It is Not About Making the Building Safe from Being Blown Away by Wind

Different Forms of Lateral Load

Man and nature cause horizontal load on a building in many forms.

Moving air becomes pressure the moment it strikes the structure: F = PA (pressure on building façade).

Earthquake constantly disturb the building’s inertia: F = ma (acceleration of building mass).

Even a driver might unintentionally test a column’s strength with his car bumper, with the impact force that can be calculated based on design code (accidental load).

These loads may take different forms, but in the end engineers change them to arrows and point them towards the vertical elements like columns or walls.

When You Push a Table the Legs Always Wobble

You might ask: how can a simple arrow represent all these scenarios? Wouldn’t this overlook something and causes engineers to under-design the structure?

Let see how the lateral loads are transferred.

Take wind load as an example. Wind pressure hits the façade of building, the stuffs behind immediately react. What are they? Perimeter beams and columns. A beam subjected to lateral load is restrained by the floor slab. This slab-and-beam assembly is generally much stiffer in its own plane than the surrounding lateral system, allowing it to behave as a rigid diaphragm in many conventional buildings. Any lateral force acting on them is transferred almost entirely to the vertical elements, namely the columns and shear walls.

For earthquake scenario, inertial force is generated when the building mass is forced to accelerate. Within a building, the floors account for a significant proportion of its mass. In other words, large share of inertial forces is generated in these floors. These inertial forces are then again, distributed to vertical column through this rigid diaphragm.

Engineers do Care What People Think and Feel

A safe design ensures the structure is strong and will not fail under the lateral load. But a complete design would ensure the building drift is well controlled.

Imagine a building sway a little too much beyond the recommended limit.

From the inside, occupants would feel uncomfortable every time the building sways, and over time, walls would start to crack.

From the outside… Well, just ask yourself: how confident would you feel for a building with noticeable sway, despite the engineer telling you the structure is strong enough under worst scenario possible?

The story does not end there. Gravity doesn't stop acting just because the building is swaying. Now, the same gravity load that used to compress a straight column, pushes down on a column that has already drifted sideways. This is known as the P-Delta effect, which further increases instability.

That is why drift limits must be satisfied, and they are controlled primarily by the stiffness of structural elements.

Three Framing Choices for Lateral Load

A moment frame is your basic lateral load system. It consists of slabs, beams and columns. Moment frame is flexible and drift-sensitive. To reduce drift, engineers often introduce shear walls as a core to increase stiffness. Another option is a braced frame, which can be thought of as a moment frame strengthened with diagonal bracing.

Both core wall and braced frame systems reduce the drift, but their effectiveness depends on how they are arranged. For a core wall system, wall location strongly affects performance. On the other hand, the effectiveness of a braced frame depends greatly on the geometry of its bracing.

How does the location of the core wall affect system performance?

Most drift problems are not uniform but localised due to torsion. Torsion occurs when the centre of mass does not coincide with the centre of stiffness. Common causes include off-centre core walls and uneven lateral elements. Under the same loading conditions and the same quantity of core wall, a framing system that induces torsion will experience greater drift than a symmetrical configuration.

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Our Engineer's Cheat Sheet – Lateral Load Framing System provides a concise, practical reference that summarises the governing behaviour, incorrect assumptions, and key review points into a practical one-page engineering reference.