Keywords: dynamic, vibrations, composite floors, computational modelling, steel structures, dynamic structural design, human rhythmic activities.

When a composite floor system incorporates a steel deck slab, the slab isotropy assumption may be easily questionable. One of the most commonly used solutions to better represent the composite floor is to consider an orthotropic system where the major direction is parallel to the slab deck ribs axis. Therefore, the main objective of this paper is to incorporate the orthotropic solution for the concrete slabs subject 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 [1,2,3,4].

A usual design assumption considers the major direction inertia as an addition of the parts 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 and the overall area (ribs plus voids). The minor direction includes only 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 [4].

This investigation focused on the computational modelling of the dynamical behaviour of current profiled composite floors. The composite floor studied in this work, spanning 14.0m and 43.7m wide, is used for gymnastics. The structural system is made of standard composite girders and the 150mm thick composite slab incorporates a 0.80mm thickness and 75mm flute height steel deck [3,4].

Numerical simulations assessed the composite floor natural frequencies and vibration modes. The investigated structural system dynamical response, obtained from finite element isotropic and orthotropic simulations, was compared to experimental results [1]. The current investigation has also considered three fundamental aspects of the dynamical composite floor response i.e. boundary condition effects, steel sheet contribution and composite floor neutral axes location on the global structural system stiffness.

Comparisons of the results between the three sets of different boundary conditions were evaluated. This was followed by an assessment of the steel sheet contribution to the composite floor global stiffness. Despite the fact that the steel sheet is thin and has small area compared to the concrete slab, the steel sheet Young's modulus is higher than the concrete and the location of the steel sheet is relatively far from the composite section neutral axis. This motivated the development of a three-dimensional composite floor model with and without the steel sheet to evaluate its true influence over the structural system response.

The composite floor neutral axes location is of major importance for evaluating the structural system global stiffness. Therefore different locations for this neutral axis were simulated to examine their effects on the composite floor dynamic behaviour. A subsequent parametric analysis was also performed to further verify the computational models accuracy. These models results were also compared to calculated and measured natural frequencies of a full-scale composite floor.

1

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

2

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

3

Silva, J.G.S. da, Soeiro, F.J. da C.P., Vellasco, P.C.G. da S., Andrade, S.A.L. de, Werneck, R.N., "Dynamical Analysis of Composite Steel Decks Floors Subjected to Rhythmic Load Actions", in Topping, B.H.V. (Editor), Proceedings of the Eighth International Conference on Civil and Structural Engineering Computing, Civil Comp Press, Stirling, UK, paper 34, 2001. doi:10.4203/ccp.73.34

4

Silva, J.G.S. da, Vellasco, P.C.G. da S., Andrade, S.A.L. de, "Dynamical Response of Steel Deck Composite Slabs with Geometric Orthotrophy Subjected to Human Rhythmic Activities", in Topping, B.H.V. and Bittnar, Z. (Editors), Proceedings of the Sixth International Conference on Computational Structures Technology, Civil Comp Press, Stirling, UK, paper 38, 2002. doi:10.4203/ccp.75.38

5

Vecci, M.A.M, Fakury R.H, Mendonça, P.V.P., "Análise de Vibrações de Pisos Submetidos a Excitações Rítmicas. Aplicação de Critérios para Conforto em Edificações", Encontro Nacional de Conforto no Ambiente Construído. III Encontro Latino-Amer. de Conforto no Ambiente Construído. Brasil, pp. 1-6, 1999.

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