Resisting uplift in light structures

Where the upward force comes from

Wind suction on the roof

Air accelerates over the roof, which causes suction. On a low-pitched roof this is the dominant effect. Wind load and terrain category.

Suction on the façade

Outward pressure arises on the sides of the building and on the leeward face.

The overturning effect of the bracing

Uplift arises at the end of a shear wall from the horizontal load. Designing a shear wall.

Why it is a bigger problem in a light building

Because the counterweight is small. In a heavy structure the self-weight amply covers the suction; in a light one not necessarily.

On top of that, the standard takes favourable self-weight at a reduced value — in other words it must not be relied on in full.

The answer: a continuous tension load path

The uplift force has to travel right through the structure:

1. Fixing of the roof covering to the structure, with closer fixings at the edges.
2. Roof-to-wall connection, designed for tension.
3. Connection between walls and between storeys.
4. Anchorage to the foundation. Hold-down anchorage.
5. Tension capacity of the foundation — from mass or from the ground.

If any link in the chain is missing, the rest is worth nothing.

What the foundation has to provide

Mass

The self-weight of a concrete foundation is a counterweight.

Ground resistance

With a ground screw or a pile, the tension resistance of the ground. The principle of ground screws.

Or a combination of the two

What matters on site

The uplift components — hold-downs, fixings, closer screw spacing — are small, cheap parts that are easy to leave out, and whose absence never shows in normal weather.

So checking them is a fixed point of the pre-covering inspection. Inspection points.

The point

In a light structure the characteristic task is the upward force, not the downward one. The solution is simple and cheap — but it only works if it is continuous throughout.