February 2001 • The FABRICATORwww.thefabricator.com

By Larry Dunville

Y ou have a shiny new building with a shinynew crane and everything looks great. Forsome reason, though, the crane won’t clear

the building columns, even though the contractorand the crane manufacturer are saying everythingis to spec and it’s not their problem. Commonsense says somebody is wrong and that somebodyshould have to pay (because it’s going to cost abundle).

Unfortunately in this case, there’s a giant crackin the building specs, and you’ve just fallenthrough it. This means that after all thearguing and legal costs, you’re still going tohave to pay to get it fixed.

If you’ve already fallen in this blackhole, there’s not much you can do, but ifyou are about to embark on a new buildingwith an overhead crane, this article will showyou where the cracks are and suggest how tobridge them safely.

What Is Required?The runway alignment specs—writ-

ten by the Crane ManufacturersAssociation of America (CMAA) andadopted by the Metal BuildingManufacturers Association (MBMA),the American Institute of Steel Con-struction (AISC), and the Associationof Iron and Steel Engineers (AISE)— fillan entire page and take considerabletime to interpret. A simplistic summaryis that runways must be ±1⁄4 inch in asingle bay and no more than ±3⁄8 inchover the full length of the runway.These tolerances must be maintained infour ways: left/right, up/down, parallel to eachother, and level in respect to each other. Figure1 shows an actual AISC/ CMAA chart.

A second set of crane-related numbers toremember are the crane-to-building tolerances.CMAA and the Occupational Safety and HealthAdministration (OSHA) require that all movingobjects (the crane and hoist) must clear all sta-tionary objects (the building) horizontally by 2inches and clear all vertical objects (roof trusses,lights, pipes, etc.) by 3 inches. Although thismeets the legal requirements, this author highlyrecommends the horizontal be increased to

4 inches and the vertical to 6inches to allow for unforeseen problems (seeSidebar).

Where’s the Villain?As in a detective story, the first move is to

round up the suspects. The problem can be foundin one of four areas:

1. Mill steel tolerances2. Building steel fabrication

tolerances3. Building erection tolerances4. Overhead crane runway tolerances (measur-

ing and verification methods) (See Sidebar.)

One big problem is that runways usually arebuilt with building steel (wide flanges), fabricatedby building steel fabricators, and installed bybuilding steel erectors, but runway steel is not build-ing steel. In fact, building steel and runway steelare incompatible in the first three ways listed pre-viously. Following is an illustration of just the firstpoint—mill steel tolerances—but the other twoitems exhibit similar shortcomings.

The mill tolerance for structural wide-flangebeams basically is 1⁄8 inch per 10 feet of length,although this oversimplifies the AmericanNational Standards Institute (ANSI)/AISC spec-ification somewhat (see Figure 2). Therefore, in acommon 30-foot bay, the wide-flange beam canhave a sweep (horizontal bow) of 3⁄8 inch, whichmeans that putting up this first piece of steelexceeds the acceptable CMAA/MBMA/AISCrunway tolerance already. To compound the prob-lem, the opposing runway can have an equal (butopposite) sweep, doubling the problem.

SolutionsHow should this seemingly simple prob-

lem be addressed? Three potential solutionsexist:

1. Adjust the rail laterally in rela-tion to the girder. Although this solu-tion is the most commonly used, it isbad engineering practice and actually isprohibited by the AISC specifications.

The runway beam/girder is thewide-flange structural shape that sup-ports the runway, while the rail(commonly American Society ofCivil Engineers (ASCE) rail, similarto railroad rail) is the track upon

which the end truck wheels traverse (see Figure3). It is a common misconception that the runwaybeams have no particular installation toleranceand that only the rail is at issue. Further, thisassumption seems to be confirmed by the lateraladjustment of the rail fasteners (for example, J-bolts/hook bolts or patented clips).

Actually, the tolerance of the beam installationis governed by the tolerance of the rail installa-tion. This is because, according to AISC DesignGuide 7, paragraph 19a, the centerline of the railshould be within ±3⁄4 inch of the girder web thick-ness. This prevents top flange rollover and subse-quent fillet cracking and possibly girder failure.

