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Over the years, methods of analysis, design, and construction for structures have gone through major modifications; however, the use of mat foundations to support building loads is ancient and very well documented. The mat foundation is a result of merging individual spread footings which support column loads and rest on soil strata, piles, or piers. The size of the mat foundation partially or fully covers the footprint of the tower structure and usually extends well beyond its footprint. The bearing strata of a mat foundation is determined by the allowable soil bearing capacity, the mat performance in terms of its settlement, and its impact on the structural behavior of the superstructure. This paper discusses the analysis of a soil-supported mat foundation with emphasis on the variation of both structural members and soil properties as they impact design. The different parameters considered in this study are structural property of concrete mat; variation of modulus of subgrade reaction, both in spatial and time domain; and variation in the loading pattern imposed on the mat. An interaction between a realistic superstructure having a perimeter tubular structural frame and a corresponding mat foundation is investigated for different parametric values, such as the stiffness of structural frame and soil properties.

Two mat foundations supporting buildings were analyzed using traditional methods for modeling soil subgrades, as well as more recently developed methods. The primary purpose of the analysis was to evaluate the relative and absolute accuracy of subgrade models that can be used in routine practice. The results indicate that some of the newer methods consistently provide significantly better agreement between calculated and observed behavior compared to the traditional methods. In addition, determination of the appropriate subgrade parameters is more rational with the newer methods. With current computer analysis capabilities, there is no reason to continue use of traditional methods that were reasonable alternatives when only manual calculations could be performed. Detailed recommendations for modeling subgrades in practice are presented, with consideration given to the capabilities of commercially available structural analysis computer software. Other factors that influence mat behavior, such as superstructure interaction effects, are also discussed.

Often, an apparently compatible relationship between mat and soil deteriorates due to the plague of construction details and the design-construction relationship. This paper reviews subgrade reaction in case studies of four landmark buildings in Houston. Concepts related to mat foundation analysis using the finite element method are discussed to acquaint the practitioner with the related soil-structure interaction concepts. Also included is an examination of structural considerations in connection with mat foundation design.

Subgrade response is the most important parameter in analyzing and designing mat foundations. Rational design of a mat foundation requires consideration of immediate and long term subgrade response. The soil response determines mat behavior, and differential movement exacerbates moments. Often, long term movement provides the most severe mat behavior characteristics. The popular use of a single modulus of subgrade reaction k, to model subgrade response is wrong and will lead to wrong designs. Mat analysis and design should be performed using the discrete area method, in which subgrade responses can be properly modeled because of the use of varying moduli of subgrade reaction. The geotechnical engineer and the structural engineer must form a solid bond to cope with mat foundation design from early planning through construction; both must work together to assure successful performance. Often, construction details and procedures will govern performance and can ruin any analysis. For this reason, the geotechnical engineer should be on site accessing conditions.

Reviews methods used in preliminary design of mat foundations, as well as procedures used in final analysis and design. Emphasis is placed on the effect that different structural and soil properties have on mat design bending moments and pressure distribution. Using results of parametric studies on two actual mat foundations, the sensitivity of mat moments and pressure distribution to changes in design parameters is investigated. The soil-dishing phenomena and its effect on mat structural design is discussed. The effect that superstructure stiffness has on mat behavior and the effect that differential settlement within the mat has in the superstructure are also presented and discussed. Finally, specific recommendations are provided regarding selection of proper structural and soil properties to be used in analysis and design of foundation mats.