Kathmandu NPTLogin | Register

Structural Engineering: How Buildings Stand Up

Every building you see is a quiet argument with gravity, and in Nepal, with earthquakes too. Structural engineering is the discipline of winning that argument: making sure loads travel safely from where they act down to the ground, through every slab, beam, column, wall and foundation, without anything breaking, bending too far, or collapsing. This pillar explains the core ideas. No software, no code clauses, just the behaviour you need to understand before any of that means anything.

How structures carry loads: the load path

Think of a building as a chain of handoffs. A person standing on a floor slab pushes down on the slab. The slab hands that push to the beams supporting it. The beams hand it to the columns. The columns hand it to the foundations. The foundations hand it to the soil. This chain is called the load path, and the single most important structural skill is the ability to trace it in any building, on paper or on site.

Loads come in three families. Dead loads are the permanent weight of the structure itself: slabs, beams, walls, finishes. They never leave, so the structure carries them every second of its life. Live loads are the temporary weights: people, furniture, stored materials, vehicles on a bridge. They come and go, so we design for the worst realistic case, not the everyday case. Lateral loads push sideways: wind and, in Nepal, earthquakes. Lateral loads are the ones that kill buildings here, so they deserve their own section below.

Two ideas govern whether the chain holds. Strength is whether members can carry the forces without breaking. Stiffness is whether they do it without moving too much. A beam can be strong enough yet too flexible, and the result is cracked finishes, jammed doors, and a building that feels unsafe even though it will not fall. Good design satisfies both.

RCC, steel, and masonry: three materials, three behaviours

Most buildings in Nepal use one of three structural systems, and each behaves differently.

Reinforced cement concrete (RCC)

Concrete is strong in compression and weak in tension, roughly ten times weaker. Steel is strong in both. RCC marries them: concrete carries the compression, steel bars (rebar) carry the tension. In a beam, the bottom stretches when loaded, so the main bars sit at the bottom. At the supports, the top stretches, so bars run over the top there too. This is why bar placement, not just bar quantity, decides whether an RCC member works. Concrete also needs steel stirrups (rings) to resist shear and to hold the main bars in place, and the concrete cover over the bars protects the steel from fire and corrosion.

RCC frames, columns and beams forming a skeleton with infill walls, dominate urban Nepal because they allow open plans and multiple storeys. Their weakness is that they demand quality: proper mix, proper compaction, proper curing, and correct bar bending and placement. An RCC frame with badly placed steel is weaker than a well-built masonry wall.

Structural steel

Steel members are light, strong, and fast to erect, which is why warehouses, industrial sheds and some commercial buildings in the Terai use them. Steel’s behaviour is predictable and it gives warning before failure by deforming. Its enemies are buckling of slender compression members, fire (steel loses strength rapidly when hot), and corrosion. Connections, bolts or welds, are where steel structures usually fail, not in the members themselves.

Masonry

Load-bearing masonry, brick or stone walls carrying the floors, built most of Nepal’s traditional housing and still builds a great deal of rural construction. Masonry is excellent in compression and terrible in tension, and it is heavy, which means large earthquake forces. Its seismic survival depends on rules of thumb that generations of builders learned the hard way: limit wall lengths between cross walls, keep openings small and away from corners, and tie everything together with horizontal bands. Our building code’s mandatory rules of thumb for masonry exist precisely because these simple measures save lives.

Seismic behaviour basics for Nepal

Nepal sits on the collision of the Indian and Eurasian plates. The 2015 Gorkha earthquake killed about 9,000 people and destroyed hundreds of thousands of buildings. Every Nepali engineer must understand, at least qualitatively, why buildings fell and what keeps them standing.

An earthquake does not push a building over like a hand. It shakes the ground, and the building’s own mass resists the shaking. That resistance is inertia, and it creates forces inside the structure proportional to the building’s weight. Heavy buildings attract larger earthquake forces, which is one reason heavy roofs and water tanks on rooftops are dangerous.

Three behaviours decide survival. Ductility is the ability to bend and deform without breaking, like a green twig versus a dry one. Ductile buildings absorb earthquake energy by controlled damage; brittle buildings snap. Regularity matters: symmetric buildings with evenly distributed stiffness twist less. Soft storeys, an open ground floor with stiff walls above, concentrate all the deformation in one floor, and that floor collapses. This is exactly what happened to countless commercial buildings in 2015. Short columns, where a partial-height wall forces a column to bend over a short length, attract huge shear forces and fail explosively.

Ductile detailing is how we buy ductility in RCC: closely spaced stirrups near beam and column ends where damage concentrates, 135-degree hooks on stirrups so they do not open up, proper lap lengths so bars do not pull apart, and the strong-column weak-beam principle so that beams yield before columns. Columns carry the whole building; if a column fails, everything above comes down. We design so that damage, when it must happen, happens in beams first.

Common structural failures and what causes them

  • Soft-storey collapse: open ground floor for shops or parking with masonry infill above. Cause: sudden stiffness drop concentrating drift in one floor. Prevention: shear walls or braced frames in the open floor, or avoid the configuration.
  • Short-column shear failure: windows or partial walls restraining part of a column. Cause: the free height shortens, shear demand multiplies. Prevention: isolate infill from columns or detail the full column for the higher shear.
  • Pounding between adjacent buildings: buildings built touching each other vibrate out of phase and hammer each other. Cause: no separation gap. Prevention: a seismic gap, typically a few centimetres per storey.
  • Foundation failure on soft or liquefied soil: tilting and sinking, seen in parts of the Kathmandu Valley in 2015. Cause: building heavy structures on weak saturated soils without proper foundation design. Prevention: soil investigation and appropriate foundations.
  • Corrosion-driven deterioration: rusting bars expand, crack the concrete, and lose cross-section. Cause: inadequate cover, porous concrete, water ingress. Prevention: proper cover, good compaction and curing, waterproofing where needed.
  • Overloading beyond design: extra floors added later without checking the frame. Cause: treating the structure as infinitely extendable. Prevention: any vertical extension needs a structural check first.

Reading structural drawings

Drawings are the contract between the designer’s intent and the site’s reality. A structural drawing set typically contains: a general arrangement or framing plan showing the column grid and beam layout; foundation plans with footing sizes and depths; column schedules listing each column’s size and reinforcement floor by floor; beam and slab details showing bar diameters, numbers, spacing and curtailment; sections and details for stairs, joints and special elements; and a bar bending schedule (BBS) listing every bar with its shape, length and quantity.

Read them in this order. First the grid and overall framing, to fix the load path in your mind. Then foundations, then columns going up, then beams and slabs at each level. Check dimensions against architectural drawings, because a beam shown 230 mm wide in the structural set and 200 mm in the architectural set means someone has to decide before concrete is poured, not after.

On site, the drawing is checked, not trusted blindly. Verify bar diameters with a gauge, count bars, check spacing with a tape, confirm cover with cover blocks, and confirm that laps are staggered and at the right locations. Concrete cover is the cheapest strength a building can buy and the most commonly skipped. When in doubt, ask the designer; a phone call is cheaper than a retrofit.

Related on Eve Construction

माथि जानुहोस्