Keywords: dynamic, vibrations, steel structures, composite floors, human comfort, dynamic structural design, rhythmic dynamical loads, human rhythmic activities.

Structural designers have long been trying to develop minimum cost solutions, as well as to increase the structure erection speed. This procedure has produced slender structural solutions, modifying the ultimate and serviceability limit states that govern their structural behaviour. A direct consequence of this new design trend is a considerable increase in the problems related to unwanted floor vibrations. This phenomenon is becoming very frequent in a wide range of structures subjected to rhythmic dynamical load actions. These load actions are generally caused by human rhythmic activities such as: sporting events, dance or even gymnastics [1,2,3,4].

Proper consideration of all the aspects earlier mentioned calls for an investigation of the structural behaviour of composite floors subjected to human rhythmic activities. The main objective of this paper is to incorporate the orthotropic solution for the concrete slabs subjected to rhythmic dynamical actions. This investigation is focused on the possibility of occurrence of unwanted vibrations that could cause human discomfort or, in extreme cases, structural failure.

When a composite floor system incorporates a steel deck slab, the hypothesis of slab isotropic behaviour can be considered questionable. One of the most commonly used solutions to better represent the composite floor is to consider an orthotropic system. The major direction is parallel to the span of the slab deck ribs.

A usual design assumption considers the major direction inertia as the addition of the part related to the concrete slab above the steel deck ribs plus an extra term that incorporates an effective width based on the ratio of concrete area present in the ribs over the overall area (ribs + voids). The minor direction only considers the first part i.e. the concrete slab above the concrete slab ribs. This simple hypothesis can be easily incorporated in any design model and the results strongly depend on the steel deck geometry. Preliminary results from an elastic linear isotropic analysis were described in Silva et al [4]. An example associated with a composite floor designed for gymnastic activities was considered. The level of dynamical effects associated to the vertical displacements can reach up to sixty times the static values, related to a resonance condition, Figure 38.1. These vertical displacements could induce excessive vibrations, compromising human comfort conditions and even the structural system.

This investigation continued with a parametric study using the orthotropic solution for the concrete slab. This investigation focused the use of different steel deck geometries and their influence in the dynamical response of the structural system. The investigated structural system response, obtained from finite element method isotropic and orthotropic simulations, was compared to current experimental evidence and the limiting values proposed by several authors [1,2,3].

1

Ellis, B.R., Ji, T., "Floor Vibration Induced by Dance-Type Loads: Theory and Verification", The Structural Engineer, Vol. 72, 1994.

2

Bachmann, H., Ammann, W.; "Vibrations in Structures Induced by Man and Machines", Structural Engineering Document 3e, International Association for Bridges and Structural Engineering, 1987.

3

Murray, T.M., Howard, J.N., "Serviceability: Lively Floors - North American and British Design Methods", Journal of Constructional Steel Research, 46 (1-3), Paper N0 251, 1998. doi:10.1016/S0143-974X(98)00155-2

4

Silva, J.G.S. da, Soeiro, F.J. da C.P., Vellasco, P.C.G. da S., Andrade, S.AL de; Werneck, R.N, "Dynamical Analysis of Composite Steel Decks Floors Subjected to Rhythmic Load Actions", The Eighth International Conference on Civil and Structural Engineering Computing, CIVIL COMP-2001, Eisenstadt, nr Vienna, Austria, Civil-Comp Press, pp. 1-16, 2001. doi:10.4203/ccp.73.34

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