Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES

Design Example 2

The minimum embedment for the helical piles (or minimum amount of pipe for the piles in Combo piles) is where the resultant shear reaches and stays very close to 0 kip. In this case, it would be 14 ft. There are some scenarios that can increase minimum embedment. If there is a soft layer of soil, the minimum embedment can require bearing below that layer. It can also be used to ensure the last helix plate is a minimum of 5 helix diameters below the surface.

When analayzing the GROUP® results, the important things to look for in Figure 7-7 are: Is the resultant deflection greater than acceptable? Is the moment greater than 0.6 Mp? Generally with a Lattice Structure, 1” lateral defection at working loads is acceptable and 0.6 Mp for resolved is 87 kip-in.

GROUP® Results Figure 7 -8

From Figure 7-8, the important things to look for include, is the total stress greater than the yield stress and is the required axial capacity smaller than half the capacity of the T-Z curve (for a FS=2). In this case the maximum total stress is 48.3 ksi which is less than (50 ksi) and the axial load is 42.2 kip which is less than 90/2=45.

At this point, if the axial capacity from the T-Z curve is considerably larger than double the required by the model, the T-Z curve is adjusted to be closer and the helical configuration is also adjusted in the Helicap model until the right capacity is found.

7-10 | www.hubbell.com/hubbellpowersystems