1810.3.3 Determination of Allowable Loads
Where required by the design, the uplift capacity of a single deep foundation element shall be determined by an approved method of analysis based on a minimum factor of safety of three or by load tests conducted in accordance with ASTM D3689. The maximum allowable uplift load shall not exceed the ultimate load capacity as determined in Section 1810.3.3.1.2, using the results of load tests conducted in accordance with ASTM D3689, divided by a factor of safety of two.
Exception: Where uplift is due to wind or seismic loading, the minimum factor of safety shall be two where capacity is determined by an analysis and one and one-half where capacity is determined by load tests.
For grouped deep foundation elements subjected to uplift, the allowable uplift load for the group shall be calculated by a generally accepted method of analysis. Where the deep foundation elements in the group are placed at a center-to-center spacing less than three times the least horizontal dimension of the largest single element, the allowable uplift load for the group is permitted to be calculated as the lesser of:
- The proposed individual allowable uplift load times the number of elements in the group.
- Two-thirds of the effective weight of the group and the soil contained within a block defined by the perimeter of the group and the length of the element, plus two-thirds of the ultimate shear resistance along the soil block.
The allowable axial design load, Pa, of helical piles shall be determined as follows:
where Pu is the least value of:
- Sum of the areas of the helical bearing plates times the ultimate bearing capacity of the soil or rock comprising the bearing stratum.
- Ultimate capacity determined from well-documented correlations with installation torque.
- Ultimate capacity determined from load tests.
- Ultimate axial capacity of pile shaft.
- Ultimate axial capacity of pile shaft couplings.
- Sum of the ultimate axial capacity of helical bearing plates affixed to pile.