The following opinion piece by Seán Breen, Chartered Engineer, was received on 29th August 2014.
Consequence Classes – What are they?
Image is credited to The Daily Telegraph, 1968.
Some may have noticed on the BCMS that the project information requires confirmation of consequence class (or building class as it used to be known). Unless you are a Structural Engineer you may not have come across this term before. So, what is it and what does it refer to?
In 1968, a 22-storey residential tower block called Ronan Point in East London suffered a gas explosion to a corner flat on the 18th floor. The gas explosion was relatively minor as the occupier was unhurt. Due to the nature of the structure and the type of construction however, the blast blew out the external walls leaving the floor above unsupported which subsequently collapsed onto the floor below. The result was a cascade effect of floor collapsing onto the floor below resulting in its collapse. The partial collapse of the building was ‘disproportionate’ in relation to the incident which occurred. To avoid this in future buildings, disproportionate collapse regulations were introduced.
©”London Over the Border” , a historical website produced by the London Borough of Newham. Image is credited to The Daily Telegraph, 1968.
So that’s the history, what about current regulations and practical considerations? TGD A defines the requirement as:
Buildings are divided into four classes of structure based on the risk associated with a collapse of the structure or a section of the structure. Primarily, the type of use and number of storeys determine the class.
•Class 1 applies to typical single occupier dwellings and is satisfied by ensuring appropriate horizontal ties to floors and roofs. TGD Part A outlines the standard restraint requirements for typical masonry dwellings (126.96.36.199 to 188.8.131.52) which will meet Class 1.
•Class 3 requires specialist attention and is reserved for very tall buildings or buildings with a large number of occupants (stadia, theatres etc.). These generally involve key element design and a bespoke risk analysis.
•Class 2A & 2B are the typical commercial building categories. Class 2A is similar to Class 1 in terms of lateral restraint but the tie arrangement will vary depending on the building construction (steel frame, concrete, timber etc.) and usually requires input from a structural engineer. Class 2B requires specialist design. These are high risk and as such the requirements are more onerous. In relation to S.I.9, mis-classification of a Class 2B as 2A building class is one to watch. A common situation in commercial projects is mixed use such as ground floor retail with 2 storey of residential over. This can fall under either 2A or 2B depending on the floor area of the retail space. Another example is two storeys of residential over two storeys of retail. For classification purposes, this is considered four storeys of retail and as such, Class 2B as the building use is defined by the most onerous use.
The main thing to remember is that Class 1 and 3 are generally correctly identified but Class 2A is often assigned when it is actually Class 2B. If this occurs and is not discovered until near completion, significant remedial works and costs may be required. With the significant responsibilities and liability attributed to certifiers under S.I.9, it is important to confirm with the structural engineer what the consequence class for the building is and what restraint details are required in order to comply with the requirements of TGD A3 and the relevant code of practice for the material involved.
Table 6 from TGD A outlines the general breakdown of classes.
In summary, the determination of consequence class is crucial in order to comply with Building Regulations Part A and if a the project involves a commercial building this role should be performed by and included on the structural engineer’s ancillary certificate. For single dwellings, where a structural engineer may not be engaged, the certifier must satisfy themselves that Class 1 applies and that the standard tying arrangements noted in TGD A are referenced on the commencement drawings and listed for inspection by the Assigned Certifier in the Inspection Plan.
Seán Breen is a Chartered Engineer practicing in Cork – @BreenSJ