AD 375: BS 2853:2011 Steel overhead runway beams

For over 50 years, steel overhead runway beams for hoists have been designed to BS 2853:1957, Specification for the design and testing of steel overhead runway beams.  Last amended in 1970, BS 2853:1957 remained largely unchanged since 1967, when it was updated to take account of the replacement of RSJs by UBs. It remained in Imperial units and Allowable Stress format, whilst continuing to refer to numerous outdated British Standards.

The publication of BS EN 1993-6: 2007, Eurocode 3: Design of steel structures. Crane supporting structures has, since 2007, provided an alternative design standard for crane supporting structures. In April 2010 it came fully into force, along with the rest of the Eurocode Parts, when the former national structural design standards were withdrawn. However, BS 2853 has not been withdrawn – instead, a new version, BS 2853:2011 Specification for the testing of steel overhead runway beams for hoist blocks was published in October 2011. The Advisory Desk has been asked why, with the Eurocodes already in force, a new edition has been published and what its continued relevance is for structural designers.

There are two answers – test loads and serviceability criteria – and these are discussed below.

Test loads
The original 1957 edition of BS 2853 covered both design and testing. With the requirement for BSI to withdraw all national standards conflicting with Eurocodes, structural design has been removed from the scope of BS 2853. However, in design to BS EN 1993-6: 2007, runway beams need to be checked under test loading if the hoist they support needs to be tested. Details of the relevant test loads are specified in BS 2853:2011 and these will govern the design of the bottom flange of the runway beam to resist local wheel loads. BS 2853:2011 is thus “non-contradictory complementary information” (NCCI) that should be used in association with BS EN 1993-6: 2007.

Serviceability criteria
The 2011 edition of BS 2853 has retained and amplified general serviceability requirements for the design of runway beams. The criteria now include:

• deflection;
• slope;
• suitability.

Deflection
The wording now clarifies that the deflection of a runway beam due to the safe working load is to be measured relative to its supports. In the past, some inspectors erroneously measured the absolute deflection. The deflection limit in BS 2853:2011 now corresponds with the design requirement in BS EN 1993-6.

Requiring a loaded runway beam to have a sufficiently large “moment of inertia” (second moment of area) to limit its deflection relative to its supports, also limits its slope due to the loaded trolley. This is an indirect way to avoid subjecting a trolley to an excessive slope. The deflections of the supports are not relevant unless they increase the maximum slope to which the trolley is subjected.

Slope
A new requirement has also been added, limiting the unintended slope of an unloaded runway beam, again to avoid subjecting a trolley to an excessive slope.

• Unintended differences in the levels of runway beam supports can arise from three sources:
• Erection tolerances;
• Differences between the deflections of each support due to static loads on the supporting structure;
• Differences between the deflections of each support due to other moving loads on the supporting structure.

Some design modification will be needed if the deflections of the supporting structure are such that the total slope of an unloaded runway beam from these three causes could exceed the limiting value. As an alternative to modifying the supporting structure, the runway beam could be treated as intentionally sloping and the trolley designed accordingly.

Suitability
The retained non-contradictory wording on general aspects of runway beam design requires the design and layout of the supporting structure to be appropriate.

Provided that the supporting structure doesn’t oscillate, its deflections due to the load on a simply supported runway beam are not normally a problem, even if some supports deflect more than others. With a simply supported beam, the slope at the trolley location will reach its maximum when the trolley is closer to one of its supports than to the other. At this point, the slope of a runway beam due to the load from the trolley is relatively insensitive to the deflection of the other support, so it is sufficient to limit the deflection of the runway beam under the load from the trolley, relative to the mean of the deflections of its supports, with the trolley at mid-span. The same is true in the case of a continuous runway beam.

However, in the case of a load on a cantilevered runway beam, it is necessary to allow for the resulting deflections of its supports, because when the trolley is on a cantilever, the remote support of the anchor arm will deflect upwards. The deflection of the cantilever relative to the mean of the deflections at its supports will thus be more than its deflection relative to the adjacent support, because the resulting slope of the anchor arm will increase the slope of the cantilever. (This is in addition to the downward deflection of the cantilever due to the upward curvature of the anchor arm.)

Accordingly, the calculated deflection of the cantilever at the trolley location needs to include its deflection due to the relative deflections of its supports.

Contact: Abdul Malik       
Tel: 01344 636525
Email: abdul.malik@steelconstruction.org

Note: Thanks are expressed to Colin Taylor for his advice in the preparation of the AD.