In-Situ Vibration Tests of Timber Frame Structures with Knee Braces

Although the natural period of vibration is a key parameter in wind and seismic engineering, it can be challenging to determine the period of vibration for traditional timber frames because there is a lack of guidance in design standards compared to other structural systems, and because it can be difficult to employ conventional measuring techniques. In this paper, a non-destructive in-situ vibration test method is presented to better understand the natural period of vibration of traditional timber frames with knee braces. A low-cost measuring technique was developed by installing an app on a smartphone to measure accelerations, placing the smartphone on the spandrel beams of the timber frame in the field, and then vibrating the frame in the principal directions using a hammer or mallet. The single-sided amplitude Fourier spectrum of the data was used to determine the period of vibration. Twelve free-standing traditional timber frames with knee braces were tested at various sites in the United States. The results indicate that the in-situ period of vibration was lower compared to an empirical estimate of the period of vibration based on ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures that is commonly used in the United States for other structural systems. The findings also suggest that the test method developed in this study may be an efficient approach for structural analysis of smaller-sized historical timber frame structures.