Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES

Design Example 3

H-Frame Structure Design with Concrete Cap and Micropile Purpose This design example is intended to assist with the design of H-Frame Structure foundations. This example will show how to design using GROUP® and a concrete cap. A grillage can be used for an H-frame structure, and an example of how to design for a grillage is given in Design Example 2 Each leg of the H-Frame Structure will be supported by a concrete pile cap. This type of structure will generally have tension, compression, shear, and moment loads that will need to be calculated or provided by the tower manufacturer After the loads for the structure have been determined, it is possible to design the piles. For this example the loading and soil is as follows:

Loads For This Example • Compression: 100 kip • Tension: 80 kip • Shear:

• Transverse: 9.5 kip • Longitudinal: 1 kip

• Moment

• Longitudinal: 150 kip-ft

Soils • Layers

Helicap Printout Figure 7-9a

• 0-10 ft: 28° Friction Angle Sand • 10-30 ft: 30° Friction Angle Sand • >30ft: 35° Friction Angle Sand • Water Table: 10 ft below surface • Required Reveal Height: 1 ft.

Pile Capacity: The first step in the design is to look at the soil profile and estimate a maximum ultimate axial capacity that can be attained. This is done by putting the boring into Helicap®, selecting the product line you wish to use, and helix configuration. This value is generally the maximum capacity for the pile you wish to use (the axial capacities of shafts can be found in the Drawings and Ratings section), but can be lower if the soils are not very good. You will use this value for the initial T-Z curve in the group model. In this example, we will use a SS175 pulldown pile with helical configuration 10/12/14/14. The estimated capacity is 100 kip (See Figure 7-9 a).

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