Configuration & Regularity

Almost all of good seismic configuration collapses into three words: simple, symmetric, regular. A simple plan is compact and close to a square or a short rectangle - a single, clean shape with no wings, notches or sprawling arms. Force entering such a building has short, direct routes to every part of the structure and down to the ground; there are no awkward junctions where it must turn a corner and pile up. A symmetric building arranges its mass and its stiffness evenly about both axes, so that the centre of mass (where the inertia force acts) sits close to the centre of rigidity (where the building resists). When those two points coincide, the building sways straight instead of twisting - it removes torsion at the source, which is the cheapest possible seismic improvement. A regular building repeats itself sensibly up its height - similar floor plans, columns that run continuously from roof to foundation, stiffness and mass that change gradually rather than in sudden jumps - so the earthquake finds no weak level to concentrate its damage.

These qualities matter because they govern how force flows. In a regular, symmetric building the lateral force is shared by many elements spread around the plan and stacked continuously down the height; each carries a modest portion, and the drift is spread evenly from floor to floor. Break the regularity and the force does not disappear - it funnels into whatever few elements remain in its path, overloading them while others sit idle. Irregularity, in other words, is not just untidy; it is a mechanism for concentrating demand exactly where the structure is least able to take it.

This is why codes worldwide, including IS 1893, formally distinguish regular from irregular buildings, demand more rigorous analysis for the irregular ones, and in the worst cases restrict them in high seismic zones. But the deepest reason to prefer simple, symmetric and regular is not to satisfy a code - it is that such buildings behave well almost automatically, forgive the inevitable imperfections of real construction, and let your engineer verify safety rather than chase it. Reach for regularity first; justify every departure from it.

If one configuration has killed more people in modern earthquakes than any other, it is the soft storey - and it is almost always created by architecture, not engineering. A soft storey is a level that is far more flexible than the ones above it, most commonly an open ground floor: slender columns alone carrying stiff, wall-filled storeys above, created for parking, shops, a lobby or pilotis. When the earthquake shakes such a building, the stiff upper block moves almost as one rigid mass, and nearly all the relative movement - the drift - is forced into the single soft level. The ground-storey columns are asked to absorb the entire building's sway; they hinge top and bottom, lose their ability to carry the weight above, and the upper block drops straight down. A closely related danger is the weak storey, where one level is not necessarily more flexible but much less strong, so it fails first.

The tragedy is that the soft storey is both extremely common and extremely avoidable. It appears wherever a design puts solid masonry or shear walls on the upper floors but leaves the ground floor open for cars or commerce - an everyday pattern in Indian cities, where the open ground storey for parking is almost a default. The physics is unforgiving: discontinue the stiff elements at one level and you have built a collapse mechanism, however good the rest of the building.

The cures are all conceptual and belong to the architect. Best of all, avoid the discontinuity - carry walls or bracing down through the ground storey, or plan the parking and retail so some stiff elements continue to the foundation. Where an open ground floor is truly unavoidable, it must be explicitly designed for: the ground storey stiffened and strengthened to compensate, often with additional shear walls or braced bays and stronger, well-confined columns, so that drift is not dumped into it. That is engineered work to IS 1893 and IS 13920, triggered by a decision you make at concept. Recognise the soft storey in a sketch and you can design it out before it ever reaches the engineer - which is the whole point of knowing it by name.

Beyond the soft storey, a family of plan and elevation irregularities recur often enough to name. Re-entrant corners are the inside corners of L, T, U, H, plus and cross-shaped plans - any plan with a notch cut into it. The wings of such a building want to vibrate differently and pull against one another, and the stresses concentrate viciously at the inside corner where they meet, cracking and tearing precisely there. The longer and more slender the wings, the worse it is. Large setbacks - where an upper part of the building steps back sharply from the lower, as in a wedding-cake tower or a podium-and-tower scheme - create a sudden jump in stiffness and mass up the height, a vertical irregularity that concentrates drift and force at the level of the change. Torsional irregularity arises whenever stiffness is lopsided in plan - heavy bracing or a core on one side only - so the building twists, as we saw in the previous lesson.

The unifying idea is that sudden changes are the enemy - abrupt changes in plan shape, in stiffness, in mass or in strength, whether across the plan or up the height. The earthquake concentrates its demand wherever the building changes character suddenly, and those concentration points become the failure points. Smoothness and gradual transition are seismic virtues; abruptness is a seismic vice.

The remedies are, once again, early design moves. Keep plans compact and convex; if a building must be L or U-shaped, either round or strengthen the re-entrant corner, or - better - split the plan into separate simple blocks with a seismic separation joint between them (the next section). Step setbacks gradually rather than in one dramatic jump, and keep stiffness changing smoothly up the height. Where an irregularity is genuinely wanted for the architecture, it is not forbidden - but it must be declared to the engineer early and designed for explicitly, with the more rigorous analysis the code reserves for irregular buildings. The irregularities themselves, and the thresholds that define them, are set out in IS 1893; the architect's task is to see them coming.

Sometimes the programme genuinely demands a complex form - a hospital with clinical wings, a school wrapping a courtyard, a mixed-use block with a tower on a podium. The most powerful tool for taming such forms is the seismic separation joint: a deliberate gap that slices an awkward building into two or more simple, regular, independent blocks, each free to sway on its own without colliding with its neighbour. An L-shaped plan becomes two clean rectangles; a podium-and-tower becomes a tower that moves independently of its podium. The joint must be wide enough that the blocks do not pound against each other when they sway out of phase - a width the engineer sets from the expected drift, per IS 1893 - and it must be carried honestly through the structure, with the architecture detailed to accommodate the movement at floors, facades and finishes. Done well, separation turns one ill-behaved building into several well-behaved ones.

Stand back, though, and the real lesson of this whole lesson is about when resilience is decided. Configuration is chosen at the earliest, most fluid, least expensive stage of design - the concept sketch - and yet it sets the ceiling on how safe and how economical the finished building can be. A brilliant structural engineer can realise the safety latent in a good configuration, refining and verifying it; no engineer can fully redeem a bad one, only reinforce heroically around its flaws at great cost, and never quite to the same level. The configuration you draw is, in effect, a budget for safety that everyone downstream must live within.

That is why seismic configuration is the architect's responsibility in a way that detailed design is not. It asks not for calculation but for judgement and discipline - to prefer the simple, symmetric and regular; to recognise the soft storey, the re-entrant corner and the harsh setback on sight; to reach for separation joints rather than tortured single forms; and to bring the structural engineer into the concept conversation, not present them with a finished shape. Own the configuration, defer every number to IS 1893 and the engineer, and you will have done the most consequential seismic work of the whole project before the first load is ever calculated.