Experimental and analytical study of vibration parameters in waffle concrete slabs

Abstract

Investigating the dynamic characteristics of slabs has become very important since modern structures, meeting economic needs and architectural tendencies, have been built lighter with longer open spans, and consequently became more susceptible to the phenomena of vibration. Therefore, addressing important parameters of vibration during the design phase of a building, such as natural frequencies and damping, can avoid vibrations problems in the future. This paper presents a research study that was undertaken to investigate the vibration parameters in waffle concrete slabs. Specifically, in situ experimental tests were carried out on twenty-six floor slabs using the heel-drop as impact source, accelerometer and mobile phone to register the natural frequencies and subsequently modal damping ratios. The natural frequencies were also obtained through the analytical calculations utilizing the numerical equations for isotropic and orthotropic plates, available in the literature. The results for fundamental frequencies show that mobile phones are effective for recording acceleration in floor slabs. Also, the two analytical methods studied in this research offered results that found to be in good agreement with the experimental ones; however, using the equations for orthotropic plates represented a more conservative alternative. As expected, natural frequencies of floor slabs tend to decrease with the increase of the span; however, the shorter span dimension seems to cause greater influence on the frequencies, especially in two-way slabs. For such two-way slabs, an exponential function can define the relationship between frequencies and the shorter open span dimension. Also, an exponential function that defines the relationship between frequencies and slab’s b/h ratio has been developed. As one of the outcomes of the study presented in this paper, it was found that using a loss factor of at least 3.4% for waffle slabs is a realistic assumption since this factor is influenced by the existence of non-structural components on the slab, e.g., partitioning walls.