Shear Walls And Lateral Stability In Apartments

A building has to carry two completely different kinds of load.

Gravity, which pushes down and is resisted by slabs, beams and columns. And lateral load, which pushes sideways and is resisted by something else entirely.

That something else is usually a set of concrete shear walls, and in an apartment building they are the elements that most constrain the plan.

Where lateral load comes from

Wind, which is the governing lateral load for most buildings in Sydney. It increases sharply with height and it is covered in [wind loading on buildings](/blog/wind-loading-on-buildings-explained).

Earthquake, which in Australian conditions is less severe than in many regions and still has to be designed for. It acts as a force proportional to the mass of the building, which means a heavy building attracts more earthquake load than a light one.

Out of plumb effects, where the building's own weight acting on a slightly deflected structure adds to the sway. Small on a low building, significant on a tall slender one.

How the load travels

Wind pushes on the facade. The facade transfers it to the floor slabs. Each slab acts as a horizontal plate, a diaphragm, and carries that force sideways to the shear walls. The shear walls carry it down to the foundations.

Every link in that chain has to work, and the one most often overlooked is the diaphragm. A floor with a large opening, a long narrow wing, or a change in level can fail to distribute load to the walls even where the walls themselves are adequate.

That chain is the lateral equivalent of the [load paths](/blog/understanding-load-paths-multi-storey) that carry gravity.

What forms the system takes

The lift and stair core. The most efficient option, because the walls around a lift shaft form a closed box that is very stiff in both directions and the space is needed anyway.

Individual shear walls, typically on party walls between apartments and around service risers, positioned to suit the plan.

A moment frame, where columns and beams are rigidly connected to resist sway without walls. Far less stiff than walls for the same material, so it is used where walls cannot be accommodated, and it results in larger columns and deeper beams.

A combination, which is what most mid-rise apartment buildings actually use.

Why position matters more than quantity

This is the part that surprises architects.

The walls have to be arranged so the building resists both twisting and swaying. If all the stiffness sits at one end of a plan, wind pushing on the building makes it rotate as well as translate, and that torsion adds substantial load to the walls furthest from the centre of stiffness.

Which means a plan with a core at one end and glass at the other is a harder engineering problem than a plan with walls distributed, even if the total wall length is the same.

An L-shaped or T-shaped plan brings the same issue, and it is one of the recurring difficulties in [mixed use developments](/blog/mixed-use-developments-engineering-challenges) where the podium and the tower want different arrangements.

The continuity requirement

A shear wall has to run continuously from the roof to the foundation.

Stopping a wall at a podium level and picking the load up somewhere else means a [transfer structure](/blog/transfer-slabs-and-transfer-beams), and transferring lateral load is considerably more demanding than transferring gravity load. It can be done and it is expensive.

Which is why the ground floor is the point of tension on almost every apartment project. Retail and car parking want the ground floor open, and the shear walls want to continue to the footing.

What it means for the design programme

Lateral stability is decided early or it is decided expensively.

Moving a shear wall after the structural design is advanced changes the whole system, because the remaining walls have to take up the load and the torsion changes. Adding a wall late is easier than removing one, and neither is cheap.

The productive sequence is to identify the lateral system while the plan is still moving, which is the clearest case for [early engineering involvement](/blog/why-early-engineering-involvement-saves-money).

The foundations underneath

Shear walls deliver large concentrated forces to the ground, including uplift at one end of a wall when the building sways.

Uplift is the condition that frequently governs the foundation. A pad footing can carry compression easily and resists uplift only through its own weight, so a shear wall on a tall building usually needs piles designed for tension. That is a [foundation design](/blog/foundation-design-raft-vs-piled-vs-strip) decision driven entirely by the lateral system.

ACSES designs structural systems for apartment and mixed use developments across Sydney. Structural engineering or talk to us about a project.