Design of Rectangular Footings in Cohesionless Soil Subjected to Torsion

This article describes a new method for the design of rectangular foundations in cohesionless soil subject to a combination of vertical and horizontal loads, overturning moments, and torsional moments. It can be seen as a supplement to the general bearing capacity equation in which possible torsional effects of horizontal loads are not included. Torsional effects have had an increasing interest and are common in the design of offshore structures, but onshore structures, such as wind turbines and masts, may also be subject to torsional loads, which can have a substantial effect on the design. The method, which is suitable for hand calculations, applies an equation proposed by Finnie and Morgan for a Tresca material and it is shown that this equation can be used for cohesionless soil for the design of rectangular footings subject to vertical and horizontal forces, overturning moments, and torsional moments. The new method has been compared with those of DNV (DNV: global quality assurance and risk management company in the maritime industry) “Recommendations on offshore soil mechanics and geotechnical engineering” and it is shown that it produces results that are considerably more advantageous than the DNV method. In addition, comparisons have been made with results produced by OptumG3 software, and it is shown that the method leads to safe design. Charts and equations suitable for practical design have been prepared and the equations have been implemented into a computer program, which can be obtained at no cost by contacting the first author.