# Why cannot static forces on a small scale model be measured in wind tunnel if the model is not rigid?

I'm studying the theory of wind tunnel tests in context of structural engineering. Often we want to predict the wind loads on a real building by placing a small scale model of the building into a wind tunnel. The wind loads can then be measured on the model, and mathematically scaled for the real building.

Common way of measuring the wind load on the building foundation is by placing the model on a force balance. It is a stiff plate-like instrument upon which the model is laid, which measures the moment applied to it by the wind, through the model.

All texts about the theory of wind tunnel testing mention that the model has to be light and very rigid. Here is a quote from one:

If the model is not rigid enough and responds dynamically to the wind force, the interaction between the dynamic response and the aerodynamic force would contaminate the base force measurements.

I don't fully understand this sentence, and it is not elaborated more in any source I can find. What is meant by the dynamic response contaminating the base force measurements? I mean, we are trying to measure the base forces. Aren't the dynamic responses of the building part of the base force? If the building, for example, vibrates, shouldn't those forces be considered anyway, so why not include them in the measurement? Or if we really want to just measure the static force on the base, is there no way to mathematically filter the measurement to obtain the static force? If, say, we assume the model vibrates sinusoidally in the wind, the inertial forces from the vibration are measured by the balance. But cannot we then simply take the average of the sinusoid to get the constant part of the force, that is, the static force on the model?