What makes a building earthquake-resistant

The first thing that surprises a non-specialist: an earthquake-resistant building is not designed to come through strong shaking undamaged. Such a building would cost an unjustifiable amount. The goal is stated differently — the building may be damaged, but it must not collapse, and it must let people out.

Hence the logic of codes worldwide: minor shaking, no damage; moderate shaking, repairable damage; strong shaking of the kind rare for that region, serious structural damage but the structure stands. Cracked columns and racked door frames after an earthquake do not always mean the builders cheated. Sometimes they are exactly what was intended.

Understanding this changes how to read the news, too. A building that stood but has been condemned and is being demolished is usually a design success rather than a failure.

An earthquake does not press down on a building the way snow does. It moves the ground underneath while the building's inertia keeps it where it was — and every force arises from that mismatch. So the greater the mass, the greater the load: a heavy building is shaken harder than a light one, all else equal.

The second quantity is stiffness. A stiff building barely deforms but takes the full blow. A flexible one deforms more and absorbs the input more gently. Neither extreme is good in itself, and the engineer's job is to find the combination where the building's own period of vibration does not coincide with the ground's. Coincidence means resonance — and it explains why in one city it is mainly, say, the eight-to-twelve-storey blocks that fall while the low and the tall ones stand.

The third and most important is ductility: the ability to deform without breaking, absorbing energy in the process. A brittle structure carries load to its limit and then disintegrates instantly. A ductile one bends, cracks, loses some capacity — and keeps standing, buying time. All modern earthquake engineering is built around ductility.

From ductility follows a rule engineers state almost as a commandment: it is the beams that should fail, not the columns. A beam carries the floor within a storey. A column carries everything above.

If a beam is the first to give under overload, the building sags locally but stays up on its columns. If a column goes first, the storey loses its support and everything above comes down. So columns are deliberately made stronger than the beams framing into them.

Breaking this rule is a classic cause of collapse, and not only on paper: when a shop owner cuts into a column for a display window, or a resident removes an inconvenient load-bearing element during a refit, what is destroyed is precisely the hierarchy the designer built in.

Concrete takes compression well and tension hardly at all. In bending, one face of a member is compressed and the other stretched — and the stretched face is carried by steel reinforcement. Hence reinforced concrete: two materials covering each other's weaknesses.

But longitudinal steel alone is not enough for earthquake resistance. The decisive role belongs to the transverse ties — steel hoops wrapping the column's section along its height. They stop the concrete spreading sideways under compression and stop the longitudinal bars buckling outwards. Concrete confined by ties is what can deform without disintegrating — that is, be ductile.

So ties are spaced closer near the joints, where demand is highest, and their ends are bent inwards into the core of the section. A tie bent outwards, or spaced too widely, opens up under strong shaking: the concrete spalls, the bars buckle, and the column loses capacity in seconds. In photographs of collapsed buildings this is the most recognisable damage of all — exposed bent bars and shattered concrete at the top or bottom of a column.

Post-earthquake investigations turn up the same set of departures again and again, and almost all of them are invisible once the finishes are on.

Saving on steel: fewer bars than the drawings show, wider tie spacing, plain bar instead of deformed. Saving on concrete: a low grade, too much water in the mix for easier placing, poorly compacted concrete full of voids. A category of its own is the aggregate: rounded river or sea sand carrying salt and silt gives poor bond and starts corroding the reinforcement from within.

Then come the alterations after handover: storeys added that were never in the calculation; partitions and parts of columns removed; heavy pools and soil placed on roofs; openings blocked up and new ones cut. Each looks like a detail on its own; together they move the building into a class of behaviour nobody ever designed it for.

A great deal is settled at sketch stage, for free. A symmetrical building with its stiff elements evenly distributed twists less than an asymmetrical one. A rectangle in plan behaves more predictably than a complex shape with re-entrant corners.

Regularity up the height matters as much as in plan. A storey noticeably softer than its neighbours collects all the deformation — the soft-storey mechanism, the commonest collapse scenario for apartment blocks. Abrupt setbacks, cantilevers and added storeys create the same problem elsewhere.

Finally, a gap is left between adjacent buildings. Blocks of different heights sway with different periods and, without a gap, hammer each other — pounding can damage floor slabs and columns at the level where they meet.

Partitions and infill are usually left out of the analysis — the frame is assumed to carry the load. In practice, stiff masonry inside a frame changes how the building behaves: it stiffens it, redistributes forces, and sometimes creates short columns where the masonry grips a column over part of its height.

Which leads somewhere unobvious: a refit that removes or adds "just partitions" can change the structure's behaviour more than the resident imagines. And a stack of identical flats in which every owner has rearranged the walls differently makes the building's behaviour unpredictable.

A large share of injuries comes not from the structure but from what is attached to it: façade cladding, parapets, ducts, suspended ceilings, glazing, external air-conditioning units, inadequately fixed stair flights.

Codes treat this as its own subject — the anchoring of non-structural elements. For a resident the practical meaning is simple: even in a sound building, it is worth looking at what hangs above the entrance, above the balcony, and above where you park.

Existing buildings are strengthened in several ways, all of which require a design. The commonest is adding reinforced concrete shear walls that take the horizontal forces and relieve the frame. Columns are jacketed — in steel, concrete or composite materials — which does from the outside what the ties do from within.

There are costlier technologies too. Base isolation, where the building sits on bearings that absorb part of the motion, is used for hospitals, important public buildings and monuments. Dampers dissipate energy the way shock absorbers do in a car. For an ordinary residential block these are usually uneconomic, but they are worth knowing about: if someone offers you "strengthening" without a design and calculations, it is not strengthening.

And an honest caveat: not every building is worth strengthening. Sometimes the sums show that demolition and rebuilding costs less and gives a more reliable result. That decision follows an assessment, not an appearance.

Only an assessment gives a real answer. But several things are open to anyone: whether the building has its permits and design documents; whether a seismic assessment has been done; whether there is a soft ground floor; whether storeys have been added or columns interfered with; whether there is a gap to the building next door.

That is enough to tell an obviously bad option from one worth showing to a specialist. Beyond that it is the engineer's call — and this is a case where the fee bears no comparison to the price of being wrong.