Experimental modal analysis of a multi-storey light-frame timber building

Abstract

A 3-storey residential building with OSB-sheathed light-frame timber walls and timber-concrete composite slabs was subjected to dynamic excitation in two different stages of construction. The experiments consisted of measurements of the accelerations resulting from forced horizontal vibrations which were excited by means of a hydraulic actuator shaking a mass of 940 kg. The exciter was positioned on the second floor of the building and rigidly anchored in the slab. It excited the building in its two main directions to vibrations with frequencies within a range of 0.2–14 Hz. Natural frequencies, modal damping ratios and mode shapes of the building were estimated using accelerations of the building measured in the four external corners of each upper floor. For increasing displacement amplitude (1/8000 mm → 0.7 mm) the natural frequencies decreased moderately (−15 %) whereas the modal damping ratios increased markedly (+100 %). Modal damping ratios computed using ambient vibration data were generally smaller than modal damping ratios obtained from forced vibration tests, which suggests that modal damping for the levels of vibration in interest in design might be significantly underestimated if based on results from ambient vibration tests. However, the mode shapes computed using ambient vibration data turned out to be reliable. The differences in dynamic properties assessed in the different stages of construction were much smaller than expected. Compared to the natural frequencies estimated by the designer by applying a simple single degree of freedom model, the measured natural frequencies were considerably higher. This may be attributed to the impact of non-structural internal walls and walls with openings not accounted for in the designer’s model.