OCC CRANE FACTOID #01: Plumb Crazy

FACTOID INTRODUCTION

This article is the first in a series of "Crane Factoids" that I will be periodically writing. They will be "short and sweet" while attempting to illustrate a single Overhead Crane related issue. An issue that is often overlooked or misunderstood.

WHAT’S CONSIDERED PLUMB?

I'm not sure if there is such a word as "Plumbness," but if not, I'll start it here. By "Plumbness," I'm referring to how perfectly perpendicular a column is to the floor and what is its acceptable tolerance. Because Overhead Crane runway alignment is so critical to proper crane operation and that crane runways are either supported by or tied-back to building columns, it's vital to establish what the acceptable requirements are for the installation of the vertical steel columns. This allowable tolerance may surprise you; or more accurately, will knock your socks off!

What is considered plumb?

According to the AISC Code of Standard Practice for Steel Buildings and Bridges (ANSI/AISC 303-16).

Paragraph 7.13.1.1 (page 16.3-49)

For an individual column shipping piece, the angular variation of the working line from plumb line shall be equal to or less than 1/500 of the distance between working points, subject to the following additional limitations:

*(Note sub-paragraphs, a) referrers to elevator shafts and b) refers to building exterior siding. I have therefore not included them here).

In other words, take your building elevation in inches and divide by 500, to determine the maximum deviation from true plumb (in inches) allowable. If you are more of a visual person than a math person, here's the AISC's graphic.

ANSI/AISC 303-16 2016 Code of Standard Practice for Steel Buildings and Bridges- Plumbness Tolerance

HOW THE MATH SHAKES OUT

If you find the AISC Fig C-75 more confusing than enlightening (which I do), I’ve made the following chart of elevations from 20 to 120 feet in 10-foot increments to help illustrate my claim that this information should “knock your socks off.”

Overhead Crane Consulting/Larry Dunville 1/500 rule illustration.

HOW THIS AFFECTS CRANES AND CRANE RUNWAYS

I don't know about you, but I find the fact that, with a 40-foot-high building, it's legit to have columns almost 1 inch out of plumb (0.96). This dimension blows me away!!! Just to make sure you are getting the full picture here, let's look at this, a) from the side (perpendicular to the runways and, b) along the length of one side of the runway.

a) Eastside Runway to Westside Runway

Your east side column (assuming a north/south running bay) can be 1-inch east of plumb while your Westside column can be 1-inch west of plumb, resulting in a column to column difference of 2 inches from the desired dimension.

b) North to South Along the Same Runway

If your first and last column along the runway are 1 inch leaning away from the crane (outbound) while one of the center columns are 1 inch leaning toward the crane (inbound), your imaginary chalk-line will be infringed upon by two inches.

SO WHAT’S THE BID DEAL?

So, what's the importance of the 1/500 factoid? I'll leave you with three observations;

1. Living in the Real World

In the real world, straight is not really straight and plumb is not really plumb.

To avoid experiencing a "come to Jesus moment," you need to think more like an Ironworker than a CAD operator. When it comes to steel straightness, etc. in the real world, you are no better off than picking out 2x4's at your local Home Depot… they're all crooked, and it's just a matter of how crooked they are.

2. What's Real and What's Not?

In a 40-foot-high building, with columns +/- 1 inch in every direction, what do you use as your measurement benchmark? It most assuredly should not be measuring “x” inches off the face of the first and last column and then drawing a chalk line between the two columns as I've seen many “so called” runway installers do. This is where the experienced, skilled tradesmen is a miracle worker in knowing how to perform a runway installation/alignment where nothing is absolutely square, level, or parallel…NOTHING!

3. Runway Tolerances Are +/- ¼ Inch

Never forget, in this world of crooked steel and 1/500 plumbness, your runways still MUST be +/- ¼ inch, vertically, horizontally, and parallel.

If that doesn't scare you, you've never actually installed an Overhead Crane runway and had to make it work.