What is structural redundancy?
Structural redundancy is the presence of more load paths in a structure than equilibrium strictly requires: the structure is statically indeterminate, so if one member is lost the forces can redistribute along an alternative load path. Redundancy is one of the means of achieving robustness, not a synonym for it.
Robustness and progressive collapse
A structure is robust if it can prevent or substantially limit the spread of failure by redistributing internal forces when local failure occurs, or when the resistance at one location drops substantially.
The underlying Eurocode principle is that a structure shall be designed so that it is not damaged by an accidental event to an extent disproportionate to the original cause. The historic trigger for this body of regulation is the collapse at Ronan Point in 1968, where a gas explosion in one flat brought down a whole corner of the block.
The Eurocode framework
Accidental actions are covered by EN 1991-1-7. Consequence classes CC1, CC2 and CC3 are set up in EN 1990 Annex B; EN 1991-1-7 Annex A develops them for buildings and splits CC2 into CC2a and CC2b.
| Consequence class | What is required |
|---|---|
| CC1 | No specific consideration of accidental actions. The robustness and stability rules of the Eurocodes themselves suffice |
| CC2a | Effective horizontal ties, or effective anchorage of floors to walls |
| CC2b | Horizontal and vertical ties. Alternatively, demonstrate that on notional removal of a column or a length of wall the damage stays within limits. If that cannot be shown, the member must be designed as a key element |
| CC3 | Systematic risk assessment covering both foreseeable and unforeseeable hazards, using refined methods such as dynamic analysis and non-linear models where needed |
Ties serve to connect columns and wall elements into the structure, so that a local loss does not immediately leave a member hanging free or falling free.
Designing redundancy in
- Choose statically indeterminate systems, for example continuous beams rather than a run of simply supported spans.
- Make connections continuous, so that redistribution is physically possible in the first place.
- Carry ties right through, horizontally and vertically, and check the anchorage at each end.
- Identify key elements, the members with no alternative load path, and design them for the accidental case.
Which edition applies
The description above follows the framework of the 2011 edition of EN 1991-1-7. A new edition of EN 1991-1-7 was published in 2025 as part of the second generation of Eurocodes. On any live project, check which edition and which National Annex the contract documents make applicable.
Related terms
- Robustness
- Statically indeterminate
- Load path
- Eurocode
- Key element
- Tie force
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