‘SWL’ is a well-known and easily understood acronym widely accepted throughout the construction industry. Its premise simply asserts that something with a Safe Working Load (SWL) of five tonnes is able to support a maximum of five tonnes. When calculating load limits in the hydraulic bracing industry, regarding earthwork support and bracing equipment, the convention is to produce values and design charts for load capacity in terms of working load. The use of (safe working) load capacity for resistance is a convenient and easily comprehended practice in which the ultimate failure values for resistance are reduced by a pre-determined safety factor, typically between 1.5 and 2.0. In this scenario, providing that actual or calculated load is less than the SWL for the piece of equipment in question, then it is sufficient for the load under consideration. This is a universally accepted system that has been the standard approach for many years. However, with the emergence of Eurocodes; most notably ‘EC7 Geotechnical Design’, the load versus resistance variation is in the process of being superseded by a paradigm shift known as Limit State Design (LSD).

Limit State Design (LSD)

Although not a totally new concept for the UK, LSD is however a more efficient method of structural analysis than the conventional use of Factor of Safety or using elastic/permissible stress methods. Concrete and structural steel work designers will be familiar with the concept and the application of partial factors to both loads and resistances. Design codes such as BS5950, CP110 and previously BS 8110 have been the standard for many years. Consequently, the transition to Eurocodes should be relatively straightforward in these defined engineering fields. However, LSD applied to geotechnical engineering, is a more alien concept to many designers as it includes unfamiliar expressions and different methodology. The use of load charts to define equipment performance is common practice in the use of construction related equipment. These easily interpreted charts typically define performance data operating within range limits. They 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.

Calculating the risk

For Eurocodes, actions and resistances are partial factors that are applied to both sides of the balancing equation. Users familiar in dealing with SWL parlance may encounter some uncertainty when drawing data from charts in terms of LSD parameters and may be unsure how to compare this data to the calculated actions. The area of concern is where the factors of safety are applied, with the overriding risk being that working loads could be compared to design resistances, therefore creating the potential for catastrophic overload. The change in parlance may cause further confusion. For example, ‘load charts’ should now be labelled as 'resistance charts’. In addition, 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.

The Transition to Eurocodes

With national standards no longer supported by revisions, Eurocodes are becoming the established practice with many major projects now using this procedure. Even small scale geotechnical designs, including those for temporary works, will be designed in this way as the new Eurocode 7 becomes the recognised norm. It is important that during this transition period, engineers and contractors fully understand the various design terms to ensure that load factors are applied correctly. There is a significant risk that safety factors may be misunderstood or applied incorrectly; especially where multiple parties are involved with calculating loads and resistances. Engineers new to the industry will no doubt fit seamlessly into the LSD system, having had no previous exposure to traditional design codes. Those more familiar with SWL however, will need to shake off the old and embrace the new. For further commentary on the Eurocode design approach watch the Groundforce Back To The Drawing Board 'Hydraulic Prop Design To Eurocodes' video. You can also view the Groundforce website.