Scaling the Mode Shapes of a Building Model by Mass Changes

It is well known, that when using natural input modal analysis, the loads are not known, and thus, the mode scaling factor that relates the magnitude of the loading to the magnitude of the response cannot be estimated. However It has been pointed out by several theoretical papers that mode shapes can be scaled by performing  several natural input modal analysis tests with different mass changes, observe the frequency shift introduced by the mass changes and then follow an estimation scheme that allows the user to estimate the scaling factor modeby- mode, i.e. only information of the particular mode of interest is used to obtain the scaling factor for that mode. The procedure is of high practical interest also in mechanical engineering since it is well known that the traditionally estimated scaling factor is often suffering from large uncertainties. In this paper it is shown how the mass change technique can be used on a ¼ scale model of a 4-storey building. The uncertainties on the estimated scaling factors are illustrated by repeating the estimation using different mass changes.