In structural engineering, the Flat Slab is identified as a
slab which is supported generally without beams by columns with or without
column heads. However, I have understood that Flat Slab designs are not quite
popular in Sri Lanka, may be owing to its complexity in design, detail and
construction. But things what I am going to discuss here are equally important in
any other column slab design problem such as pile cap etc.
Generally the slab can be analyzed by dividing it in to
longitudinal and transverse frames consisting of columns and strips of slab.
This is identified as Equivalent Frame method and frames are analyzed by moment
distribution. However these manual methods are things for the past. More
sophisticated and user friendly Finite Element Methods (FEM) and software
are widely used to solve the problem.
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| Plate mesh and the columns : STADD Pro |
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| 3D view of the Model |
However it can be seen in FEM that the support moments in slab column joint are sometimes unacceptable owing to its unrealistic high values. The anomaly is varying as accordance with span, column sizes and slab thicknesses.
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| Longitudinal Bending Moment Diagram |
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| Transverse Bending Moment Diagram |
However , most FE software offers 'peak smoothing' techniques to addresst this problems.
In General, the number of elements used in a FEM model can greatly affect the accuracy of the solution. As the number of elements, or the fineness of the mesh, is increased, the accuracy of the model increases as well. However in this particular Slab Column problem finer mesh would deteriorate the quality of the results more.
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| Deflection pattern : scale is adjusted |
The solution for this problem lies on the
realistic modeling of boundary conditions of the
slab.
In this regard, I usually adopt two methods;
1.) As the first method, Slab may modeled as
a plate/shell element while column may modeled as a
three dimensional solid object. By doing so, we get closer to
the real world scenario of the slab column joint. However analysis of
this model may be time consuming and required higher degree of computer
resources.
2.) In the second method, the flat plate
is modeled as a plane of finite elements, and columns are simplified
to “pin” or “fixed” supports applied at the elevation of the plane of the
slab. Sometimes these supports are modeled as springs with a finite
elastic stiffness to improve the behavior of the model.
In this method it is essential to connect nodes rigidly which lies within the area of
the column section to the end of the frame member. This technique is
otherwise referred to as a master-slave technique, where the end of the frame
member column is the master joint, and the joints in the slab within the column
area are slaves.
This technique offers an advantage over the use of three
dimensional solids with respect to computational efficiency and
Analyzing time of the model.
