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CivilComp Proceedings
ISSN 17593433 CCP: 81
PROCEEDINGS OF THE TENTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING Edited by: B.H.V. Topping
Paper 170
Computerized Analyses of Alternative Mechanical Models of Waffle Slabs using the Finite Element Method R.H. Dias+, J.B. Paiva* and J.S. Giongo*
+Paraná Catholic University (PUCPR), Curitiba, PR, Brazil
R.H. Dias, J.B. Paiva, J.S. Giongo, "Computerized Analyses of Alternative Mechanical Models of Waffle Slabs using the Finite Element Method", in B.H.V. Topping, (Editor), "Proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing", CivilComp Press, Stirlingshire, UK, Paper 170, 2005. doi:10.4203/ccp.81.170
Keywords: ribbed plates, building slabs, rib to plate eccentricity, finite element method, reinforced concrete.
Summary
Although exhaustedly discussed in other fields of engineering, the
eccentricity existing between the ribs axes and the average cover plane in
beamstiffened building slabs is always considered as simplified in analysis models or
completely mistakenly disregarded when computational tools for simulations using
more realistic models are available. Designers of civil structures generally do not
know much about the influence of the eccentricity on the behaviour of these systems
(displacement and strength responses of these structures).
The technical design codes allow the structural analysis of reinforced concrete ribbedslabs to be considered as elastic according to two main mechanical models, without mentioning the existence of the eccentricity:
The consideration of Tsections is a resource widely employed in the calculation of slab concrete beams monolithic to the slabs, and the best way to quantify the geometric properties of the sections for the application in the calculation models are discussed in the codes. A more exact result is obtained by the integration of the normal stress over the cover plate next to the coverrib bonding, once they decrease as they apart from the bonding. Thus, due to the complexity, the standards provide resources for the Tdimensions to be obtained in a simplified way. The plane grid model in the simulation of reinforced concrete slabs is based on the ACI435 (1989) [1], which demonstrates the process developed by Ewell at al. [2]. With regard to the application of the same process for the simulation of ribbedslabs, no comments were made, however, this process is accepted in practice by many researchers as feasible, once ribbedslabs present a behaviour closer to the grid than the reinforced slab itself. The Brazilian reinforced concrete code NBR6118:2004 [3] indicates it as a suitable model for the analysis. However, some authors such as Sheik & Mukhopadhyay [4], affirm that the modeling of stiffened plates by means of a grid has failed in the evolution of a satisfactory generic solution. The most realistic models that generally use software applying the Finite Element Model with a diversity of numerical resources such as the concept of master and slave displacements, optimum resource in the eccentricity simulation, are not mentioned in the technical standards. Designers of civil structures have easy access to these computational tools, however, many of them ignore the importance of considering the eccentricity between ribs and the ribbedcement slab building's pavements, being the publication of comparative results of numeric simulations and physical tests necessary. In the context of this discussion, this article presents results obtained by the computer modeling of waffle slabs, varying the ratio of the interrib gap to the length of the slab (a_{1}/l), by increasing or decreasing the number of ribs along each side of the slab, employing diverse mechanical models and various types of finite element, and taking into account or ignoring the eccentricity between the parts. The differences found between these cases and models are evaluated and the choices to be made when interpreting the results to work out the correct dimensions for structural elements are enumerated. References
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