or, how to lose a quarter-million dollars in the blink of an eye

OVERHEAD CRANE CLEARANCE REQUIREMENTS

Any crane professional worth their salt can quote CMAA 70 crane clearance requirements off the top of their head, 2-inches on the side, 3-inches over the top. 

End of Story…or is it???

 When's the last time you read, carefully, what Spec 70 says about overhead crane clearances? 

 It's not as "cut and dried" as you might think.

WHAT THE BOOK REQUIRES

CMAA SPEC 70-2015

1.3 CLEARANCE 

1.3.1 Clearance shall be maintained between the crane and the building, as well as cranes operating at different elevations, under all normal operating conditions. In the design of new cranes, all factors that influence clearance, such as roof/ceiling deflection, girder camber, trolley positions, and configurations, shall be considered. As a minimum, the clearance between the highest point of the crane and the lowest overhead obstruction shall not be less than 3 inches with the crane unloaded. Pipes, conduits, lights, etc., must not reduce this clearance. 

1.3.2 Clearance shall be maintained between the crane and the building, as well as parallel running cranes, under all normal operating conditions. In the design of new cranes, all factors that influence clearance, such as wheel float, bridge skewing, or trolley positions and configurations, shall be considered. As a minimum, the clearance between the end of the crane and the closest side obstruction shall not be less than 2 inches with crane centered on runway rails. Pipes, conduits, lights, etc., must not reduce this clearance. 

READING BETWEEN THE LINES

Now, let's parse it out, sentence by sentence, to digest the phrases most of us inadvertently skip over.

1.3 CLEARANCE 

1.3.1 Vertical Overhead Crane Clearance

Clearance shall be maintained between the crane and the building, as well as cranes operating at different elevations, under all normal operating conditions.

In the design of new cranes, all factors that influence clearance, such as roof/ceiling deflection, girder camber, trolley positions, and configurations, SHALL be considered. 

As a minimum, the clearance between the highest point of the crane and the lowest overhead obstruction SHALL not be less than 3 inches with the crane unloaded. 

Pipes, conduits, lights, etc., must not reduce this clearance. 

 

1.3.2 Horizontal Overhead Crane Clearance

Clearance shall be maintained between the crane and the building, as well as parallel running cranes, under all normal operating conditions. 

In the design of new cranes, all factors that influence clearance, such as wheel float, bridge skewing, or trolley positions and configurations, SHALL be considered. 

As a minimum, the clearance between the end of the crane and the closest side obstruction SHALL not be less than 2 inches with crane centered on runway rails. 

Pipes, conduits, lights, etc., must not reduce this clearance. 

Unfortunately, most look upon the 2 & 3-inch minimum as a fixed number, and it's not.

Minimum= the lowest or the smallest amount of a VARYING quantity

The CMAA’s 2 & 3 inch numbers are just a starting point. Remember, these are both known and unanticipated issues that may pop up through the course of the project.

REAL-WORLD TOLERANCES

Let me give you an example. I'm currently working on an EOT overhead crane project in which the crane span is 228 feet, and the runways are at an elevation of 110 feet. For a project with these abnormally long dimensions, here are some facts I keep in the back of my mind;

• AISC (American Institute of Steel Construction) building column installation is considered "plumb" when the vertical alignment is equal to or better than 1/500. In other words, .24 inches out of plumb for every ten vertical feet is totally legit.

• AISC rolling mill tolerances for straightness is 1/8 inch for every 10 feet in length. In other words, a 50-foot piece of steel can have 5/8 inch of lateral sweep and still meet mill tolerance for straightness.

 So now I'm armed with the reality that straight steel is not straight, compounded by the fact that the longer the steel, the greater the total variance. Why use a tolerance dimension that assumes absolute perfection in all materials, fabrication, and installation? I maintain that we are willing to accept perfection as the norm because as “office rats,” we’ve been indoctrinated by the CAD mindset.

THE CURSE OF A CAD MINDSET

For over a couple of decades now, we have been living in the era of CAD drawings. An age in which a line representing a 1000 ft building wall is perfectly straight, plus nothing and minus nothing! But in the real world, the pieces of steel we use to build those walls are just like the 2x4's we buy from Home Depot. Or, as my father would say, "These 2x4's are as crooked as a dog's hind leg."

Crooked 2x4’s and bowed steel are a reality; the model of CAD perfection is a dangerous illusion. To accept a CAD mindset of straight steel and plumb columns sets us up for inevitable failure.

ACCOMODATING THE REAL WORLD

So, what can we do about the reality of "crooked steel" in a perfect CAD world? I would start by using the ASME NOG-1 description for clearance. NOG-1 is the US crane specs for Nuclear Cranes. For the most part, it's unnecessary for non-nuclear cranes, but some of their wording is more informative than is CMAA Spec 70. Their explanation of crane clearances is better than OSHA, ANSI B30.2, or CMAA wording. 

ASME NOG-1 (2015)

1146 Clearances.

The crane shall be designed to provide clearance between the crane components and the building and surrounding obstructions. At least 3 inches of clearance shall be provided between the highest point of the crane and the lowest overhead obstruction, after taking into account variables that affect the elevation of overhead structure relative to the crane, including but not limited to external loads such as snow and wind. The clearance between the ends of the crane and the building columns and other obstructions shall not be less than 2 inches with the crane centered on the runway rails. In addition, the crane must clear the obstructions when operational variables, including but not limited to maximum wheel float, crane skew, rail alignment, civil installation tolerances, and thermal expansion, are considered.

It may seem like I'm splitting hairs, but the differences in wording between NOG-1 and CMAA 70 is noteworthy.

• NOG-1 calls out "at least" 3 inches of clearance.

  •  Further, it follows up by saying, "after taking into account variables that affect the elevation of an overhead structure relative to the crane, including but not limited to external loads such as snow and wind."

• NOG-1 calls out "shall not be less than 2 inches."

  • …with the crane centered on the runway rails

  • …the crane must clear the obstructions when operational variables, including but not limited to maximum wheel float, crane skew, rail alignment, civil installation tolerances, and thermal expansion, are considered.

CONCLUSION

We need to change our perspective. The CAD mindset sees the world as square, plumb, and level. Since we use their drawings, we basically reside in their world. Conversely, the Ironworkers that install the building steel know the reality of the shop floor. They know that nothing is square, plumb, or level and have to plan accordingly. To increase our odds for success, we need to think more like Ironworkers and less like CAD operators.

Adding an inch or two to both horizontal and vertical clearance dimensions, at the drawing stage, doesn't cost a penny. Fixing a problem after the steel is up can cost a quarter million in the blink of an eye. 

Let me throw you one more curve. The above discussion considers the risks of using the proscribed minimum vertical and horizontal clearance tolerances WHEN ALL THE STEEL, ALL FABRICATION, AND ALL INSTALLATION TOLERANCES HAVE BEEN SUCCESSFULLY ACCOMPLISHED. What about those situations when you find a half dozen columns that don't meet either mill steel tolerances or AISC installation tolerances for plumbness or both? What can it cost in project completion timing, inevitable legal costs, and damage to personal reputation for the lack of a single inch of insurance?

Bottom Line…

Barring exceptional circumstances, using the proscribed minimum Overhead Crane clearance tolerance is a fool's errand.

Always add an inch (or more) to both horizontal and vertical dimensions and make everyone else at the table fight you to take it out. Most of the time, they won't even know it's there, and you'll find you are sleeping much better in the meantime.