This conventional wisdom is so commonly

44

accepted that it has evolved intogenerally accepted practice.Unfortunately, like so much conven-tional wisdom, it’s wrong, it’s bad forthe equipment, it will result in signif-icantly shorter service life, and it canbe dangerous.

2. Augment the specs. Just be-cause the generally accepted specshave left the crane runways as anorphan does not mean that you as aprospective new building ownershould not include a stop-gap page ofspecs to cover yourself. If you buy thesteel from the same vendor, fabricatewith the same fabricator, and installwith the same installation crew, youvery likely will end up with the sameproblem.

It defies reason that any efficientcontractor can buy, fabricate, and in-stall 20+ pieces of apparently identicalred primed steel to a tolerance two tofour times tighter than the other sev-eral thousand pieces of red steel inthat same building. This is not meantto slight building contractors.Successful contractors have set up awell-disciplined system to produce

The FABRICATOR • February 2001www.thefabricator.com

45

Horizontal and VerticalClearances

As referenced in the body ofthe article, OSHA/CMAA requirea horizontal clearance of 2 inchesand a vertical clearance of 3 inch-es. However, because of the toler-ances of mill steel, fabrication, andinstallation, the top of a 50-footcolumn can meet spec and still beoff dimension by several inches.

Regarding vertical clearance,long-span buildings can haveexcess deflection while the cranecould have excessive camber,thereby encroaching on therequired 3-inch clearance—not tomention the obstacles of forgottenwater pipes, electrical runs, andlight fixtures.

For all but the most headroom-critical situations, using 4-inchhorizontal and 6-inch verticalclearances will prove to be a veryinexpensive insurance policyagainst unforeseen dimensionalchanges.

Figure 1Overhead crane and gantry crane and their structural support framing systems shall be designed, fabricated, andinstalled in accordance with the current applicable revisions of the following codes and standards:

1) “Specifications for Electrical Overhead Traveling Cranes,” CMAA Specification Number 70; and Specification for Top Running and Under Running Single Girder Electric Overhead Traveling Cranes,” CMAA Specification Number 74, published by the Crane Manufacturers Association of America, Inc.

2) American National Standards Institute and American Society of Mechanical Engineers publications including but not limited to the following: ANSI/ASME B30.2, B30.2a, B30.2b, B30.10, and B30.17.

3) “Specification for the Design, Fabrication, and Erection of the Structural Steel for Buildings,” published by the American Institute of Steel Construction, Inc.

4) “Industrial Buildings Steel Design Guide Series 7,” published by the American Institute of Steel Construction, Inc.

5) “MBMA Low Rise Metal Building Design Manual 1996,” published by the Metal Building Manufacturers Association.

Runway Installation Tolerances

Item Overall Tolerance Maximum Rate ofChange

Figure

Span

Straightness

Elevation

Rail-to-RailElevation

AdjacentBeams

Rail-to-Runway GirderCenterline

Rail Separation N/A N/A< 1⁄16''

e < 3⁄4 tw1⁄8'' in 20' -0''

C of GirderL

C of RailL

tw

1⁄4'' in 20' -0''

N/A

1⁄4'' in 20' -0''

1⁄4'' in 20' -0''

1⁄4'' in 20' -0''

F = 1⁄8''

L < 50' D = 3⁄16''L > 50' < 100' D = 1⁄4''L > 100' D = 3⁄8''

L < 50' E = 3⁄16''L > 50' < 100' E = 1⁄4''L > 100' E = 3⁄8''

C = 3⁄8''

B = 3⁄8''

L < 50' A = 3⁄16''L > 50' < 100' A = 1⁄4''L > 100' A = 3⁄8''

Web

L = L + A(Max.)

Support Points(Typical)

LTheoretical

Span

L = L - A(Min.)

