Section 2 Disproportionate collapse
A3 Disproportionate
(1). A building shall be designed and constructed, with due regard Collapse. to the theory and practice of structural engineering, so as to ensure that in the event of an accident the structure will not be damaged to an extent disproportionate to the cause of the damage.
(2). (For the purposes of sub-paragraph (1), where a building is rendered structurally discontinuous by a vertical joint, the building on each side of the joint may be treated as a separate building whether or not such joint passes through the substructure.
Introduction
2.1 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 I.S. EN 1991-1-7 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.
Disproportionate collapse
2.2 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 I.S. EN 1991-1-7 or the method set out below. Other additional material specific requirements may be outlined in I.S. EN1992-1-1 to I.S. EN 1996-1-1 and I.S. EN 1999-1-1.
Any reference to Eurocodes must be taken to include the relevant Irish National Annex.
The Requirement A3 will be met by adopting the following approach for ensuring that the building is sufficiently robust to sustain a limited amount of damage or failure, depending on the consequence class of the building, without collapse.
Step 1 - Determine the building consequence class from Table 6;
Step 2 - Assess additional measures;
Step 3 - Design and construct additional measures.
Step 1 - Determine building consequence
2.3 With reference to Table 6, the issues to be considered with respect to assessing the consequence class (risk group) of a building are its occupancy level, use, the number of storeys and floor areas.
Step 2 - Assess additional measures
2.4 The additional measures which should be provided vary extensively according to building type and use. The actual measures should be designed in accordance with the relevant sections of the design codes, e.g. high rise hotels or apartment buildings 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 Table 6 are set out below.
Consequence Class 1 buildings
Provided the building has been designed and constructed in accordance with the rules given in this Technical Guidance Document for meeting compliance with A1 and A2 in normal use, no additional measures are likely to be necessary.
Consequence Class 2a buildings (Lower Risk Group)
In addition to the Consequence Class 1 measures, provide effective horizontal ties, or effective anchorage of suspended floors to walls, as described in the codes and standards listed in par. 2.5, for framed and load-bearing wall construction.
Consequence Class 2b buildings (Upper Risk Group)
In addition to the Consequence Class 1 measures, provide effective horizontal ties for framed and load-bearing wall construction, together with effective vertical ties, in all supporting columns and walls. Horizontal and vertical ties should be in accordance with the codes and standards listed in par. 2.5.
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 100 m2, whichever is smaller and does not extend further than the immediate adjacent storeys (see Diagram 15).
Where the notional removal (one at a time in each storey of the building) of such columns, beams supporting one or more columns, or any nominal lengths of load-bearing walls, would result in an extent of damage in excess of the above limit, then such elements should be designed as key elements (see par. 2.6).
Consequence Class 3 buildings
A systematic risk assessment of the building should be carried out taking into account as far as is reasonably practicable, all the normal hazards which can be foreseen together with any abnormal hazards.
Critical situations for design should be selected that reflect the conditions that can be reasonably 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 standards given in par. 2.5.
Further guidance is given in Annexes A and B to I.S. EN 1997-1-7 and I.S. EN 1990
Step 3 - Design and construct additional measures
2.5 Details of the effective horizontal and vertical ties where appropriate together with the design approach for checking the integrity of the building following the notional removal of vertical members and the design of key elements are given in the following standards and their National Annexes:
I.S. EN 1990:2002+A1: 2005
I.S. EN 1991-1-7:2006
I.S. EN 1992-1-1:2005
I.S. EN 1993-1-1:2005
I.S. EN 1994-1-1:2005
I.S. EN 1995-1-1:2005+A1: 2008
I.S. EN 1996-1-1:2005
I.S. EN 1999-1-1:2007+A1: 2009
Definitions
2.6 The following definitions apply to Section 2:
Key element - A key element is a structural member whose removal would cause damage in excess of the limits stipulated in par. 2.4 and should be capable of sustaining an accidental design loading of 34 kN/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 applied in accordance with Expression 6.11 (b) of I.S. EN 1990 and may be a concentrated or distributed load (refer to Clause A.8 of I.S. EN 1991-1-7:2006).
Load-bearing construction - For the purposes of this Guidance the term 'load-bearing wall construction' includes masonry cross-wall construction and walls comprising close-centred timber or lightweight steel section studs.
Nominal length of load-bearing wall - The nominal length of load-bearing wall construction referred to for Consequence Class 2b buildings (Upper Risk Group) should be taken as follows:
in the case of a reinforced concrete wall, the distance between lateral supports subject to a maximum length not exceeding 2.25H;
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 exceeding 2.25H; where H is the storey height in metres.
Note: Annex A of I.S. EN 1991-1-7 provides corresponding guidance.
Table HA6 - Building consequence classes - Extract from TGD A
Diagram HA15 - Area at risk of collapse in the event of an accident - Extract from TGD A
