Reliability Analysis of Anchor Foundations Subject to Vertical Uplift Forces
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ORIGINAL PAPER
Reliability Analysis of Anchor Foundations Subject to Vertical Uplift Forces Sougata Mukherjee1
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G. L. Sivakumar Babu1
Received: 18 February 2020 / Accepted: 27 May 2020 Ó Indian Geotechnical Society 2020
Abstract Foundations of transmission towers are subject to uplift forces, and the designs are based on deterministic procedures. Present design guidelines on design of anchor foundations are discussed, and limitations as well as need for incorporation of reliability-based design are brought out. Reliability analysis of anchor foundations subjected to vertical uplift forces using the first-order reliability method is performed. The results of the reliability analysis for different distributions of shear strength parameters of soil and the uplift loads in terms of Hasofer–Lind reliability index have been presented. Load and resistance factors have also been presented to enable reliability-based design. A design example is presented to compare the results obtained from this study. The results show that reliabilitybased approach can lead to an economic design than the conventional factor of safety approach. It is suggested that codes need to incorporate reliability-based designs in practice. Keywords Reliability analysis Reliability index Load and resistance factors Transmission tower Anchor foundation Uplift capacity
& Sougata Mukherjee [email protected] G. L. Sivakumar Babu [email protected] 1
Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India
Introduction In tall structures, like transmission towers, chimneys, wind turbines, utility poles, etc., the predominant force is the horizontal force coming from the wind, earthquake, seepage, etc., which causes uplift at the base. To withstand these uplift forces, different forms of foundations are used. Among these, plate anchors are the most commonly used [19] as they can resist huge uplift forces at shallow depth. The first comprehensive study on pullout resistance of horizontal anchors was reported by Balla [1]. Since then, an extensive research was carried out in this field by several researchers [4, 8, 16, 19, 21, 28, 29, 37, 41]. Evaluation of Uplift Capacity The available design procedures are based on three types of failure surface within the soil placed over the plate. The earliest methods include the friction cylinder method [27]. In this method, the failure surface is assumed to be vertical (Fig. 1a). The ultimate uplift capacity is the sum of weight of the soil within the failure zone and the frictional resistance mobilized along the failure surface. Mors [30] assumed a failure surface approximated as a truncated cone extending above the base with an apex angle of 90° ? u/2, where u is the friction angle of soil (Fig. 1b). In this case, the net ultimate uplift capacity is equal to the weight of the soil present inside the failure zone. But none of these methods provide reliable estimate of the ultimate uplift capacity [29]. Turner [40] pointed out that these methods give the conservative results
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