Computerized Analyses of Alternative Mechanical Models of Waffle Slabs using the Finite Element Method

摘要

In this paper several alternative of numerical models of waffle slabs, currently a popular system for the construction of floors in large buildings, are assessed in the analysis, by the finite-element method (FEM), of vertical displacements, bending moments and tensile stress in the ribs. These tests are performed over a range of ratios of the inter-rib gap to the length of side of the slab (a1/l), as in a number of cited studies. The set of models elaborated here is designed to point up the effects of the offset of the axes of the ribs from the median plane of the plate. This eccentricity of the stiffeners is supremely important in the analysis of the behavior of plates stiffened with beams and is always taken into account in naval and aeronautic engineering problems, yet it is totally ignored in most civil structure analyses by civil engineers, or else approximated in a highly simplistic manner. In the current study we use the computer Finite Element Method (FEM) package ANSYS 5.5, to simulate the slabs according to each model; the models incorporate the eccentricity in several ways: by discretizing the ribs with a 3D beam finite element that accepts fixed offsets between its centroid and the node that locates it on the median plane of the plate; by 3D modeling, in which both plate and ribs are discretized into shell finite elements; by using a simplified model based on the Theory of Equivalent Orthotropic Plate, in which the waffle slab is represented by a solid plate of equivalent stiffness to bending, and finally, by adopting a simplified grid model, where each rib and a piece of attached plate form an equivalent T-section, placed concentrically across the median plane. In addition, the slab is represented in one model without allowing for eccentricity, to observe how this affects the result. The elastic behavior adopted throughout all the analysis. The design of the beams and plates are then estimated in accordance with each model and the decisions a designer has to take are enumerated.