City.  The hotel had a large, open atrium, with two levels of concrete 
"skyways" for walking above.  The "skyways" were hung from the roof.  As the structural engineer 
specified their attachment, a long threaded steel rod ran from the roof, through one skyway - with the skyway held on 
by a nut - and then down to the second skyway, also held on by a nut.  The builder, realizing that he would have to 
thread the nut for the upper skyway up many feet of rod, made a "minor" change:  He instead used two threaded 
rods, one from roof to upper skyway, one from upper skyway to lower skyway.  It's all the same, right?  Well, no:  In 
the original design, the upper nut holds the weight of just the upper skyway.  In the m
fied version, it holds the weight of *both* skyways.  The upper fastening 
failed, the structure collapsed, and as I recall several people on the skyways 
at the time were killed.  So ... not even a factor of two safety margin there.  
(The take-away from the story as delivered to future structural engineers was 
*not* that there wasn't a large enough safety margin - the calculations were 
accurate and well within the margins used in building such structures.  The 
issue was that no one checked that the structure was actually built as 

This would be the 1981 Kansas City Hyatt Regency walkway collapse 

Which says of the original design: "Investigators determined eventually that this design supported only 60 percent of the minimum load required by Kansas City building codes.[19]", though the reference seems to be a dead link. (And as built it supported 30% or the required minimum.)

So even if it had been built as designed, the safety margin would not
have been "well within the margins used in building such structures".

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