Comparative Analysis of Annular Raft and Solid Raft

Abstract

The goal of the study is annular mat foundation in various modern structures: Circular tower-shaped structures, like, microwave towers,TV tower, cooling towers, overhead water tanks, etc For this purpose, three raft cases are considered, which are mentioned below. (1) Intze overhead tank Annular raft with net SBC 24Kn/m2 (2)Intze overhead tank Circular Solid raft This study aims to evaluate these foundation systems\' structural behavior and performance aft\'s outer radius is also taken as a variable for obtaining different results in our thesis work. An F.E.M.-based analysis is done to determine critical moments and deflections in both annular and circular rafts. Comparison of this base pressure critical moments, shear force, bending stress, and deflections are made based on the above-mentioned case, including the cost comparison foundation. The concrete grade is M-30, and HYSD FE 500 steel reinforcement is used in design calculation. The load combination is followed as IS 1893-2016 and IS 456-2000—seismic zone 3. The software package staad pro v8i and connect edition is used. To provide relevant calculations, Microsoft Excel is used.

Introduction

I. INTRODUCTION

The foundation is a mediator to effectively transfer the load from the superstructure to the soil, without any failure for both the structure and the soil. Annular Mat foundation is one of the effective types of shallow foundations, which carries the load to the soil without any differential settlement in the soil. Annular foundation may be used where the base soil has a low bearing capacity and/or the column loads are so large that more than 50 percent of the area is covered by conventional spread footings.

In the present era, factors like rapid urbanization and massive developments taking place at one place or the other have also given rise to instances of failure of the structures, which in turn, sometimes lead to colossal destruction and loss of lives. For this reason, the study of the bearing capacity of soil has attained paramount significance. On one hand, overestimation of the bearing capacity results in structures potentially prone to disastrous collapse, while its underestimation can make the foundation uneconomic.

There are various theories for the design of the Annular foundation. Those vary from conventional manual calculation methods to most modern computer-based methods. The finite element method is one of the effective and economical numerical methods for analyzing these foundations. For the Annular Mat foundation, advanced numerical modeling techniques are utilized by dividing the MAT into grid elements and predicting the behavior of the structure under loading for critical elements projects.

II. TYPE FOUNDATION

Following are the different types of Foundation.

1. Shallow Foundation                                         2.            Deep Foundations

• Isolated footing                                                        Pile Foundation                                                       
• Combined Footing
• Strip Foundation
• Raft Foundation

III. ANNULAR AND SOLID RAFT

The raft foundation is generally recommended in underground structure, or when the footing area exceeds 50% of the building than raft foundation is provided.

The Annular or raft foundation also supports the control the differential settlement.

Annular raft foundation in various modern structures: Circular tower-shaped structures, like TV towers, microwave towers, chimneys, cooling towers, overhead water tanks, etc.

IV. NECESSARY INFORMATION REQUIRED FOR DESIGN A FOUNDATION

For satisfactory design and construction of the annular foundation, the following information a neighbouring structure.

1. Site plan- a site plan showing the location of the proposed as well neighboring structures.
2. Building plan- showing the detail of height, staging column, etc.
3. Loading condition- dead load, wind or earthquake loads, shown on the schematic plan indicating the design combination of the load transmitted to the foundation.
4. Environmental factor information relating to the geologic history of the area, seismicity of the region, hydrological, information indicating groundwater condition and its seasonal variation, climatic factor like the vulnerability of the site to sudden flooding by surface runoff, erosion, etc.
5. Geotechnical information – subsurface profile with stratification details engineering properties of the founding, strata, index properties, effective shear parameters determined under appropriate drainage conditions, compressibility characteristics, swelling properties, results of field tests like static and dynamic penetration tests, pressure meter tests, etc.
6. Modulus of elasticity and modulus of subgrade reaction.
7. Limiting values of the angular distortion and differential settlement, the superstructure can withstand.

Conclusion

Based on the study carried out, the following conclusions are drawn:. 1) The distribution of pressure below the foundation is very with the change subgrade modulus. 2) Maximum reduction in the negative moment along the MY and MX is more at top solid raft 3) The overturning resisting moment is increased in case soild raft 4) The steel is reduced by providing ring Beam system to the annular foundation. 5) During the analysis it has been observed that moments of the raft changes with respect to SBC. 6) The variation in ring beam size effect moment and shear force annular raft foundation. 7) It is also observed that the saving in cost of construction is 12% by the provision of Beam over the annular raft. There is about 10% saving in steel.

References

[1] Comparative study of annular raft foundation & solid circular raft foundation for different diameter of water tank e-issn: 2395-0056. [2] IS: 875(part 1-5): “code of practice for the structural safety of building loading standards”. [3] IS 1893(part-1):2016, “Criteria for earthquake resistance design of structures”. [4] IS: 456-2000: “Code of practice for plain and reinforced concrete”. [5] Agarwal. P. and Shirkhande.M, “Earthquake resistant Design of Structures” Printice- Hall of India Private Ltd. New Delhi, India. [6] Bhavikatti S.S, “Advance R.C.C. Design” , New Age International (p) Limited, Publishers, New Delhi, India. [7] B.C. Punmia, Ashok Kumar Jain, Arun Kumar Jain (2003), “Soil Mechanics and Foundations”, Published by Laxmi Publications (P) Ltd. [8] N Krishna Raju , Book of advanced reinforced concrete design”. [9] Sajjad Sameer U and Sudhir K. Jain, “Lateral Load Analysis of Frame Staging for Elevated Water Tanks”, Structural Engineering, ASCE, May- 1994, Vol-120, Pg-1375. [2] S.C.Dut

Copyright

Copyright © 2024 Rohit Kumar, Anubhav Rai. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.