Wind Tunnel Testing And When A Building Needs It

The Australian wind loading standard gives a method for calculating wind actions on buildings. It works well, and it is built on assumptions.

It assumes a reasonably regular shape, a reasonably typical surrounding, and behaviour that does not depend on the building moving.

Once a project falls outside those assumptions, the code answer becomes conservative, unreliable, or both.

When testing earns its place

Tall and slender buildings. As height and slenderness increase, dynamic response starts to matter. The building moves in the wind, and that movement changes the loading. The code handles this approximately. A tunnel handles it properly.

Unusual shapes. Curves, tapers, setbacks, large openings and anything sculptural. The code's pressure coefficients come from tested regular shapes, and they do not extend to a form nobody has tested.

Complex surroundings. A tower in a group of towers experiences accelerated flow, shielding, and interference effects from its neighbours. In a dense CBD this can be the dominant factor, and the code cannot represent it.

Occupant comfort. People notice building motion long before it is structurally significant. Acceleration criteria at the upper levels are frequently what governs the design of a residential tower, not strength. Assessing it needs a dynamic model.

Pedestrian wind conditions. Councils regularly require a wind environment assessment for plazas, entries and footpaths around a tall building. Downwash off a tower face can make a street unusable, and mitigation is far cheaper designed in than added later.

Facade pressures. Local peak pressures at corners, parapets and setbacks drive the facade design, and the facade is a large proportion of the cost. Code values at these locations are deliberately conservative.

What testing gives back

The usual outcome is a better answer rather than only a safer one.

Overall base shear and overturning frequently come down, because the code's conservatism for a specific shape in a specific setting is removed.

Facade pressures get resolved zone by zone, which means the glass and the fixings are specified for what they actually see rather than for a blanket worst case. On a large tower that is a significant saving.

Acceleration is quantified against comfort criteria, which either confirms the structure is adequate or tells you early that damping is needed. That informs whether a [mass damper](/blog/vortex-shedding-mass-dampers-tall-buildings) is required, and it is the kind of decision that has to be made while the structure is still being sized.

The methods

A rigid pressure model, instrumented with pressure taps across the surface, for facade loads and overall forces.

A high frequency force balance model, measuring base forces to derive the dynamic response, which is the common approach for overall structural loading.

An aeroelastic model, which physically reproduces the building's stiffness and mass so it responds like the real thing. Used on the most sensitive projects.

Pedestrian level testing, measuring wind speeds at ground level around the model.

Computational modelling is used alongside physical testing, particularly for pedestrian level work, and for the primary structural loading on a significant tower a physical test remains the accepted standard.

The timing problem

Testing needs a model, the model needs a form, and the results change the structure.

Too early and the building shape is still moving, so the test is wasted. Too late and the results arrive after the structure has been committed.

The workable point is once the massing and the facade geometry are settled but before the structural design is locked. Surrounding buildings, including approved but unbuilt ones, are modelled as well, which means the planning context has to be understood first.

When it is not needed

Most buildings. Low and mid rise buildings of regular shape in ordinary surroundings are exactly what the code was written for, and [wind loading on buildings](/blog/wind-loading-on-buildings-explained) covers how that calculation works.

The test is for the projects where the code either does not apply or costs more than the testing does.

ACSES provides structural engineering for multi-storey and high rise development across Sydney. Talk to us about a project.