Abstract:
The current practice in the design of mat foundation is to provide it with constant thickness.
Analysis of mat foundation and study of the results reveal that mat foundation is extensively
over-designed in the current approach. Mat foundation of constant thickness is designed by
ACI method, Conventional method or computer analysis by Finite Difference method, Finite
Grid method or Finite Element method. This over-design is due to uncertainty in the analysis
methods and lack of understanding of mat behaviour. Use of thick shell finite elements of
Ahmad lead to realistic analysis and more rational design procedure for mat foundation with
variable thickness, as proposed by Morshed and Sutradhar. To improve upon this procedure,
an extensive investigation has been carried out here on the behaviour of mat foundation
using finite element program named ANSYS. The most significant parameters for mat
foundation with variable thickness are mat thickness and modulus of subgrade reaction. A
reshaping scheme proposed earlier has been improved.
The reshaping scheme of mat foundation with variable thickness is compared with flat slab
having drop panels. Column loads applied on the mat foundation generates a distributed
reactive soil pressure. A mat foundation can be considered as an inverted flat slab with nonuniform
pressure on it. For high-rise buildings upto fifteen story, design of mat foundation is
normally governed by vertical load that is dead load plus reduced live load. Usually
structures have regular column arrangement. An approximate method of analysis has been
proposed for mat foundation having column arrangements in a regular grid pattern. Most
residential and office buildings are of this type. For these types of structures, one can design
the mat foundation with variable thickness economically and speedily using hand
calculation. A simplified arrangement of reinforcement distribution has also been proposed.
Finally, a complete analysis of mat foundation with variable thickness for a three bay ten
storied building has been done by the proposed approximate method and Finite Element
method. Results of both these methods have been compared. An economic evaluation of the
mat foundation with variable thickness has been compared with mat of constant thickness.
An economy of 25% to 40% can be achieved by the proposed method for the mat
foundation.
