What This Requirement Covers
Approved Document A, Requirement A3 requires that buildings are designed and constructed so that, in the event of an accident, the building will not suffer collapse to an extent disproportionate to the cause. This provision was introduced following the Ronan Point collapse in 1968 and aims to ensure structural robustness.
Key Requirements
Building Classification
Buildings are classified according to their consequences of failure
- Class 1: Houses up to 4 storeys, agricultural buildings, single-storey buildings (no additional measures beyond normal design)
- Class 2A (Lower risk): 5-storey residential buildings, 4-storey offices, industrial buildings up to 3 storeys (effective horizontal ties required)
- Class 2B (Upper risk): 6-15 storey residential, hospitals, educational buildings over 2 storeys, public buildings (horizontal ties, vertical ties, and notional member removal analysis or key element design)
- Class 3: Buildings over 15 storeys, stadiums, buildings of unusual form or occupancy (systematic risk assessment using BS EN 1991-1-7)
Horizontal Ties
For Class 2A and above
- Floor ties: Continuous ties must be provided in two directions at each floor level
- Minimum tie force: 60 kN/m width (or Ft = 20 + 4n kN/m, where n is the number of storeys, whichever is less)
- Peripheral ties: Continuous ties around the perimeter of each floor at each level
- Internal ties: Ties connecting columns and walls to the floor structure
Vertical Ties
For Class 2B and above
- Each column and loadbearing wall must be capable of being removed without causing disproportionate collapse of the structure beyond the immediate area
- Where this cannot be demonstrated, the element must be designed as a key element to resist an accidental load of 34 kN/m²
Risk Assessment (Class 3)
- A systematic risk assessment must identify all foreseeable and unforeseeable hazards
- The assessment must demonstrate that the building can sustain localised damage without disproportionate collapse
- This typically involves non-linear structural analysis and consideration of progressive collapse scenarios
Practical Compliance Tips
- Determine the building class at the earliest design stage; it affects the structural design throughout
- For Class 2A buildings, ensure the structural engineer includes horizontal tie calculations in the design package
- For Class 2B and Class 3 buildings, the disproportionate collapse analysis should be a distinct section of the structural calculations
- Precast concrete structures require particular attention to ties and connections between elements
- Steel frame buildings achieve tying requirements through bolted or welded connections between beams and columns
- Transfer structures (where upper floors are supported by beams spanning over open spaces below) are particularly sensitive to disproportionate collapse and require careful analysis
- Submit the disproportionate collapse strategy to Building Control with the structural calculations