Improved design approach for mat foundation with variable thickness

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.