“A little dab’ll do ya”
No, no, wait, that’s for Brylcream
You could have a rod that’s threaded for its entire length, and you could also have a rod with smooth sections in between threaded sections if the smooth section matches the smaller diameter of the threads rather than the larger diameter.
That’s certainly correct as a matter of geometry.
The rod in question was supposed to be IIRC 120+ feet long. If it was semi-threaded as you propose, there’d be IIRC 10 or 12 threaded sections each a couple feet long at WAG 10-15 foot intervals. Each of those diameter changes where threads begin and end and the diameter changes would be a point of stress concentration. So about 20 of those. Per rod and there were many rods (10? 20?) required
Difficult to analyze, difficult to manufacture, difficult to transport, difficult to assemble. Gonna be difficult.
Now the actual design was a rod threaded full length as @Dewey_Finn says just below. Which implies a need to run a lot of large nuts up 120 feet of a lot of rods. Then another set of nuts up 100 feet 110. Then 100, then … Notso hotso either.
I thought from reading Henry Petroski’s book that the hanging rod in the case of the Hyatt Regency walkway collapse was supposed to be threaded throughout, with the nuts needing to be spun up dozens of feet to support each level.
Petter actually addressed that in the video I linked to earlier. Cutting threads that long would be extremely difficult, and the threads could be easily damaged during transport and assembly. Not to mention what a tedious pain in the butt it would be for the construction worker who has to thread a nut up a probably a nearly 30 foot length of rod.
A job to give the jobsite rookie.
You probably want the deep socket wrench for that.
What everyone just said. The original design for the rods sucked. Supposedly (according to the Wikipedia article) the original design was just a preliminary sketch. Since it wasn’t constructable as shown, it’s not at all surprising the fabricator would propose separate rods instead. (Threading the rods the whole length was reportedly considered but it was thought they would be too easily damaged during transport.)
What is surprising is that nobody checked to see what the new loads were on the various components with the revised design. And that is why the project engineer and the engineering firm were sanctioned and permanently lost their licenses to practice in that state. They were lucky to not to be held criminally responsible for their gross negligence.
When I taught physics I used the following analogy. The original design was like kids holding onto a vertical hanging rope (like in gym class), one above the other. The revised design was like one kid holding onto a vertical rope, with a second kid holding onto their legs. 
It’s obvious that the load on the top kid’s hands just doubled in the second case. Just like the load on the walkway nuts.
Oh, yeah, the original design was totally impractical. It just wasn’t actually impossible, like some claim.
Well, your physics students were not machinists or engineers. It is quite possible to make such a rod. but it takes extra machining. Just thread the rod and then machine off the threads where you don’t want them using a lathe. This doesn’t affect strength much since you only machine down to the roots of the threads, you are not reducing the core diameter.
The 2nd method is better which is to roll thread the threaded portions. This expands the threads on the rod. Used all the time for high strength fasteners.
At least it wasn’t using glue to hold 26 Ton concrete slabs above your head.
There are companies that do nothing but manufacture threaded rods. Piece of cake. As for running the nuts into position, use a powered nut runner.
Was that the case in the late 1970s when the building in question was being constructed, though?
Whether it is technically feasible, then or now, having an exposed 30 foot threaded rod on a public bridge looks incredibly cheesy. It looks like you couldn’t be bothered to make a proper part. “Let’s just use allthread!” And people will be grabbing on to it.
And yes, this was taught to us in engineering class, too, an a bad example. At the time, I thought it was something from the dead past, like the Tacoma Narrows bridge, instead of something from the last 12 months!
The Hyatt walkway collapse was in 1981. So I suppose fresh from your college days.
Yup. Probably in an '82 class, either stress or engineering materials.
The Washington Post is reporting the building as being stabilized (article behind paywall):
NEW YORK — An under-construction Manhattan high-rise at risk of collapse was stabilized late Tuesday and some evacuations of nearby buildings were lifted.
“We’ve been monitoring the building for many hours and have not seen any movement,” Ahmed Tigani, commissioner of the New York City Department of Buildings, said during a news conference Tuesday.
By nighttime, residents were allowed back into several of the seven buildings that were evacuated as a precaution.
I’m surprised and relieved they apparently saved the building.
Steel beams in building aren’t supposed to bend. That’s usually a high pucker sign.
It’ll be interesting to see if they eventually take down the building.
Adding the extra floors may have severely compromised the foundation.
However the developer, Berman thinks otherwise.
Link Developer of unstable building in NYC says it was never in danger of collapse - CBS New York
I hope they don’t proceed with the conversion, and if they do, nobody wants to move in.
You have the right idea but the wrong product. What’s needed here is epoxy resin – my miracle cure for anything broken! Just squirt some 5-minute epoxy into the crack in that V-shaped column, and have a strong guy (possibly Superman, if necessary) push it back into a vertical shape. Problem solved!