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19th June 2017 : A Guide to the Specification of Execution Class for Building Structures

By M. Denham
Chairman & Managing Director
Elland Steel Structures Ltd
19th June 2017

The introduction of the execution standard for structural steelwork, BS EN 1090-2 first introduced the concept of an execution class for a component of a structural frame or building. A common error is that the structure is specified an execution class and that equally applies to all of the structural components in the structure, this is not the case and can cause an inordinate amount of money being wasted due to over specification. The structural engineer responsible for the frame is able to specify that some components may be specified to execution class EXC2 and other more critical components may be specified as EXC3. The Consulting Engineer responsible for the structural frame is expected to specify the execution class. On the introduction of BS EN 1090-2, if the execution class of the structure or components of the structure was not specified then the default execution class would be deemed to be EXC2. Annex B of BS EN 1090-2 gave guidance on the determination of the execution class, however this annex was only informative. There was however a problem with this guidance as it was located in an execution code rather than a design guide used by the Consulting Engineers, as such many Consulting Engineers were unaware of their responsibility to specify the execution class.


In 2014, the UK NA to BS EN 1993-1-1 was published, namely “NA+A1:2014 to BS EN 1993-1-1:2005+A1:2014”. The new code changed the method of determining the execution class of the structure (or component) to clause NA2.27.3 of the new NA, the informative method in Annex B of BS EN 1090-2 was withdrawn. Every Consulting Engineer should now have knowledge of the NA to BS EN 1993-1-1, so the only obstacle now is to understand the new process in clause NA 2.27.3. The specifier still needs to be able to first define the consequence class of the component and/or structure, this is done by referencing Table B1 from EN 1990:2002.

Definition Of Consequence Class Table

Table B1 from BS EN 1990:2002 is shown above and its use is pretty straight forward. Once the specifier has the consequence class, they then need to consider whether the structure is sensitive to fatigue (BS EN 1993-1-9) or the component is made using a steel product with a design strength above 460 N/mm2 up to 700 N/mm2. The latter of the two is easily known by the Consulting Engineer, the former, the “fatigue sensitive” consideration is more difficult to define. Unfortunately, there is no “magic bullet” for the determination of whether the structure is “fatigue sensitive” or “quasi-static”, the designer is required to assess this by checking the design using BS EN 1993-1-9. “Quasi-static” is the term used to define a structure that is subject to such low levels of fatigue loading that this level of fatigue loading will not have a detrimental effect on the structure for its intended design life.

Scope and conditions of application of execution class table

Table NA.4 of the NA+A1:2014 to BS EN 1993-1-1:2005+A1:2014 is shown, but the reader is directed to the commentary in clause NA2.27.3 for more guidance.


If the structure (or component) is “fatigue sensitive” and it’s consequence of failure is assessed as CC2, then clearly the structure (or component) is defined as “Generally EXC3”. If the structure or component was “fatigue sensitive” and it’s consequence of failure is assessed as CC3, then clearly the structure (or component) is defined as “Minimum EXC3”. What is “Minimum EXC3”, this means that the structure (or component) is expected to be subject to fatigue and there is a high consequence of failure. In this case specifying EXC3 is as an absolute minimum, but the consulting Engineer may wish to improve the execution quality by explicitly specifying additional clauses in BS EN 1090-2 which ordinarily are not undertaken.


There is now no longer an EXC4 definition, rather an EXC3 plus additional specified clauses to improve the product as specified by the Consulting Engineer. No guidance on the scope of EXC1 is given in the NA, other than a warning that, “specifying EXC1 may mean that there will be a higher probability of failure of the structure”.


If for “fatigue sensitive” structures, NOTE 1 of NA 2.27.3 states, “In order to ensure full compatibility with the design resistance values in BS EN 1993, technical requirements in BS EN 1090-2 relating to certain execution classes might need to be enhanced. Guidance is given in PD 6705-2 for designs to BS EN 1993-2”. BS EN 1993-2 is the design code for bridges and the guidance in PD 6705-2 was based on bridge design, but it can be applied to other “fatigue sensitive” structures if specified by the Consulting Engineer.


The author is aware that the definition of whether a structure is “fatigue sensitive” or not is confusing to the market and there is even more confusion in the market in relation to how to adequately deal with structures that are sensitive to fatigue. That is why it is of vital importance that clients use the services of competent structural steelwork contractors, such as Elland Steel Structures Ltd who have the capabilities to undertake these challenging contracts. If an inappropriate steelwork contractor is appointed and the design work is not done correctly, the structure will simply fail before its intended design life and the consequences could be catastrophic for the general public.



 

Elland Steel Structures Ltd
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