Determination of the Execution Class
The procedure for determining the Execution Class is a straightforward four step process:
Step 1: Determine the Consequence Class
The purpose of categorising the Consequence Class is to ensure that buildings and other structures are constructed with the appropriate level of quality control within the fabrication process. Consequence Classes are derived on the basis of building type, building height (number of storeys), floor plan area per storey (for retail) and occupancy. A structure, or a part of it, could also contain components with different Consequence Classes.
BS EN 1990 gives guidelines for the choice of Consequence Class in Table B1.
Table A.1 of BS EN 1991-1-7 gives examples of categorisation of building type and occupancy according to Consequence Classes that assist with the implementation of Annex B of BS EN 1990.
SCI Guide P391 provides guidance on classification of mixed use buildings and buildings with basements.
Step 2: Define the Service Category
Service categories are the method used in BS EN 1090-2 to consider the risk from the actions to which the structure and its parts are likely to be exposed to during erection and use, such as fatigue and likelihood of seismic actions. They also consider the stress levels in the components in relation to their resistance.
Service categories are determined from Table B.1 of BS EN 1090-2 but for most buildings in the UK, SC1 will be appropriate.
Step 3: Define the Production Category
Production categories are the method used in BS EN 1090-2 to consider the risk from the complexity of the fabrication of the structure and its components, e.g. application of particular techniques, procedures or controls.
Production categories are determined from Table B.2 of BS EN 1090-2 and it should be noted that a structure or part of a structure may contain components or structural details that belong to different production categories.
However, in all cases, the Execution Class is not sensitive to the Production Category selected.
Step 4: Derive the Execution Class
Having determined the Consequence Class, Service Category and Production Category for a building, the required Execution Class is derived simply from Table B.3 of BS EN 1090-2.
Table B.3 - Recommended Matrix for Determination of Execution Classes
* EXC4 should be applied to special structures or structures with extreme consequences of a structural failure as required by national provisions.
Annex B of BS EN 1090-2 is classed as ‘informative’. Therefore the approach described is not mandatory and the engineer can base the selection of Execution Class on experience provided they can support their decision.
For the majority of buildings constructed in the UK, EXC2 will be the appropriate requirement. Where no Execution Class is specified, Clause 4.1.2 of BS EN 1090-2 states that EXC2 shall apply.
The engineer should always derive the Execution Class based on the design parameters appropriate to each project. The requirements to each Execution Class are listed in Table A3 of BS EN 1090-2 and can be reviewed by the engineer if desired.
However, the engineer should avoid over-specification of the Execution Class wherever possible to avoid unnecessary costs being introduced. For example, EXC2 is the Execution Class derived for a project but the engineer requires full traceability (an EXC3 requirement) instead of the partial (or batch) traceability requirement of EXC2. Rather than specifying EXC3 on the basis of achieving this single Clause requirement, it is suggested that EXC2 is still specified but with the higher level of traceability added to the specification.
NB Further detailed information can be found on the Steel Construction web site.
The Execution Class is the sole responsibility of the Structural Engineer.
voestalpine Metsec plc takes no responsibility for the Execution Class derived.