Transmission And Substation Foundations - Technical Design Manual

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SECTION 7: DESIGN EXAMPLES

Design Example 3

There are a few options for consideration at this point in design. Are the piles to have a fixed or pinned end condition? Batter or no batter? Embed into a concentrated pile cap? A fixed head condition will make the foundation more rigid and result in smaller defections with lateral loads. However, it also results in greater moments. Battered piles will also make a foundation more rigid and result in less deflection. This results in the ability to uses smaller shafts to resist lateral loads, but also required an axial load to work. It is acceptable to embed the pile cap, but there are many variables that have to be considered before doing so. Can it be guaranteed that the cap will always have soil around it? Will the soil around it have the same properties as has been assumed for the top layer? Is the soil disturbed? These are just a few of the items that need to be considered before GROUP®. At this point, the data is input into Group®. Some of the inputs include: the soils, the loads (including different load cases for tension/compression as well as different directions the loads can act), T-Z curve, and the pile configuration/ properties. The pile configuration is going to be made up of 2 sections. The first is a cased pulldown pile (to resist moment) and the next will be an uncased pulldown pile. You want the cut off between the two to be at the point where estimated moment in the pile is less than the cracking moment of the uncased column.

Generally the loads put into GROUP® are working loads. Because GROUP® is estimating lateral deflection; the best way to get a factor of safety is to apply it to the GROUP® results. Please consult the GROUP® Manual for any further information about how to use GROUP®. With the data in GROUP®, the design becomes an iterative process to come up with a pile configuration that works well. If the moment in the piles is too high, they can be spaced further apart or battered at a different angle to relieve it. If the piles have too much axial loading, spacing a little closer together can fix that issue. Sometimes, the loads will just require larger diameter pipe, greater diameter column, or more piles to have an acceptable model. If the axial capacity from the T-Z curve is considerably larger than double the required by the model, the T-Z curve can be adjusted down to get a more cost effective pile. In this case the T-Z curve was adjusted from 100 to 85 kips. Here are the results of the analysis with the T-Z curve capacity of 85 kips on 4 piles battered 10 degrees from vertical, away from the center, and spaced on the corners of a 5’ square.

Resultant Deflection (in)

Resultant Moment (kips-in) 20 2400 60 80 100 120 140

Resultant shear (kips)

0.02

0.04 0.06 0.08 0.01 0.12 0.14 0.16 0.18

0.2 0.4 0.6 0.8 1

1.2

1.4 1.6 1.8

2.2 2.4 2.6 2.8

Distance from PileTop (ft)

Resultant Deflection, Moment, and Shear Figure 7-10

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