Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY Factor of Safety The equations discussed above are used to obtain the ultimate capacity of a helical pile/anchor. For allowable (working) stress design (ASD), an appropriate Factor of Safety must be applied to reduce the ultimate capacity to an acceptable design (or working) capacity. The designer determines the Factor of Safety to be used. In general, a minimum Factor of Safety of 2 is recommended. For tieback applications, the Factor of Safety typically ranges between 1.25 and 2. Design or working loads are sometimes referred to as unfactored loads and do not include any Factor of Safety. They may arise from dead loads, live loads, snow loads, and/or earthquake loads for bearing (compression) loading conditions; from dead loads, live loads, snow loads, and/ or wind loads for anchor loading conditions; and from earth pressure, water pressure, and surcharge loads (from buildings, etc.) for helical tieback or Soil Screw® earth retention anchor conditions. Ultimate loads, sometimes referred to as fully factored loads, already fully incorporate a Factor of Safety for the loading conditions described above. Hubbell Power Systems, Inc., recommends a minimum Factor of Safety of 2.0 for

permanent loading conditions and 1.5 for temporary loading conditions. This Factor of Safety is applied to the design or working loads as defined above to achieve the ultimate load requirement. National and local building code regulations may require more stringent Factors of Safety on certain projects. Most current structural design standards in Canada use a limit states design (LSD) approach for the structural design of helical piles/anchors, rather than working or allowable stress design (ASD). All specified loads (dead, live, snow, wind, seismic, etc.) are factored in accordance with appropriate load factors, and load combinations should be considered. In addition, the geotechnical resistance of the helical pile/anchor must be factored. Geotechnical resistance factors for helical piles/anchors are not yet clearly defined. Therefore, a rational approach should be taken by the designer and resistance factors should be considered that are suitable to specific requirements. These are typical geotechnical resistance factors for helical piles: Compression: 0.65 to 0.75 Tension: 0.55 to 0.65

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