B

B

Web

Support Points(Typical) Theoretical

Support Points(Typical) Theoretical Height

C

C

D

E

F

F

Top of beam for top running crane.Bottom of beam for underhung crane.

Top Running

Underhung

Top Running Underhung

LC

LC

LC

and install building steel, but runwaysteel, although similar-looking, is asignificantly different animal.

While it is unlikely the contrac-tor would adopt this more stringentstandard temporarily, it is not impos-sible. The silver lining for you, thebuyer, in using the augmented specsas part of the contract is that the cor-rections no longer are your problemor expense.

3. Redefine the scope of build-ing contractor and crane supplierresponsibilities. This technicallycorrect, practically viable solution isthe least used of the three, simplybecause of lack of knowledge andhigher up-front costs.

The common scope of the cranebuilder’s contract is to supply and in-stall the crane, runway rail, and con-ductor bar. This leaves a critical gapin which the buyer is exposed to thepreviously mentioned problems. Thescope should be changed to moveresponsibility for the runway girdersfrom the building contractor to thecrane builder.

Chances are, the crane builderwill insist on very tight tolerancesfrom the steel supplier and will takeprecautions to account for reasonablefloor and column tolerances. Also,having the crane builder’s employees

install the runways can help to im-prove installation accuracy becausethis job is their specialty. If the plantis a union plant, however, the runwayconductor bar installation should beawarded to a local electrical contrac-tor, while the crane builder remainsresponsible for the bar.

Get It Right the First TimeIn summary, runway steel is not

building steel. Poor runways will re-sult in premature wheel failure, mo-tor and or gearbox failure, and pre-mature runway replacement. Witha typical wheel replacement costing$8,000 and new runways costing$50,000 or more, not to mentiondowntime, getting it right the firsttime can be a real bargain. Using aug-mented overhead crane runway specsin conjunction with the informationprovided here can help you to stayout of court and maintain good rela-tions with valuable vendors. ■

Larry Dunville is President of DearbornCrane & Engineering, 1133 E. 5th Street,Mishawaka, Indiana 46544, phone 219-259-2444, fax 219-256-6612, e-mailLDunville@DearbornCrane.com, Web sitewww.DearbornCrane.com. Dearborn Crane& Engineering designs, manufactures, in-stalls, and services electric overhead travelingcranes and crane runways.

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February 2001 • The FABRICATORwww.thefabricator.com

46

Other Permissible Variations

Area and weight variation: ±2.5 percent theoretical or specified amount.

Ends out-of-square: 1⁄64 in. per in. of depth, or of flange width if it is greater than the depth.

Camber and Sweep

Permissible Variation, In.

Sizes Length Camber Sweep

Sizes with flange width Allequal to or greater than

6 in.

Sizes with flange width Allless than 6 in.

W Shapes, HP Shapes

1⁄8 in. x(total length ft.)

101⁄8 in. x

(total length ft.)5

1⁄8 in. x(total length ft.)

10

Figure 2This specification is for the design, fabrication, and erection of the structural steelfor buildings as published by the American Institute of Steel Construction, Inc.

MeasuringMeasuring a runway to ensure proper installation is not as simple as it

may seem. Three issues are critical to a proper analysis:1. Proper tools2. Correct measuring points3. Acceptable measuring processTaking into consideration the sag in an outstretched tape measure or

measuring to the web and not the edge of a wide flange are just two of themany points to consider when measuring a crane runway properly.Remember that you can’t fix a measurement problem until you determinewhat the problem is. You could spend tens of thousands of dollars, only tofind you’ve made the problem worse. Proper runway measurement is essen-tial for preventing subsequent problems.

Primary FrameColumn

Spacer Rods

Crane Rail

CraneRunwayBeam

Separate CraneColumn

Bracing as RequiredBetween Building Columnand Crane Column

Stiffeners

Figure 3The runway beam/girder is the wide-flange structural shape that supports the run-way, while the rail is the track upon which the end truck wheels traverse.

Source: Copyright© American Institute of Steel Construction, Inc. Reprinted with permission. All rights reserved.

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