SWL v Design Load

06 Oct

The initials “SWL” are one of the most easily recognised and understood acronyms in the construction industry; you know that something marked “SWL 5 Tonnes” can safely support a maximum load of 5 tonnes in real terms. Apply this philosophy to proprietary support equipment such as the hydraulic bracing systems used for earthwork support, it is commonplace to quote values and produce design charts for load capacity of bracing equipment in working load terms.

The use of (safe working) load capacity terminology for resistance is a convenient and easily comprehended practice in which the ultimate or failure values for resistance are reduced by a lumped safety factor typically of between 1.5 and 2. Providing that actual or calculated (unfactored) loads are less than safe working load for the piece of equipment in question then it is strong enough to do the job. It’s a universally understood system that has served us well for generations. However with the arrival of the suite of Eurocodes on the scene, most notably EC7 Geotechnical design, this load – resistance balancing act is in the process of being superseded by a different concept associated with limit state design (LSD).

LSD is not a new concept the UK. It is a more efficient method of structural analysis than traditional lumped factor of safety or elastic / permissible stress methods. Concrete and structural steel work designers will be more than familiar with the concept and the application of partial factors to both loads and resistances. As codes such as BS5950, CP110 and its successor BS 8110 have been with us for many years. Consequently the transition across to Eurocodes should be relatively straightforward in these engineering disciplines. Limit state methods applied to geotechnical engineering is a far more unfamiliar concept to many if not most designers, involving unfamiliar expressions and completely different methodology.

The use of load charts to define equipment performance is very common practice with construction related equipment. These charts typically define component performance data over operating range limits. These charts are produced by either structural calculation or by actual test data; this being reduced by a lumped factor of safety to produce allowable or working values. Users, specifiers and engineers can understand and interpret data from these charts without confusion.

By contrast, in Eurospeak, we now need to start thinking about actions and resistances as partial factors are applied to both sides of the “equals” sign of the balancing equation. For most users familiar in dealing with SWL parlance, charts conveying component performance data in terms of LSD parameters will cause some uncertainty as how this data needs to be compared to the calculated actions. The most common question that springs to mind is “where are the factors of safety applied”?. The big danger is that working loads are compared to design resistances with potential catastrophic overload as a result.

The difference in nomenclature alone adds to the confusion. “load” charts should really now be labelled up as “resistance” charts, terms such as ultimate, characteristic and design when applied to both actions and resistance, need to be clearly understood together with the application of partial factors.

Eurocodes are now with us. National standard were officially withdrawn as of March this year, all major projects are now being designed using Eurocodes. Even small scale geotechnical designs including those for temporary works will be designed this way as new Eurocode 7 becomes established.

It is important during this transition period that engineers and contractors fully understand the various design terms to ensure that load factors are applied correctly. There are potential pitfalls and a significant risk that safety factors might be misunderstood or applied wrongly; particularly where loads and resistances are calculated by different parties.

For young engineers trained in the philosophy of LSD, this should not be a problem. However there are plenty of engineers and contractors out there who have been used to SWL for many years and for them it’s thinking caps on.

Tony Gould
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