1.2 Disproportionate Collapse
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All buildings must be designed to accommodate unforeseen or accidental actions in such a way as to prevent the extent of any resulting collapse being disproportionate to the cause of the collapse. Buildings should be designed so that they are robust which is defined in BS EN-1991-1-7:2006 as the ability of a structure to withstand events like fire, explosions, impact or the consequences of human error without being damaged to an extent disproportionate to the original cause.
Explanation of terms
Nominal length of load-bearing wall construction should be taken as:
in the case of a reinforced concrete wall, the distance between lateral supports subject to a length not more than 2.25 x storey height
in the case of an external masonry wall, or timber or steel stud wall, the length measured between vertical lateral supports
in the case of an internal masonry wall, or timber or steel stud wall, a length not more than 2.25 x storey height.
Storey height is the distance from the underside of one floor to the underside of the floor immediately above.
Key element is a structural member upon which the stability of the remainder of the structure depends and should be capable of sustaining an accidental design loading of 34kN/m2 applied in the horizontal and vertical directions (in one direction at a time) to the member and any attached components such as cladding, having regard to the ultimate strength of such components and their connections. Such accidental design loading should be assumed to act simultaneously with 1/3rd of all normal characteristic loading.
Load bearing wall construction includes masonry cross-wall construction and walls comprising close centred timber or lightweight steel section studs.
Fire - the protection to be afforded to the structure of a building when it is exposed to the action of fire is dealt with by mandatory Standard 2.3. The guidance within Section 2 refers to relevant codes that should be used for the design of the structure in a fire.
Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).
A building which is susceptible to disproportionate collapse is one where the effects of accidents and, in particular, situations where damage to small areas of a structure or failure of single elements could lead to collapse of major parts of the structure.
Buildings should be provided with a level of robustness by adopting the principles of risk analysis, categorising buildings, taking into account both the risk of the hazard and its consequences and providing additional measures commensurate to the level of risk and consequences of such collapse of the building. The risk level and accidental actions that should be considered when undertaking the structural design of a building for disproportionate collapse should be in accordance with the recommendations of BS EN 1991-1-7:2006 or the method set out below.
Any reference to European Standards for Structure (Structural Eurocodes) in this section must be taken to include the relevant UK National Annex.
To ensure that buildings are designed and constructed to sustain a limited extent of damage or failure without a disproportionate level of collapse from an unspecified cause, the following procedure should be followed:
The issues to be considered with respect to assessing the risk group of a building are its occupancy level, use, the number of storeys and floor areas.
The risk of an extreme event such as an explosion or other incident occurring would not be decreased simply by providing these measures and there is no certainty that demolition or building alteration would be carried out in accordance with good practice but the consequences of such an incident occurring would be considerably reduced.
Table 1.1. Determine building risk group
| Risk Group | Building Type |
|---|---|
| 1 | Houses not more than 4 storeys |
| Carports, Conservatories and Greenhouses | |
| Domestic garages and other small single leaf buildings not more than 1 storey | |
| 2A | 5 storey houses |
| Flats and maisonettes not more than 4 storeys | |
| 2B | Flats and maisonettes more than 4 storeys but not more than 15 storeys |
| 3 | Every domestic building not covered in Risk Groups 1, 2A and 2B |
The nomenclature of the Risk Groups 1, 2A, 2B and 3 are synonymous with the consequence classes in Table A.1 – Categorisation of consequence classes of BS EN 1991-1-7:2006.
In determining the number of storeys in a building, basement storeys may be excluded provided such basement storeys fulfil the robustness of Risk Group 2B buildings.
For buildings intended for more than one type of use the Risk Group should be that pertaining to the most onerous Risk Group.
The additional measures which should be provided vary extensively according to building type and use and the actual measures should be designed in accordance with the relevant sections of the design codes. For example, high rise hotels or flats or assembly buildings or grandstands require a different level of robustness than low rise buildings or storage buildings.
The additional measures which should be applied to buildings of the risk groups derived from the above table are set out below:
Risk Group 1 buildings - no additional measures are likely to be necessary when the building has been designed and constructed in accordance with the rules given in this Technical Handbook, or other guidance referenced under Section 1, for complying with Standard 1.1 in normal use.
Risk Group 2A buildings - provide effective horizontal ties, or effective anchorage of suspended floors to walls, for framed and load-bearing wall construction.
Risk Group 2B buildings - provide effective horizontal ties for framed and load-bearing wall construction, together with effective vertical ties, in all supporting columns and walls.
Alternatively, check that upon the notional removal of each supporting column and each beam supporting one or more columns, or any nominal length of load-bearing wall (one at a time in each storey of the building) the building should remain stable and that the area of floor at any storey at risk of collapse should be not more than 15% of the floor area of that storey or 100m2, whichever is the less and does not extend further than the immediate adjacent storeys (see diagram below).
Where the notional removal of such columns and lengths of walls would result in an extent of damage in excess of the above limit, then such elements should be designed as ‘key elements’.
Risk Group 3 buildings - a systematic risk assessment of the building should be carried out, taking into account all the normal hazards that can be foreseen as far as possible together with any abnormal hazards.
Critical situations for design should be selected that reflect the conditions that can be foreseen as far as possible during the life of the building.
The structural form and concept and any protective measures should then be chosen and the detailed design of the structure and its elements undertaken in accordance with the recommendations in the codes and standards in clause 1.2.4.
The structural design and construction to take account of the additional measures including horizontal and vertical ties where appropriate and checking the integrity of the building following the notional removal of vertical members and the design of key elements, should be carried out in accordance with the design recommendations contained in Annex A of BS EN 1991-1-7:2006.
More detailed information has been produced by organisations on disproportionate collapse as follows:
Technical Guidance Note ‘The Building Regulations 2004 Edition – England and Wales Requirement A3 – Disproportionate Collapse’, National House Building Council (NHBC)
Technical Bulletin Number 3 ‘Design Guidance for Disproportionate Collapse’, UK Timber Frame Association
‘Masonry Design for Disproportionate Collapse Requirements under Regulation A3 of the Building Regulations (England & Wales)’, Brick Development Association
'Guidance on meeting the Robustness Requirements in Approved Document A', Steel Construction Institute
'How to design concrete buildings to satisfy disproportionate collapse requirements' http://www.concretecentre.com/
The above guidance is based on England & Wales Regulation A3 and should be interpreted in relation to Standard 1.2. In particular, references to building classes should be risk groups and the building types and occupancy should be interpreted as the building types set out in the table to clause 1.2.2.