Chance Technical Design Manual

5.9 HELICAL PILE DEFLECTION AT WORKING LOAD Most of the discussion thus far has focused on evaluating the ultimate load capacity of helical piles/anchors in axial compression or tension. This is considered as the load limit state and gives the upper bound on the load capacity. The deflections of the pile/anchor at this load state will be very large (> 2 inches [>51 mm]), and the pile/anchor deflection will continue to increasewith no additional increase in load capacity. It is also of great interest to most engineers to consider the behavior of a helical pile/anchor at a lower working load or serviceability state which is well below the load limit state. We can consider a typical load-deflection curve as shown in Figure 5-27. This plot shows the test results of a 1.5-inch square shaft helical anchor with a single 12-inch helix installed to a depth of 10 feet in a medium-dense silty sand. The test was performed in tension. According to the IBC, the ultimate capacity is the load producing a net deflection of 10% of the helix diameter. In this case, the ultimate capacity occurs at 1.20 inches of deflection, which corresponds to 19,500 lb. It is obvious that in this case, as in most cases, the anchor can actually hold load up to a deflection of as much as 20% of the helix diameter.

Using an ASD Factor of Safety of 2.0, the working load for this anchor is (19,500 lb)/2.0 = 9,750 lb. Because the load-deflection curves of helical piles/anchors are generally nonlinear, it would be expected that the deflection at the working load would be less than one half of the deflection at the ultimate load (1.20 inches). In this case, the deflection at the working load of 9,750 lb is on the order of 0.36 inches. Using a lower Factor of Safety results in a higher deflection at the working load. For example, if a Factor of Safety of 1.5 is used, the working load becomes (19,500 lb)/1.5 = 13,000 lb, and the deflection corresponding to this load is on the order of 0.55 inches. Based on a review of a number of tests performed on single-helix piles/anchors in Colorado, Cherry and Perko (2012) suggested that for many piles/anchors, the deflection at the working loads (FS = 2.0) averaged about 0.25 inches. Additional work is needed to determine how this may vary for multi-helix piles/anchors and if other soils produce different behavior.

(in)

DESIGN METHODOLOGY

DEFLECTION VS. LOAD PLOT FIGURE 5-27

Page 5-34 | Hubbell Power Systems, Inc. | All Rights Reserved | Copyright © 2023