Robustness is the property that stops a local failure becoming a disproportionate collapse - the difference between losing a single bay and losing a building. It is also the design check most often waved through, because in normal conditions nothing ever loads it. Yet the few clauses that govern it, in EN 1991-1-7, are among the cheapest insurance a structure can carry.
What "disproportionate" means
The principle behind robustness is simple: the damage should be proportionate to the cause. A gas explosion, a vehicle impact, a one-off accidental overload may legitimately destroy the member it strikes - but it should not bring down a structure far larger than the original event. The textbook cautionary tale is the kind of progressive collapse where the loss of one element cascades through a building floor by floor.
The goal is not to make every member indestructible. It is to give the structure alternative load paths, so that if one element is lost, the load it carried finds another way to the ground.
The tying route
For most ordinary buildings, the codes offer a prescriptive shortcut rather than a full alternative-load-path analysis: tying. Continuous horizontal ties through the floors and vertical ties up the columns knit the structure together so that, if a member is removed, the surrounding members and the floor system can bridge over or hang across the gap.
Ties cost almost nothing to detail, and can be the difference between a damaged bay and a collapsed building.
That is the central economic argument. The tie forces are modest, they are usually satisfied by reinforcement and connections that are present anyway, and the marginal cost of checking and detailing them is tiny against the consequence they guard against.
Working the ties
For a frame - say a six-storey building - the internal tie force is a function of the floor loading and the span, and the procedure is to:
- Calculate the required horizontal tie forces from the loading and geometry, for internal ties, perimeter ties and ties to columns.
- Carry the tie continuously through the beams and into the columns - a tie is only a tie if it is continuous and anchored; a bar that stops short does nothing.
- Check the connections can actually deliver the force. This is the step most often missed: the beams may have the capacity, but the end connections have to transmit the tie force, and a nominally pinned connection may not.
That last point is where tying designs most often fall down on paper - the tie is traced through the members but never checked at the joints that have to carry it.
Why it deserves more than a tick
Three reminders keep robustness honest:
- It is a real check, not a formality. Treat the tie forces and their connection demands with the same seriousness as a gravity load case.
- Continuity is everything. A discontinuous tie, or one that dies in a connection that cannot anchor it, provides no alternative load path at all.
- It is cheap relative to the risk. Few other checks buy so much resilience for so little.
The EN 1991-1-7 provisions sit in the standards reference, and the load-combinations article covers the accidental design situation that the robustness rules sit alongside.