#02: Following The Rules Can Be Lethal
Crane Factoid #02:
How really “straight” is “straight”? “Spoiler alert … not very straight!”
Crane Factoid #02 is really just an extension of the previous Crane Factoid #01. In the last Factoid, we talked about the AISC 1/500 Rule for the installation tolerance of plumb columns. In that Factoid, we found that at an elevation of just 30 feet, columns can be almost one inch out of plumb and still meet the AISC 1/500 installation tolerance requirement. Although adequate for building steel, this can present a real problem for crane runway steel.
Crane Factoid #01: Plumb Crazy
In other words, a steel erector can fully follow “AISC Good Practice,” and still produce a crappy building, from a crane runway point of view.
THE SKY IS FALLING SYNDROME
It’s vital to put Factoid #1 and #2 in their proper perspective. I’m not Chicken Little screaming that the Sky Is Falling, that all the standards are junk, and that I know better than an organization like AISC. What I am saying is that Building Steel and Crane Runway Steel are not the same thing and that AISC is all about Building Steel and NOT about Crane Runway Steel. Building Steel and Crane Runway Steel may both show up as primer red wide-flange steel, but that’s where the similarity ends. You cannot, or at least should not, use the same rolling tolerances, fabrication tolerances, or installation tolerances for both Building and Crane Steel. If you choose to use AISC Specs for Crane Steel, be prepared to eventually pay a big bill to fix your faulty runways because you chronically have serious crane problems that never get better.
SWEEP AND CAMBER
Unfortunately, there’s another situation, like the 1/500 Plumbness rule, in which you can follow the rules and still get into big trouble. A beam can deviate from true straight in two ways, Camber which is measured up and down, and Sweep, which is measured left and right.
Although excessive deviation in either plane can be a problem, it’s my experience that Sweep is a more common problem with overhead crane runways.
ASTM/AISC MILL TOLERANCES FOR STRUCTURAL SHAPES
Tolerances Illustrated, By Erika Winters Downey and Jason Ericksen, S.E, “Modern Steel Construction,” October 2006
SO, WHY IS THIS A BIG DEAL?
Looking at tolerance requirements in a vacuum doesn’t shake you up. But when you apply these tolerances to a real-world situation, and it starts to draw your attention like a flashing neon sign. Here’s a table I made showing the AISC 1/8-inch sweep per 10-foot Rule for support centers from 20 feet to 65 feet.
Calculations using ⅛ inch per 10 feet, based on just one runway beam.
BUT SPAN MEASUREMENTS REQUIRE TWO RUNWAYS
If you remember, the CMAA Sweep tolerance is 1/8 inch per bay and a total of no more than 3/8 inch over the full length of the runway. As you can see here, even at a 20-foot support center, you can have an AISC acceptable rolling tolerance while blowing away the CMAA Sweep tolerance. The longer the support center, the worse the condition.
As if that’s not bad enough, remember overhead crane runways come in pairs. Take a look at how bad it can get when you look at both the right and left-hand runways as a set.
Exceeding CMAA at every support center length
In other words, on a crane system with runway support centers at 50 feet, you could have an additive deviation of 1-¼ inches while your installer fully conformed to the AISC tolerance requirements!!! This deviation is a prescription for a perfectly good crane that will not function because of poor runways. A case in which the building owner would likely sue the runway installer, lose the case in court and then pay, not only for the lost legal case but also for a contractor to fix his building.
THEN WHAT SHOULD I DO?
There are four steps to protecting yourself.
1) Building steel is not the same as overhead crane runway steel. Until you and your contractors faithfully buy this argument, you are doomed to continual frustration.
2) AISC rolling tolerances, fabrication tolerances, and installation tolerances are building steel tolerances and should NOT be used for crane runways.
3) Nowhere does it say you are required to use AISC tolerances for the fabrication and installation of your crane runways.
4) Write your own fabrication and installation specs for runways. Start with the AISC requirements and tighten them up for;
i) Plumb column tolerances
ii) Acceptable Wide-Flange rolling tolerance. (FYI, my purchasing agent told our local steel warehouses that we were willing to pay a premium for very, very straight steel, and we would send back any steel that did not conform. We eventually found two local warehouses that would “play ball” and used them exclusively for a decade.)
iii) Installation sweep and camber limits
Remember, each of these parameters are a function of length. Therefore, the longer the span, the higher the runway elevation, and the longer the runway support centers, the more precarious your situation. If anyone of these parameters are long, you’ve got a problem; if two or more are long, hire an expert to help guide you through the maze.
NOT JUST EXPERIENCE, BUT THE RIGHT KIND OF EXPERIENCE
The most critical piece of advice is to hire a runway supplier and installation company with plenty of runway experience. We’re not talking building steel experience here, you specifically want runway experience. You need a firm that does overhead crane runways 24/7 and has done them for years. Specifically engage a firm that has an installation superintendent that’s been doing runway installations for years. Don’t be bashful, ask for the installers resume. After all, the success of your project, in no small part, depends on this person. You want an installer with a lot of gray hair, or maybe no hair at all. With runway installation, especially installations with long dimensions, there’s no substitute for experience… PERIOD!
For those that want to learn more about this topic, here are a couple of great reference articles;
Tolerances Illustrated, By Erika Winters Downey and Jason Ericksen, S.E, “Modern Steel Construction,” October 2006
On Target and Within Tolerance, By Larry S. Muir, P.E., Modern Steel Construction, September 2015